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Geological Society, London, Memoirs
Growth patterns and emplacement of the andesitic lava dome at Soufrière Hills
Volcano, Montserrat
R. B. Watts, R. A. Herd, R. S. J. Sparks and S. R. Young
Geological Society, London, Memoirs 2002, v.21; p115-152.
doi: 10.1144/GSL.MEM.2002.021.01.06
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Growth patterns and emplacement of the andesitic lava dome at Soufri~re Hills Volcano, Montserrat
R. B. W A T T S l, R. A. H E R D 2, R. S. J. S P A R K S 1 & S. R. Y O U N G 2
1 Department o f Earth Sciences, Wills M e m o r i a l Building, University o f Bristol, Queens Road, Bristol B S 8 1R J, U K
(e-mail: Rob. Watts@bris.ac.uk)
2 Montserrat Volcano Observatory, M o n g o Hill, Montserrat, West Indies
Abstract: Eruption of the Soufri+reHills Volcano on Montserrat allowed the detailed documentation of a Pel~an dome-forming
eruption. Dome growth between November 1995 and March 1998 produced over 0.3km3 of crystal-rich andesitic lava.
Discharge rates gradually accelerated from < 1m3s-1 during the first few months to >5 m3s-1 in the later stages. Early dome
growth (November 1995 to September 1996) was dominated by the diffuse extrusion of large spines and mounds of blocky lava.
A major dome collapse (17 September 1996) culminated in a magmatic explosive eruption, which unroofed the main conduit.
Subsequent dome growth was dominated by the extrusion of broad lobes, here termed shear lobes. These lobes developed
through a combination of exogenous and endogenous growth over many weeks, with movement accommodated along curved
shear faults within the dome interior. Growth cycles were recognized, with each cycle initiated by the slow emplacement of a
large shear lobe, constructing a steep flank on one sector of the dome. A growth spurt, heralded by the onset of intense hybrid
seismicity, pushed the lobe rapidly out, triggering dome collapse. Extrusion of another lobe within the resulting collapse scar
reconstructed the steep dome flanks prior to the next cycle.
In recent decades, phenomena observed during the growth of lava
domes have been closely monitored, the most notable examples
being at Mount St Helens, USA, between 1980 and 1986 (Swanson et al. 1987), Mount Pinatubo, Philippines, in 1991-1992 (Daag
et al. 1996) and Mount Unzen, Japan, in 1991-1995 (Nakada et al.
1999). As a result, many emplacement features and the processes
controlling their formation have been described (e.g. Anderson &
Fink 1992). The ongoing eruption of Soufri~re Hills Volcano on
Montserrat has involved the construction of an andesitic lava
dome, with alternating phases of growth and gravitational collapse
(Young et al. 1998). The eruption required an intense scientific
monitoring effort, due to the associated hazards and their threat to
the local population, and this has yielded extensive data records on
the eruption. These records have been used to distinguish patterns
of growth and to develop a better understanding of the mechanisms
controlling lava-dome eruptions (Sparks 1997; Voight et al. 1999;
Melnik & Sparks 1999, 2002; Wylie et al. 1999; Sparks et al. 2000).
Because of near-daily helicopter observation flights during the
eruption, an impressive photographic and video collection has
accumulated, documenting the growth and collapse of the lava
dome. Detailed monitoring of these morphological changes was
also achieved using various surveying techniques, such that an
accurate record of dome growth and magma production rate is
available (Sparks et al. 1998). All of these data have been used, in
conjunction with ground observations, theodolite, and electronic
distance measurements, to produce maps detailing the complex
development of the dome in time and space.
This paper describes the chronological evolution of the dome
throughout the first episode of dome growth (November 1995 to
March 1998) with the aid of maps and photographs, showing
examples of the different structures that were extruded at particular stages of the eruption. We discuss the relationships between the
formation of these structures and the controlling mechanisms
during magma ascent and emplacement of the dome. Understanding of changing rates and styles of dome growth is vital
in successful hazard assessment during dome-forming eruptions.
We demonstrate that different growth styles are intimately associated with the generation of pyroclastic flows and the inception of
explosive eruptions. We also develop an interpretation of the observations that attributes much of the morphological variation and
behaviour to rheological stiffening of the magma caused by degassing and associated crystallization and also to deeper processes in
the magma chamber, which periodically supplies pulses of fresh,
gas-rich magma.
The terminology used is as follows. There was one lava dome
extruded between 15 November 1995 and 10 March 1998. Extrusion of a second dome began in November 1999, but is not dealt
with in any detail in this paper. Individual extrusions during the
November 1995 to March 1998 period are termed lobes, and each is
named by its date of first appearance.
Geological setting
Montserrat is a mountainous, diamond-shaped island, 16 km long
and 9 km wide, that lies towards the northern end of the Lesser
Antilles island arc. It is almost entirely volcanic in origin and
is dominated by three volcanic centres. Recent 4~
dating
(Harford et al. 2002) highlights a southerly shift in the focus
of magmatism with time, from the low-lying Silver Hills (e. 2600
to 1200kaBp) in the north, through the Centre Hills (c. 950 to
550kaBP) and the South Soufri6re Hills-Soufri6re Hills complex
(c. 160 ka to the present) in the south. The most recent activity has
focused on the Soufri6re Hills region in the south-central sector of
the island (Fig. 1). This activity has involved the sporadic growth
and collapse of at least five andesitic lava domes and the formation of associated pyroclastic aprons surrounding these domes. The
youngest and smallest dome, Castle Peak, was probably formed in a
small eruption e. 350 years ago just prior to colonization of the
island (Robertson et al. 2000; Harford et al. 2002). This dome
nestled centrally within English's Crater (Fig. 2), a large structure
breached to the east, which is believed to have formed by a c. 4 ka
sector collapse (Roobol & Smith 1998). A moat thus circled Castle
Peak dome on two-thirds of its circumference, with the final third
facing into the eastward-trending Tar River valley. The 1995-1998
episode of dome growth involved the near-continuous extrusion of
300 x 106 m 3 of andesitic lava, constructing a complex Pel6an dome
on top of Castle Peak dome and within English's Crater. A further
100 x 106 m 3 of lava has been extruded since November 1999 up to
the time of writing (October 2000). A consequence of this eruptive
activity has been the partial destruction and burial of both Castle
Peak and the rim of English's Crater.
Eruption chronology
Since the onset ofphreatic activity on 18 July 1995, the eruption has
progressed in an atypical manner in comparison with other well
documented historical dome eruptions (Newhall & Melson 1983).
From the initial phreatic period continuing through to the present
day, a sequence of distinct eruptive phases has been experienced
(see Table I). These phases highlight a fluctuating magma discharge
DRUITT, T. H. & KOKELAAR,B. P. (eds) 2002. The Eruption of Soufridre Hills Volcano, Montserrat, from 1995 to 1999.
Geological Society, London, Memoirs, 21, 115-152. 0435-4052/02/$15 9 The Geological Society of London 2002.
115
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116
R . B . WATTS E T AL.
Fig. 1. Pre-eruption map showing the
location of andesitic domes (light shaded
areas) of the Soufri+re Hills-South
Soufri+re Hills complex in southern
Montserrat. Dashed line marks the outline
of the rim of English's Crater and the sides
of the Tar River valley. Darker shaded
areas are older uplifted pyroclastic
sequences. Inset map shows the
entire island.
Fig. 2. Early January 1996. View of Castle
Peak (CP) sitting within English's Crater
(EC), a c. 1 km diameter structure, looking
west from above the Tar River valley. Pale
lava on top of Castle Peak is new dome
growth (D), and brown-stained vegetation
results from early phreatic activity. Gages
dome (G) is seen in background to right.
Table 1. Growth rates and associated surface/extrusive phenomena observed during the 8 eruptive stages of the current eruption
Stage
Time period
Growth rate (m 3 s -l)
Surface phenomena and extrusive features
I
18 July 1995 to 14 November 1995
Pre-dome
Phreatic explosions
II
15 November 1995 to 16 February 1996
0.1-0.5
Spines, whaleback structures
III
16 February 1996 to 30 September 1996
1-4 with daily spurts of >5 in July and
August
Spines § megaspines, Type 1 + Type 2
shear lobes
IV
1 October 1996 to 12 December 1996
0.5-2
Blocky lava § spines, endogenous activity
V
13 December 1996 to 13 May 1997
2-4 with daily spurts of >5 in December
and January
Megaspines + blocky lava, Type 1 + Type 2
shear lobes
VI
14 May 1997 to 10 March 1998
>5 with two phases of post-collapse explosive
activity
Type 1 shear lobes, blocky lava
VII
11 March 1998 to mid-November 1999
No surface extrusion
Sporadic dome collapse, sporadic explosions
VIII
Mid-November 1999 to date
2-5
Spines, Type 1 shear lobes + blocky lava
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GROWTH PATTERNS AND DOME EMPLACEMENT
1100
(a)
Cessation
of growth
.t
25 June
collapse
1000,
..'J..
30131 July
collapse
~
*,
Z
$
900,
to~
.~
9
e-
._~
800
f
9~'""
i
26 December
collapse
17 September *
collapse ~ ~
"1-
,0% 11oo
~oo
21 September
collapse --->-
300 ,ool
~o
1996
/
60o ,oo-I~oo-~o-1ooo
1997
1998
120
117
ones sending clouds of steam and dark ash aloft, which were blown
westward towards the capital town of Plymouth. The first large
phreatic explosion on 21 August initiated the first full evacuation of
southern Montserrat. Of significance were the explosion craters of
July, August and November, which formed a N N W - S S E alignment
across Castle Peak (Fig. 4). Much of the subsequent magma discharge was focused along this zone, which suggests a fundamental
fracture control on magma ascent. Extrusion of the first fresh lava
occurred on the SW flanks of Castle Peak (Fig. 4), although it
was initially interpreted as a cryptodome due to its highly oxidized nature. This small dome extruded on 25 September 1995 and
continued to extrude for the next four or five days, generating
small rockfalls prior to the growth stopping. A central spine of
less oxidized material was extruded last, and degradation of the
surrounding dome during October 1995 increased the prominence
of this spine.
I
(b)
VII
100
Stage H." 15 November 1995 to 16 February 1996
VI
E
%
TX
80
60
V
[ IV ] .,.. "'~
40
III ..,...
aO
20
II
o o,..i."lOO
,.-
.s
200
1996
300
404
500
6()0
700
1997
Eruption Days
1800
960
1000
1998
Fig. 3. (a) Graph showing the change in height of the active focus of growth
throughout the eruption. Day one represents the onset of dome growth on
15 November 1995. (b) Graph showing the change in dome volume
throughout the eruption (after Sparks et al. 1998 with more recent updates)
and the different eruptive stages of the 1995-1998 period.
rate or pulses in magmatic activity that overprint a gradual escalation in eruptive vigour and background magma discharge rate
(Sparks et al. 1998). As each phase of activity occurred, lava was
extruded in a variety of styles and morphologies. Figure 3 details
the volume of the dome and height of the active area of dome
growth with time in the 1995-1998 episode of dome growth. Here
we have divided the history of dome growth into stages, which are
identified on the basis of prominent changes in dome growth patterns or significant volcanic and/or seismic events (Table 1). The
main shifts in dome growth are given in Table 2 with notes on the
different styles of growth during these shifts. Below we describe
the dome evolution in chronological order, drawing attention to
major styles of growth and morphological development. This paper
does not consider in detail the new episode of dome growth,
which started in November 1999, although similar patterns of dome
growth are being observed to those described here.
Stage L" 18 July 1995 to 14 November 1995
The start of the eruption on the 18 July 1995 was heralded by a
vigorous phreatic vent opening at the site of the poorly defined
Langs Soufri+re on the N W flanks of Castle Peak (Fig. 4). Islanders
were disturbed by a continuous roaring sound and the sight of a
near-continuous jet of steam from within English's Crater. In later
weeks, further new vents opened around Castle Peak (Fig. 4) and
phreatic activity continued for over four months on a near-daily
basis. Phreatic explosions were of a variable intensity, the larger
Phreatic activity continued throughout October and on until 15
November, when two small piles of fresh lava blocks were observed,
one within the twin craters of the 18 July phreatic vent and the other
between it and the September spine (Fig. 4). Growth of a new dome
was confirmed, both by the presence of incandescent lava blocks and
the onset of hybrid seismicity (Miller et al. 1998). The lava blocks
were pale grey and generally <5 m in diameter, with larger, curving
spines jutting out from the crater floor. The main growth occurred in
the more southerly crater of July 1995, which was filled completely
with new lava by the end of November. Spines were the main
extrusive feature of this early stage of dome growth (Table 3), with
typical widths of 30m and heights up to 35m. They exhibited a
curved, outer surface covered by a thin breccia coating, with distinct
subparallel grooves or striations running along their length. After
several days of growth (typically at a few metres per day), spine
collapse would occur due to gravitational stresses, forming piles of
rock debris around the stubby base of the spine. Spines typically
grew to an altitude of 810-860 m above sea level (a.s.1.) in this period
of low magma discharge rates (0.1-0.5m 3 s-l). Growth of these
spines was focused along the N N W - S S E zone defined by the early
phreatic vents, particularly in the vicinity of the July 1995 craters.
In Stage II, many spines emerged (Table 3) and grew on the western
flanks of Castle Peak, such that the new dome developed as coalescing piles of spine debris (Fig. 5a). Fumarolic activity commonly
occurred at the edges of these piles with associated rockfall activity increasing as the dome grew. A prominent feature of the freshly
extruded lava was the apparent lack of flow structures and nearsolid appearance of the blocks and spines.
An interesting feature, formed between 24 January and 6 February, was the formation of whaleback structures coincident with
a period of seismic tremor and slightly raised extrusion rates of
around 1 m 3 s -l. These elongate bodies extruded individually as
extremely viscous lava from the N N W - S S E vent pattern in different directions (Fig. 5b). Each whaleback was pushed across the
surface of Castle Peak by continued magmatic pressure (in a manner analogous to toothpaste being squeezed out of a tube). Whaleback structures would reach up to 200m long, 30m wide and
35 m high, and exhibit a smooth surface with a grooved appearance
and striations aligned parallel with the direction of extrusion (Fig.
6a, b). Growth of each whaleback structure occurred sporadically
at an estimated 20-30 m day -1 and continued to move for up to a
week prior to growth stagnating and the subsequent break-up of
the structure into a chaotic jumble of blocks (Fig. 7). During
growth, as the whaleback pushed forward, rockfalls spalled off
its steep headwall, exposing incandescent lava within the interior. Whalebacks are previously undocumented, although they have
been witnessed during other eruptions (e.g. Santiaguito dome
in Guatemala, W. I. Rose, pers. comm.). Less distinct rubbly lava
also extruded in January 1996, although poor visibility prevented
detailed documentation.
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R . B . WATTS ET AL.
118
Table 2. Chronology of activity in the 1995-1998 period of dome growth at the SoufriOre Hills Volcano
Date
Activity
25-29 Sep. 1995
First juvenile lava extruded as small pile of blocks and spines on SW flanks of Castle Peak, but extrusion rapidly
stagnated.
15Nov. 1995
Onset of continuous extrusion of juvenile lava as spines on NW flanks of Castle Peak.
24 Jan. 1996 to 3 Feb. 1996
Extrusion of northern whaleback over several days followed by southern whaleback.
15-18 Feb. 1996
Extrusion of southeastern whaleback.
Mid-Feb. to early April 1996
Repeated extrusion of large vertical spines in central area of Castle Peak, spine collapse produces first pyroclastic flows
down Tar River valley.
Late April 1996
Growth spurt preceded by hybrid earthquake swarm, forming shear lobe of blocky lava (i.e. 25 April 1996 lobe) at
summit directed to NE flanks.
1-3 June 1996
Extrusion of megaspine to form northern peak.
22-25 June 1996
Extrusion of megaspine to form southern peak.
12-15 July 1996
Extrusion of megaspine to form southwestern peak.
Mid-July to early Sep. 1996
Vigorous extrusion of shear lobes of blocky lava towards NE dome flanks during growth spurts triggering dome
collapses on 29-31 July, 11-14 Aug. and 2-3 Sep.
17 Sep. 1996
Directed explosion following major dome collapse down Tar River valley removing one-third of dome volume.
1 Oct. 1996
Onset of renewed dome growth (i.e. 1 Oct. 1996 lobe) at base of explosion crater at low extrusion rate.
Early Nov. to early Dec. 1996
Intense seismicity accompanying minor dome growth, suggesting period of endogenous activity. Localized doming on
southern dome flanks.
13Dec. to 24Dec. 1996
Rapid extrusion of 13 Dec. 1996 megaspine along shear zone in southeast sector. This megaspine is quickly
overwhelmed by blocky lava generating dome collapse on 19 Dec.
25Dec. 1996 to 10 Jan. 1997
Rapid extrusion of 25 Dec. 1996 lobe forming 'pancake' morphology in central part of dome following localized
endogenous doming. This rapidly overwhelms 13 Dec. 1996 lobe and 1 Oct. 1996 lobe.
Mid Jan. to late Jan. 1997
Rapid blocky growth from central area directed across SE flanks, generating semi-continuous pyroclastic flow activity
and large collapses on 16, 19 and 20 Jan.
Early Feb. to 26 March 1997
Blocky lava of 21 Jan. 1997 lobe directed down southeastern and eastern flanks from central area forming a conical
dome morphology.
27 March to mid-May 1997
Growth of 27 March 1997 lobe guided southwards triggering collapse of southern flanks on 30-31 March and 11 April
leading to inundation of White River valley.
16May 1997
Large vertical spine extruded at dome summit.
17May to early July 1997
Growth of 17 May 1997 lobe across northern flanks triggering dome collapse down northern flanks on 25 June
followed by regrowth of 27 June 1997 lobe to north.
Mid-July to late July 1997
Stagnation of active lobe, growth slightly switched to NW flanks (14 July 1997 lobe).
Late July to 12 Aug. 1997
Repeated small collapses down western flanks lead to major collapse on 3 Aug. Entire 14 July 1997 lobe collapsed
triggering period of 13 repetitive Vulcanian explosions.
13Aug. 1997
Renewed blocky growth in crater followed by formation of active lobe to the west.
8 Sep. 1997
Stagnation of 13 Aug. 1997 lobe with fresh lobe growth directed northwards. Rockfall and pyroclastic flow activity
completely fills Mosquito Ghaut.
21Sep. 1997
Rapid extrusion of 8 Sep. 1997 lobe to the north triggers major dome collapse directed down Tuitt's Ghaut forming
amphitheatre-shaped edifice. This triggers a period of 75 Vulcanian explosions (22 Sep. to 21 Oct.).
22 Oct. to 2 Nov. 1997
Renewed blocky growth in crater developing into northward growing 22 Oct. 1997 lobe.
3-6 Nov. 1997
Stagnation of 22 Oct. 1997 lobe contemporaneous with three collapses concentrated on the southern flanks attributed
to switch in active lobe to the south.
Mid-Nov. to 25 Dec. 1997
Continuous growth of southerly directed 4 Nov. 1997 lobe constructing a large peak straddling former Galway's
Wall area.
26 Dec. 1997
Major debris avalanche and sector collapse removing the entire southern flanks (i.e. 4 Nov. 1997 lobe) formed in
previous two months. Violent pyroclastic density currents.
27 Dec. 1997 to late Feb. 1998
Redevelopment of southerly directed lobe (i.e. 27 Dec. 1997 lobe) rebuilding the large peak on the southern flanks.
1-10 March 1998
Cessation of first period of dome growth signalled by 50m high spine at summit of 27 Dec. 1997 lobe.
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380500
381400
1847600
~
O
O
LEGEND
Explosionpit
Avalanchescari
Sept1995
dome
Nov 1995
dome
Contour Interval - 100 feet
1 ft=0.3048 m \
Fig. 4. Map of English's Crater on
25 November 1995 showing the location of
phreatic vents (with dates when they first
appeared) and the initial extrusions of fresh
andesitic lava in the early eruptive stages.
Note that the 21 August phreatic vent
opened up along a fracture-controlled line
of smaller vents. Farrell's Wall, Gages Wall
and Galway's Wall represent different
sectors of the steep avalanche scar defined
by English's Crater. The rim of the crater is
marked by a dashed line. Peaks A B and C
on this rim were used as prominent
topographic features in surveys of the dome.
Map co-ordinates are part of the
Montserrat Grid System.
Aug
Chances ~
v
Peak
e" ~ . ~ , , . ~ ~ - - ~ ~ ' ~ ' - ~ ~
0
I
25 Sept Dorr
e + tal u s
100m
I
1846600
Dome Volume = 0.1x106 m 3
23 N O V E M B E R 1995
3 -1
Extrusion Rate = ~0.17 m s
Table 3. Theodolite data collected during Stages H and I l l of the eruption to monitor the height and growth rate of spines extruded at
this time
Date (eruption day)
Theodolite location
15/11/95 (1)
5/12/95 (20)
10/12/95 (25)
17/12/95 (32)
20/12/95 (35)
Long Ground
Long Ground
Long Ground
Whites
Whites
24/12/95 (39)
26/12/95 (41)
28/12/95 (43)
30/12/95 (45)
3/1/96 (49)
8/1/96 (54)
11/1/96 (57)
22/1/96 (68)
5/2/96 (82)
21/2/96 (98)
11/3/96 (117)
19/3/96 (125)
22/3/96 (128)
25/3/96 (131)
26/3/96 (132)
Whites
Whites
Whites
Whites
Whites
Whites
Long Ground
Photo method*
Photo method*
Whites
Observatory
Whites
Whites
Whites
Whites
29/3/96 (135)
30/3/96 (136)
5/4/96(142)
Whites
Whites
Long Ground
18/4/96 (155)
20/4/96 (157)
30/4/96 (167)
Long Ground
Long Ground
Whites
Height of spine
(m a.s.l.)
Height of summit
(m a.s.1.)
Spine growth rate
(m day -1)
760
814
805
805
825
>7
Cathedral spine
9
Highest of 3 spines
2.5
817
814
812
817
810
814
842
4
>9
823
825
821
866
847
857
874
885
9
5
11
Spine width = 35 m
855
862
883
14
Spine width = 44 m
905
903
929
* Measurements obtained by a technique used to calculate changes in dome volume through comparison of photographs taken
from the same location. Dates in European format (5/12/95--5 December 1995). a.s.l., aboe sea level.
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120
R. B. WATTS E T AL.
Fig. 5. Sequence of maps showing gradual development of the dome during the initial three months of dome growth leading to onset of pyroclastic flows at the
end of March 1996. Shaded area marks the new dome growth. X-Y represents the line of section used in Figure 7.
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GROWTH PATTERNS AND DOME EMPLACEMENT
121
Fig. 6. (a) 2 February 1996. View looking west above Castle Peak showing two whaleback structures on the new dome: the southern whaleback is marked S
and the northern whaleback N (see Fig. 5b). Note the steep walls of English's Crater (EC) and the dome of Chances Peak (ChP) and telecommunications
tower in the centre background. Gages Wall (G) is on the right with Plymouth Town (P) in the distance. (b) 1 February 1996. View looking east from
crater rim near Chances Peak. Elongate structure in centre is the southern whaleback (S), which extruded subhorizontally away from the central area over
several days (see Fig. 5b). Note the smooth outer surface of the whaleback, with rocks spilling off the leading edge on the right. Loose blocks sitting on its
top are carried along during growth. CP is the main spine of Castle Peak dome.
Stage IlL" 17 February 1996 to 30 September 1996
In mid-February, a marked rise in the average background extrusion rate (r 2 m 3 s -1) was experienced at the start of this period,
following a volcanotectonic earthquake swarm on l l February
1996. There was also a steady increase in dome height, which
reached 960m a.s.1, by the end of June 1996 (Fig. 3a). Spectacular
spines with prominent vertical striations scored along their smooth,
outer surfaces, and exhibiting characteristic curved-horn shapes
were commonplace throughout March and April (Fig. 8a, b).
Growth rates of these spines averaged 6 - 7 m day -1, sometimes
over 10m day -1, attaining heights of c. 40m before collapsing
(Table 3). Spine growth was generally concentrated in the central area of Castle Peak, a slight southerly shift from the initial
growth area of mid-November 1995. The continuous formation of
spine debris enlarged the area covered by the new dome. By the
end of March, the entire western half of Castle Peak had been
overwhelmed by fresh lava (Fig. 5c) and blocky talus was rapidly
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122
R.B. WATTS E T AL.
Fig. 7. Schematic diagram showing the development of whaleback
structures in January 1996, along section X-Y in Figure 5b. (a) 15 January
1996. Dome growth occurring as spine extrusion and gravitational collapse.
(b) 24 January 1996. Whaleback structure extrudes northwards away from
central area averaging 30 m day -l . (e) 29 January 1996. Growth of northern
whaleback stops and another whaleback starts to drag across the dome
surface southwards away from the centre at 20-30m day 1. (d) 5 February
1996. Both whalebacks have stagnated and gradually break up. Spine
growth and collapse recommences around summit area.
filling the moat of English's Crater. Rockfalls were no longer
confined by the walls of English's Crater on the NE flanks, and they
developed into small pyroclastic flows down the upper parts of the
Tar River valley.
The pulsatory character and most common style of dome
growth can be illustrated by. observations in the second half of
April. Two spines were extruded around 14-16 April from the July
1995 crater area coincident with a period of elevated hybridearthquake seismicity. The spines toppled over, one to the west and
the other to the east. While the westerly spine remained stagnant,
the easterly spine started to break up on 25 April as new lava
started to extrude. This blocky lobe then expanded to the NE and
formed a steep headwall at its advancing front from which rockfalls
cascaded down into the Tar River valley. The flow front position
stagnated when it reached the steep edge of Castle Peak with
generation of rockfalls at the flow front matching supply of lava
from the lobe. The flow front developed a furrowed appearance of
ridges and chutes related to repeated generation of erosive rockfalls
spilling down from the summit of the lobe. This blocky lobe, here
termed a shear lobe to highlight this process of directed extrusion
(Fig. 9a), continued to produce lava until 2 May, when growth
became more focused around the summit. This formation of shear
lobes was repeated many times throughout the eruption although
the style of lobe development was markedly different in the later
eruptive stages (Fig. 9b, c), allowing a classification between earlystage Type 2 lobes and late-stage Type 1 lobes. A feature common
to both types was that the summit of a lobe was commonly shifted
away from the vent area during growth, giving an illusion of
shifting vent positions as an individual lobe developed. A similar
illusion occurred when a lobe stagnated and a fresh lobe was
initiated, growing in a different direction from the previous lobe.
Formation of Type 2 lobes was apparent after extended periods
of stagnation, while Type 1 lobes predominated during periods of
more steady growth, although fluctuations in discharge rate were
evident during the development of both structures.
From June to July 1996, noticeable shifts in the focus of
dome growth followed the slow extrusion of broad features (up to
100m wide) that are best described as fault-bounded megaspines
(Fig. 10a, b). A megaspine is characterized by two contrasting parts.
One side of a megaspine consists of a smooth, striated and curving
wall which is interpreted as moving along a large fault in the dome.
The other side consists of a headwall of massive, blocky material
that breaks up during growth. A megaspine grows by upward or
subhorizontal movement along the fault structure with lava blocks
spalling off the main headwall as growth occurs (Fig. 10c). Emplacement of such a large structure often stopped after a few days,
with renewed activity taking place in another localized part of the
dome. The most notable examples of megaspine growth occurred
during 1-3 June (to the north), 22-25 June (to the south) and 12-15
July 1996 (to the SW). Each of these extrusions formed a prominent
peak on the dome and originated from the same central focus of
growth. At this central focus, extrusion of fresh lava was guided
along a curvilinear shear fault that directed the lava in a specific
direction, sometimes over a hundred metres away from the previous
growth area (Fig. 11).
By mid-July 1996, the new dome was a substantial size (c. 30 x
106m 3) and had multiple peaks due to the repeated formation of
megaspines. Vigorous spurts of dome growth were also evident at
this time in pulses lasting several hours, at estimated discharge rates
of >5 m 3 s-1. The focus of growth was located in the central area,
with the 17 July 1996 lobe directing fresh lava down the NE flanks of
Castle Peak and the upper reaches of Tar River valley. At these rapid
growth rates, distinctive piles of blocky lava (c. 4-5 m diameter) were
extruded, exhibiting a curvilinear shape with occasional larger spines
projecting out. The coincidence of this vigorous growth and the
focus of growth directing lava to the NE resulted in a series of major
dome collapses producing large pyroclastic flows on 29-31 July,
I I August, 20-21 August and 2-3 September 1996. This involved
the repeated collapse and reconstruction of the NE dome flanks
(Fig. 12a, b) and the gradual inundation of the entire Tar River
valley (Cole et al. 2002). These repeated collapses led to a decrease
in the height of the active area of growth (Fig. 3a). Renewed growth
was always focused at the base of the spoon-shaped scar formed by
each collapse, often accompanied by a vigorous, semi-continuous
ash plume from the central area. A new lava lobe, often bounded by
a shear surface at the backwall, would then extrude as large, curved
blocks that rapidly filled and engulfed the entire scoop (Fig. 12c).
The lava lobe always expanded asymmetrically towards the open
side of a previous collapse scar, guiding it away from the vent area.
A particularly long episode of near-continuous collapse (c. 9 hours)
occurred on 17 September, culminating in sub-Plinian magmatic
explosive activity (Robertson et al. 1998). This sustained collapse
removed over one-third of the new dome (c. 11.8 x 106 m 3 volume
non-dense rock equivalent, all volumes quoted here are collapse
volumes calculated from deposit volumes and/or collapse scar
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Fig. 8. (a) 9 April 1996. View looking north across English's Crater and the new dome, with the northeastern flanks and airport in the background. Large spine
of fresh lava (P) is c. 40 m high and c. 35 m broad. Broad spine in centre (Q) is the main spine of Castle Peak dome. Note Galway's Wall (GW) in foreground.
(b) 12 April 1996. Typical curved-horn spine extruded in Stages II and III. Note the semi-cylindrical shape with smooth and striated outer surface. Other half of
spine is massive lava of the spine interior that spalls off to form a basal skirt of debris around the base of the spine. This spine is estimated to be 40 m high and
35 m broad. Note Perches Mountain (PM) in background to the right.
volumes (Calder et al. 2002), and a substantial portion of the underlying Castle Peak dome, leaving a large steep-sided scar in the central area open to the east (Figs 13a and 14a).
Stage IV." 1 October 1996 to 12 December 1996
Extrusion of fresh lava did not resume for two weeks following the
17 September 1996 explosive eruption, which was estimated to have
reamed out the conduit to a depth of 4 km (Robertson et al. 1998).
This event involved substantial widening of the upper conduit,
with abundant ballistic ejecta of vent-wall breccia, hydrothermally
altered rocks of Castle Peak and dense, juvenile blocks thought to
have originated in the upper conduit (Robertson et al. 1998). The
explosion was directed to the east as evidenced by strong asymmetrical distribution of ballistic ejecta, and this activity is believed
to have flared open the upper conduit. Lava started to extrude at
the base of the scar on 1 October with an initial discharge rate of
1.8 m 3 s -1. The focus of upwelling was noticeably shifted c. 150 m to
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R.B. WATTS ET AL.
Fig. 9. Characteristic features of Type 1 and Type 2 shear lobes shown in
schematic form. (a) Development of a Type 2 shear lobe: following an
extended period of stagnation, a fresh pulse of magma pushed out a viscous
plug that was emplaced as a large spine or megaspine. The hotter magma
that forced out the plug continued to ascend along another shear fault that
detached from the conduit wall and magma extruded in the direction of least
resistance (see Fig. 11). During extrusion, the lobe broke up into curving
blocks with the highest point often displaced laterally away from the
conduit, giving the illusion of shifting vent locations when a new lobe moved
in another direction. The steep flow front advanced until it reached a steep
slope (e.g. the margin of Castle Peak or the rim of English's Crater) where
the front stagnated and generated rockfalls and pyroclastic flows as lava was
supplied from behind. (b) Early stage development of a Type 1 shear lobe:
following shorter periods of stagnation, a new shear lobe extruded with a
large, coherent, smooth and striated upper surface with a broad headwall of
massive lava. The upper surface developed a broad, semi-cylindrical shape
supported by the surrounding dome flanks. This switch in the focus of
activity and early stage development of a new lobe often triggered a dome
collapse, e.g. 4 November 1997. (e) Late stage development of a Type 1
shear lobe: following stagnation of the viscous plug, hotter and more ductile
lava would rise up along the same shear zone. Broad, curving spines
extruded and broke up at the rear of the lobe and this activity alternated
with the injection of magma into the core of the lobe. This latter process
expanded the summit area and triggered rockfalls off the leading edge of the
lobe. The repeated nature of these processes over several weeks developed a
conical summit with a broad skirt of blocky talus. Formation of a Type 1
lobe was most apparent during periods of steady-state growth when only
minor fluctuations in the average discharge rate were experienced.
the east in comparison to the location of spines and upwellings
before 17 September 1996. From this time, the dimensions of lobes
generally became substantially greater. This temporary shift in
growth foci and the larger dimensions of subsequent lobes are
attributed to the widening of the upper conduit asymmetrically to
the east.
Initially, the new lobe consisted of a slab of smooth lava overlying loose talus (Fig. 14a, b). The morphology of this lava exhibited
a transition over several days from smooth (Fig. 14b) to an unusual
darker, rubbly surface (Fig. 15a) and eventually to the blocky and
spiny appearance that had previously characterized the dome. This
period of activity shows an excellent example of the morphology
formed by new growth infilling the scar after a major collapse. The
early growth of the 1 October 1996 lobe was also the first example in
this eruption of a lava morphology affected by lateral spreading
(Fig. 15a). As growth continued, the lobe gradually filled the scar
at a decreasing discharge rate and had apparently stagnated by
20 October 1996. Renewed extrusion, still at a reduced rate, took
place on 22 October, but focused only on the central part of the
lobe. This formed a central raised area of blocks and small spines
surrounded by the lower abandoned sectors of the lobe (Fig. 13b).
In early November 1996, intense shallow earthquake swarms
occurred, dome growth rates dwindled to <1 m3s -1, and dome
height stagnated at 900 m a.s.1. By mid-November, further earthquake swarms triggered landslides off the steep outer face of
Galway's Wail on the southern rim of English's Crater (Fig. 15b).
This apparent intrusion into the dome, causing localized uplift of
over 30 m on the southern dome flanks, was the first clear evidence
for endogenous activity during the eruption. This raised levels
of concern that the threat of sudden collapse of Galway's Wall
might trigger rapid decompression of the dome interior (Young
et al. 2002).
Stage V: 13 December 1996 to 13 M a y 1997
The crisis related to the instability of Galway's Wall was temporarily
relieved on 13 December 1996 when the southeastern margin of
the 1 October 1996 lobe was uplifted along a major ductile shear
zone bounded by a steep, striated fault with a trend of 110 ~ The
13 December 1996 lobe (also known as the 'Venus' lobe) initially
extruded as a megaspine near the zone of endogenous uplift. This
fault-bounded feature extruded rapidly in a southeasterly direction
overnight and continued growth along the same trend for several
days, forming a single entity at least 150 m long, 100 m wide and
100 m high (Figs 16a and 17a). Within days, the megaspine had been
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125
Fig. 10. (a) and (b) Maps highlighting the switches in the focus of activity during June and July 1996 following the emplacement of megaspines. Dotted lines
represent approximate boundary between massive lava and loose talus blocks (for clarity, this distinction is not marked in later maps). (c) 19 August 1996,
looking east from summit of Chances Peak. Megaspine extruded in early July 1996 (c. 40 m high and c. 100 m broad) showing a smooth, curved northern face to
the left and massive, crystalline lava breaking off its broad headwall (H) as large blocks to the right. This large feature was subvertically emplaced in the
northwest sector of English's Crater (see a) and remained a prominent peak on the dome for the following six months. Also in view is another megaspine (S)
emplaced on the southern sector of the dome in late June 1996 (see b).
overwhelmed by the pulse of magma that had initiated its extrusion.
Near-continuous rapid extrusion of blocky lava (c. 4 - 6 m 3 s -1) was
guided along the same southeasterly directed shear fault that
extruded the megaspine, and pyroclastic flows spilled off the SE
dome flanks.
On 25 December 1996 a pronounced pulse of activity, heralded
by localized endogenous doming and episodes of banded seismic
tremor (Miller et al. 1998), led to the emplacement of the
25 December 1996 lobe, also known as the 'Santa' lobe (Fig. 16b).
This new lobe initially punched through the central summit of the
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126
•
R. B. WATTS E T AL .
u
Fig. 11. Schematic diagram illustrating the emplacement of a megaspine
and the switching of activity that was apparent in June and July 1996.
Diagrams represent a cross-section along the line X-Y in Figure 10b.
(a) 30 June 1996. This was a period of very slow growth with only minor
rockfall activity occurring at the summit. Lava within the upper conduit is
crystallizing and forming a near-solid plug within the dome. (b) 13 July
1996. Increased seismic activity heralds a pulse of fresh magma through the
conduit. This increased pressurization results in the lava plug being pushed
out along a curved shear fault within the dome. This plug is emplaced as a
megaspine at the summit producing rockfalls that break away from its
headwall during growth. (c) 27 July 1996. Following emplacement of the
megaspine, the hotter magma beneath is redirected along another shear fault
that provides an easier pathway to the surface. The rapid rise from depth of
hot magma at the head of the fresh magma pulse does not allow enough
time for a plug to develop in the upper conduit. The 17 July 1996 lobe was a
typical example of a Type 2 shear lobe comprising large blocks and stubby
spines sourcing pyroclastic flows.
13 December 1996 lobe at a vigorous pace, with field observations
suggesting a discharge rate between 6 and 9 m 3 s -1. This fresh lobe
spread out laterally across the summit to form pancake-shaped
lava on top of the 13 December lobe. Notably, some of the most
spectacular examples of incandescence during the night-time and
even daytime were experienced during this period associated with
rapid discharge rates (Fig. 17b). In the following two weeks, this
lobe rapidly overwhelmed both the 1 October 1996 lobe and the
13 December 1996 lobe through lateral growth (Fig. 18). The
25 December 1996 lobe provides the best example of the more fluidlike behaviour exhibited by the lava when dome growth was more
vigorous. The top third of the dome (c. 50-70 m) on 28 December
1996 consisted of a front of incandescent blocky lava which con-
tinually generated incandescent rockfalls down the lower two-thirds
of the dome. The circular plan-form and slightly raised central
summit of this lobe indicated that lava was extruded in the summit
region but was able to spread laterally, forming an overall pancakelike morphology. Emplacement of this somewhat more fluid lava
contrasts with the predominant extrusion of spines and lobes along
ductile shear faults.
The 25 December 1996 lobe initially spread symmetrically,
spilling lava down the entire eastern flanks, but by mid-January its
advance had become more focused towards the SE. The continued
advance of the lobe guided lava down the SE dome flanks and
triggered large pyroclastic flows down the SE side of the Tar River
valley on 9 16 and 19-20 January 1997. With the northern half of
the 25 December 1996 lobe now effectively abandoned, subsequent
blocky growth then rapidly infilled the SE-facing scar, producing a
series of small collapses (Figs 19c and 20a, b).
By early February 1997, Castle Peak had been partially destroyed and completely buried by rockfalls and pyroclastic flows,
although the discharge rate had declined to more moderate levels
( 2 - 4 m 3 s-l). A slight switch in steady growth of the southeasterly
directed 21 January 1997 lobe to a more easterly trend, focused
rockfall activity down the eastern flanks throughout late February
and early March. The conical dome that developed in the latter half of
March illustrated well the effect of rockfalls on dome morphology.
This period provided a very good example of the evolution of an
asymmetric Type 1 shear lobe over an extended period of sustained
growth. Large, curving spines of fresh lava would extrude away from
the rear of the lobe, then gradually push forward and break up,
spilling blocks onto the lobe summit. This activity alternated with
magma pushing into the molten core of the lobe, expanding this
area and triggering rockfalls off the lobe headwall (Fig. 9c). A swath
of rockfalls from both collapse of spine debris and disintegration of
the lobe headwall gradually formed a lobe with a furrowed appearance as rockfall chutes developed. This repeated process of alternating spine growth and magma injection into the core gradually
formed a lobe with a broad, conical form (Figs 21a and 22a).
The end of March witnessed the extrusion of a remarkable
structure, formed in the early development of the 27 March 1997
lobe (also known as the 'Easter' lobe). The growth of this lobe
occurred at a time of good visibility and its development was well
documented. The appearance of this structure immediately preceded
a dome collapse on 30-31 March (c. 3.6 x 106 m 3 non-DRE deposit
volume) focused on the southern flanks; this destabilization is attributed to the initial growth of the 27 March 1997 lobe. A broad mass
of lava started to project out in a subvertical manner on 27 March,
originating from about 50-70 m below the summit area of the conical dome lobe extruded throughout March. Its movement took
place in a stick-slip fashion at estimated rates of 25-30m day -1,
with movement of the lobe accommodated by a southerly directed
ductile shear fault in the dome. By 3 April, the 27 March 1997 lobe
exhibited a smooth, yet striated, curved upper surface (c. 100 m long
and c. 120 m wide) and a near-vertical headwall of massive lava
almost c. 150 m high (Figs 21b and 22b).
Growth of this structure involved gradual rotation guided along
by the shear fault surface forming a semi-cylindrical cross-section.
Vigorous degassing emanated from around the horseshoe-shaped
boundary zone between the lobe and the inactive parts of the dome.
On initial extrusion, the smooth backwall was directed subvertically, and as growth continued this changed down into a more
subhorizontal position (Fig. 22c). This mode of extrusion can be
closely correlated to the shear lobes of blocky lava evident in mid1996. In the case of the 27 March 1997 lobe, however, the smooth,
semi-cylindrical upper surface remained as one large coherent structure during extrusion instead of breaking up into smaller, curving
blocks that gradually rotated forwards during growth (Fig. 9b). This
difference may be partly attributed to the fact that the 27 March
1997 lobe was buttressed and supported by much larger surrounding dome flanks than the blocky lobes apparent in the earlier
eruptive stages.
The 27 March 1997 lobe continued to grow in the same manner,
bulldozing through the southern flanks of the new dome and
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127
Fig. 12. (a) and (b) Maps highlighting the scales and locations of collapse scars of 12 August 1996 and 2 September 1996, both formed as a result of the
growth spurts prevalent in the Stage III period. Shaded area represents the new dome. Legend as in Figure 5. (c) 25 August 1996. View from point Y on (a)
looking at NE flanks of new dome. Fresh lava blocks are extruding from the summit area (marked by arrow) and being directed down the NE flanks.
Such growth was typical during spurts of activity that were commonly experienced during late July and August 1996. Note the ash-venting near the focus
of extrusion at the rear of the lobe. Dashed line marks the scar rim from the 11 August 1996 collapse within which fresh lava of the 12 August 1996 lobe has
filled up the scar in a two-week period. CP marks the prominent two-pronged spine of Castle Peak.
destroying the u p p e r ramparts of Galway's Wall (Fig. 23a). Large
blocks of massive lava spalled off the headwall of this lobe as it
jerked f o r w a r d to generate rockfalls and pyroclastic flows d o w n
Galway's Wall and into the W h i t e River valley. W i t h i n a week,
d o m e talus and pyroclastic flows h a d o v e r w h e l m e d w h a t remained
of G a l w a y ' s Wall. Following a n o t h e r d o m e collapse (c. 3 x 106 m 3
deposit volume) on the southern flanks on 11 April, a well developed lobe was observed extruding in a similar m a n n e r and trend as
the 27 M a r c h 1997 lobe. The s m o o t h backwall of this 13 April 1997
lobe, however, was steeply angled at c. 45 ~ whilst the leading edge
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R. B. WATTS E T AL.
Fig. 13. (a) and (b) Maps highlighting the scale and configuration of the 1 October 1996 lobe growth following the 17 September 1996 explosion. Light-shaded
areas mark lava extruded prior to the September explosion; darker shaded areas mark the 1 October 1996 lobe. Legend as in Figure 5.
continued to push outward and break up during growth. Slow
growth of this lobe continued for a few weeks until stagnation in
mid-May, when the headwall of massive lava remained as the steep,
upper ramparts on the southern dome flanks. A broad skirt of talus
fanned away from these ramparts (Fig. 23b) onto the deposits from
the associated pyroclastic activity, which had destroyed most of the
White River valley (Cole et al. 2002).
S t a g e VI." 14 M a y 1997 to 10 M a r c h 1998
This period commenced with a gradually accelerating background
rate of extrusion (c. 4 - 5 m 3 s -1) and an increase in dome height
to 1000m a.s.1. Between 14 and 17 May 1997, a distinct switch in
the focus of growth took place, returning to the central area with
an impressive c. 50-m-high vertical spine evident at the summit on
16 May (Fig. 24a, b). Within two days, this spine had collapsed and
lava extrusion was guided in a northerly direction, with rockfalls
spilling down a broad swath covering the southeastern-to-eastern
sector and on around to the NW flanks (Fig. 24c). Despite limited
visibility, the stark contrast between the shutdown of activity on the
southern flanks and the broad spread of activity across the entire
northern flanks indicated the development of the 17 May 1997 lobe
directing lava to the north. This rapid switch in dome growth also
indicated that vertical extrusion of lava was only evident between
major switches in activity, with the preferred mode of emplacement taking place through subhorizontal shear lobes. This semicontinuous rockfall activity continued unabated at a steady rate for
several weeks, broadening the dome flanks against the northern
walls of English's Crater. By early June 1997, talus was level with
Farrell's Wall on the northern rim, and rockfalls tumbled directly
down the northern dome flanks into Tuitt's Ghaut and Mosquito
Ghaut. The ensuing weeks involved increasing pyroclastic flow
activity down these ghauts, threatening the communities of the
northeastern slopes of the volcano (Loughlin et al. 2002).
On 22 June 1997, a marked change in seismicity occurred, with
an 8-hour cyclic pattern of intense hybrid earthquake swarms and
associated cycles of ground deformation recorded by near-vent
tiltmeters (Voight et al. 1999). This period of increased pressurization continued for several days and culminated in the rapid destabilization of the entire 17 May 1997 lobe. A major dome collapse
(c. 6.4 • 106m 3 deposit volume) on 25 June resulted in the first
fatalities of the crisis (Loughlin et al. 2002). Large pyroclastic flows
were funnelled down Mosquito Ghaut, reaching about 6 km to the
NE, lowering the dome summit by 100m and excavating a steepsided scar on the northern dome flanks. Within a few days the scar
was refilling with blocky lava and stubby spines to construct the
27 June 1997 lobe. By 10 July, a broad headwall of massive lava
had risen up within the scar indicating the northerly directed lobe
had re-established itself (Fig. 25a).
A gradual switch in activity then became apparent throughout
late July, with rockfalls and pyroclastic flows initially coursing down
Mosquito Ghaut. By 14 July rockfall activity was concentrated
more to the west, directing pyroclastic flows down Gages valley.
This shift is attributed to the stagnation of the 27 June 1997 lobe and
the formation of the 14 July 1997 lobe directing an active headwall
to the west. A further pulse in activity on 31 July was again marked
by an abrupt increase in amplitude of tilt cycles in conjunction with
intense hybrid earthquake swarms (Voight et al. 1999). This was
associated with a growth spurt that produced a westerly directed
dome collapse engulfing Gages valley and Plymouth Town in
pyroclastic flow deposits on 3 August (Cole et al. 2002; Druitt et al.
2002). In a similar scenario to 25 June, the rapid influx of magma
into the upper conduit had pushed out the entire shear lobe that was
already actively growing to the west. This event produced a deep
central crater in the dome with a breached, open scar facing to
the west. The rapid removal of the 14 July 1997 lobe, caused by the
collapse on 3 August, perturbed the magmatic system in the upper
conduit and triggered a week-long series of repetitive Vulcanian
explosions (Druitt et al. 2002). By mid-August, blocky lava was
apparent within the crater and the emplacement of a westerly
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Fig. 14. (a) 2 October 1996. Photo looking west from above the Tar River valley (see Fig. 13a). Castle Peak (CP) is the dark material in the left foreground
surrounded by fresh, pale lava of the new dome, which appears as a rugged ridge defining the rim of the horseshoe-shaped collapse scar (CS) and
explosion crater from 17 September 1996 collapse and explosive eruption. Small bun-shaped feature in the centre of this scar is the 1 October 1996 lobe
(OL) extruded 15 days after explosive event had evacuated the upper conduit. Megaspine (M) of early July (see Fig. 10c) can be seen behind the central growth
and Peak C on English's Crater is on far right. Trench in foreground is the main exit channel eroded by pyroclastic flow activity on 17 September
1996. (b) 2 October 1996. Close-up of new growth (1 October 1996 lobe, OL) seen in (a) showing smooth-topped massive lava (c. 20m thick) resting atop
a mantle of rock debris. This feature, named informally 'the brain', gradually rose and infilled the 17 September 1996 explosion scar over the following month.
Note the fumarolic activity around the perimeter of the fresh lobe.
directed 13 August 1997 lobe rapidly ensued. Elevated seismicity
and extrusion rates (estimated at > 5 m 3 s-1) continued with gradual
collapse of the headwall continuing to generate pyroclastic flows
towards Plymouth. By late August, the western dome flanks had
been rebuilt (Fig. 25b, c), with the height of the dome nearly reaching 1000m a.s.1.
In early September 1997, the broad headwall of the 14 August
1997 lobe was widening its span to generate rockfalls down the
northern flanks, as well as down Gages valley. By 8 September,
rockfall activity inundated the northern flanks whilst activity had
shut down on the western flanks, indicating stagnation of the
14 August 1997 lobe. Lava blocks spilled off the active headwall of
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Fig. 15. (a) 10 October 1996. View looking west from above NE dome flanks showing the 1 October 1996 lobe (OL) infilling the scar (ES) formed by the
17 September 1996 explosive eruption. Small scar (S, in foreground) was formed by small rockfalls tumbling down the eastern flanks of the dome. Note
the rubbly carapace (a few metres thick) that characterized the flat surface of the 1 October 1996 lobe, c. 160m in diameter here. (b) 3 December 1996.
View looking WNW at the dome from above southern rim of English's Crater (EC). The 1 October 1996 lobe (OL) growing in the 17 September 1996 explosion
crater and collapse scar can be seen in the centre. The steep rim (S) of the spoon-shaped scar of 17 September is well seen here. ED marks the approximate
zone of localized doming noted during volume surveys at this time and interpreted as a site of endogenous growth. ChP marks the summit of Chances
Peak dome (summit height c. 909 m a.s.l.). Galway's Wall (GW) with fresh avalanche debris at its base is centre left, and a small remnant of Castle Peak (CP) is
visible in central foreground.
the 8 September 1997 lobe n o w growing directly n o r t h w a r d and
debris completely filled M o s q u i t o G h a u t at this time thus extending the limits of Farrell's Plain (Fig. 25c). The active lobe continued to spall away blocky lava onto the n o r t h e r n d o m e flanks and
d o w n Tuitt's G h a u t . The d o m e at this stage was v o l u m i n o u s
(c. 85 x 106 m3), with the n o r t h e r n flanks growing as a single lobe.
On the m o r n i n g o f 21 September 1997, a s w a r m o f hybrid earthquakes preceded a large d o m e collapse. A n estimated 14.3 x 106 m 3
o f material was funnelled d o w n Tuitt's G h a u t , forming an
a m p h i t h e a t r e - s h a p e d scar in the dome, with a large crater open to
the n o r t h (Figs 26a and 27a). This collapse was a further example of
the rapid extrusion of a n e w shear lobe destabilizing the sector o f the
volcano in which it was actively growing. A second series of repetitive Vulcanian explosions followed the collapse, due to rapid depressurization of m a g m a in the m a i n conduit (Druitt et al. 2002). This
m o n t h - l o n g period p r o d u c e d only very m i n o r changes to the overall
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Fig. 16. (a) to (c) Maps highlighting extrusion of the 13 December 1996 lobe, the 25 December 1996 lobe, the 21 January 1997 lobe and the location of vigorous
pyroclastic flow activity in mid- to late January 1997. Legend as in Figure 5.
dome morphology, although explosive activity reamed out a deep,
funnel-shaped central crater, almost 300 m in diameter. The onset of
renewed dome growth on 22 October involved the slow extrusion
of another lobe to the north. This feature initially extruded vertically, with the back wall of the explosion crater acting as the shear
zone accommodating its growth. As it continued to rise, the 22 October 1997 lobe gradually projected to the north, partly infilling the
scar produced by the 21 September collapse (Figs 26b and 27b).
Growth of the 22 October 1997 lobe stagnated on 3 November
and rockfall activity started to spill down the southern flanks of
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Fig. 17. (a) 18 December 1996. View looking west from southern edge of Tar River valley marked by point Z on Figure 16a. Large structure in centre is
the headwall of the 13 December 1996 lobe, around 150 m high and 200 m broad. VL marks the smooth, curved outer surface of the megaspine extruded in the
early stages of this lobe. CS marks the scar rim of the September explosion crater and collapse scar; CP is the main spine of Castle Peak. Pale blocky
talus in the foreground is rockfall debris produced by extrusion of the 13 December 1996 lobe. (b) 28 December 1996. Night-time view of glowing dome
seen from the Whites area to the northeast, c. 2 km from the dome (Fig. 1). Incandescent blocks of the 25 December 1996 lobe are evident, spilling down the
eastern flanks of the dome. Note the silhouetted twin-pronged spine of Castle Peak in the left foreground.
the dome, coincident with a period of intense shallow hybrid earthquakes. Three major collapses (c. 8 x 106m 3 total volume of
deposits) then occurred on 4 and 6 November, each concentrated
on the southern flanks and sending pyroclastic flows coursing down
the White River valley. These collapses are attributed to the
22 October 1997 lobe dislocating (hence stagnating), with redirected
extrusion to the south (Fig. 28). The bulldozing effect of the
4 November 1997 lobe instigated collapse of older lava on the southern dome flanks (the two collapses on 4 November) and collapse of
freshly extruded blocky lava from the headwall of the 4 November
lobe itself (the 6 November collapse; Figs 29a and 30a). Rapid
advance of this lobe infilled the collapse scar formed by these events,
pushing a broad headwall of massive lava southwards. Lava continued issuing from this southerly directed shear zone for many
weeks. Throughout this period, activity alternated between the
extrusion of broad, curving spines at the back of the lobe or rockfalls spilling away from the summit headwall. This latter activity
suggested a pulse of magma feeding directly into the core of the shear
lobe and triggering rockfalls off the summit headwall (Fig. 9c). This
alternation between spine extrusion and headwall break-up, on a
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Fig. 18. Schematic diagram illustrating extrusion of the 13 December 1996
lobe. Section is along line X-Y on Figure 16a. (a) Dome configuration on
3 December 1996, with earthquake swarms indicating endogenous activity
but no clear evidence for dome growth at surface. Magma within conduit
continues to degas and crystallize until remobilized. (b) Dome configuration
on 13 December 1996. A pulse of fresh magma has uplifted part of the
1 October 1996 lobe, resulting in the extrusion of a megaspine, the initial
growth of the 13 December 1996 lobe. This structure is pushed to the SE,
bulldozing through the dome flanks generating rockfalls down the Tar River
valley. (e) Dome configuration on 19 December 1996. The 13 December
1996 lobe is overwhelmed and broken up by the extrusion of fresh, hot,
blocky lava. Pyroclastic flows inundate the Tar River valley until the waning
stages of the pulse. (d) Dome configuration on 28 December 1996. The shear
zone feeding the 13 December 1996 lobe starts to plug with crystalline lava.
A further pulse of magma emplaces more fluid lava at the summit of the
dome (i.e. the 25 December 1996 lobe), rapidly spreading over the l October
1996 lobe and 13 December 1996 lobe and spilling lava blocks down the
eastern flanks.
near-daily basis, suggested short-term fluctuations in the discharge
rate and alternating pulses of more viscous and less viscous magma.
An immense peak was gradually constructed that straddled
the Galway's Wall area (Figs 29b and 30b). By 21 December, the
summit of the 4 November 1997 lobe was c. 1030m high and
the loading of this structure was undermining the strength of the
Galway's Soufri~re area, which was progressively buried. During
a period of intense hybrid seismicity on 26 December, a debris
avalanche and major dome collapse (known as the 26 December
1997 or 'Boxing Day' collapse) removed the entire 4 November 1997
lobe (c. 55 x 106 m 3 n o n - D R E of dome rock and talus) that had
grown since early November (Sparks et al. 2002; Voight et al. 2002).
Remarkably, the other sectors of the dome were unaffected by this
major flank failure and dome collapse.
Vigorous extrusion of blocky lava began to infill the resulting
collapse scar immediately (Fig. 31a). Renewed development of
a southerly directed lobe, the 27 December 1997 lobe became
apparent, and within two months, the southern flanks were rebuilt
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134
R. B. WATTS ET AL.
Fig. 19. Cartoon illustrating the development of the 25 December 1996 lobe (see Fig. 16b). Sketches on the left represent a NW-SE cross-section through the
dome; sketches on the right represent plan views at the same time. (a) 28 December 1996. The 13 December 1996 lobe has stagnated. The 25 December 1996
lobe punches vertically through the 1 October 1996 lobe and proceeds to spread symmetrically across the summit, overwhelming the 13 December 1996 lobe.
(b) 5 January 1997. Lava extrusion becomes more directed, guiding curved slabs to the east and overwhelming the 13 December 1996 lobe and spilling lava
blocks down the eastern flanks. The western edge of the 25 December 1996 lobe has become abandoned from the rest of the lobe. (e) 17 January 1997.
Oversteepening of the SE dome flanks triggers a dome collapse on 16 January, and ensuing blocky lava infills the horseshoe-shaped scar. Lava extrusion is
focused within the scar and directed by a southeasterly shear lobe as a large sector of the 25 December 1996 lobe is abandoned.
(Fig. 30b), although the growth rate had noticeably waned as this
period progressed. Development of this Type 1 shear lobe occurred
as alternating spine extrusion and rockfalls off the summit headwall
suggesting endogenous activity, a similar pattern to that observed
in November and December 1997. By late February, several large
spines had extruded prior to the formation, in early March, of a
prominent 50-m-high spine (informally termed the Galway's Spine)
that towered atop the 27 December 1997 lobe (Figs 31b and 32a).
Coincident with extrusion of this feature was a marked reduction in rockfall activity, seismicity and ground deformation; all of
these factors signalled the cessation of the first episode of dome
growth on about 10 March.
Stage VII." 11 March 1998 to mid-November 1999
A full account of this 20-month interim period between the two
phases of dome growth is presented by Norton et al. (2002). Despite
no extrusion of fresh lava at the surface throughout this period,
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GROWTH PATTERNS AND DOME EMPLACEMENT
135
Fig. 20. (a) 22 January 1997. View of same area as Figure 17a but closer and one month later on, following a period of vigorous blocky growth and subsequent
dome collapse. Flower-shaped structure (with radial cracks) is extrusion of massive lava (c. 30 m high) of the 21 January 1997 lobe sitting within the 20 January
collapse scar (CS). Note the spine of Castle Peak (CP) in right foreground and the nearly vertical fractures within this spine. (b) 25 January 1997. Similar view
taken three days later than (a), with the same features marked. The 21 January 1997 lobe developed into a pile of curvilinear, massive lava blocks (typically
c. 5 m in size) which has continued to infill and engulf the collapse scar (see Fig. 16c). This blocky morphology was characteristic of a Type 2 shear lobe formed
during periods of relatively high discharge rate (>5 m 3 s-i). Many of these blocks tumbled down the SE dome flanks (in foreground) to generate moderate-sized
pyroclastic flows down the Tar River valley.
there were still intermittent periods of increased activity, notably the
3 July 1998 dome collapse (Fig. 32b). This large-volume collapse
followed three months of quiescence with only rare rockfalls, small
pyroclastic flows and subdued seismicity. The collapse involved
20-25 • 106 m 3 of lava and formation of a 200-m-deep horseshoeshaped canyon through the dome on its southeastern flank. There
was no seismic precursor to this event, which is believed to have
been predominantly gravitationally influenced, although heavy
rainfall may have been a contributory factor. The collapse was
initially focused in an area weakened by continuous intense fumarolic activity. The southeastern flank was also a steep ridge of loose,
blocky lava that formed the eastern rim of the scar from the collapse of 26 December 1997, and was therefore prone to instability.
Following the collapse there occurred sporadic degassing and small
explosions from a crater at the central base of the scar. A smaller
collapse on 12 November 1998 removed c. 3 x 106 m 3 of lava from
the western dome flanks. The collapse eroded away part of the
western dome flanks and joined the 3 July 1998 scar forming a deep
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136
R . B . WATTS ET AL.
Fig. 21. (a) 1 April 1997. View looking west above Tar River valley showing the conical form of the dome that developed throughout March 1997. As blocks
tumbled eastwards off the active area they eroded deep rockfall channels and chutes into the talus of the lower dome flanks. Note that Castle Peak has been
completely overwhelmed by fresh lava. Four days previously, the easterly directed 21 January lobe (JL) stagnated and the 27 March 1997 lobe (EL) started to
grow towards Galway's Wall (G). Summit height of dome c. 970 m a.s.l. (b) 6 April 1997, from near the same location as (a). This photo highlights the dramatic
contrast between the conical-shaped dome from growth in February and March 1997 (JL) and the initial, smooth appearance of the active 27 March 1997 lobe
(EL) sliding out southwards (to the left) from the summit area. This also highlights the contrast in morphology of a Type 1 shear lobe in the early stages (EL),
and the later stages (JL) of lobe development (see Fig. 9b, c). In this view, the 27 March 1997 lobe was around 200 m long, and 150 m wide, with a c. 150 m high
headwall that generated dome collapses down the White River valley as it grew. The arrow indicates motion. Note the gas plume emerging from along the
boundary between the two parts of the dome.
corridor-shaped gorge trending E S E - W N W t h r o u g h the entire
dome. A further collapse on 20 July 1999, this time directed to
the east, r e m o v e d c. 5 x 106 m 3 of lava, mainly off the 27 D e c e m b e r
1997 lobe. This event excavated a smaller c a n y o n t h r o u g h the
n o r t h e r n flanks that linked up with the m a i n gorge ( N o r t o n et al.
2002). Sporadic explosions and m i n o r gravitational collapses continued to occur until the onset of renewed d o m e g r o w t h in midN o v e m b e r 1999 (onset of Stage VIII; Table 1).
Discussion
Petrological and rheological variations
Lava d o m e extrusion is p r o f o u n d l y influenced by m a g m a rheology.
W e begin our discussion o f the observations of the Soufri~re Hills
d o m e evolution by summarizing the petrological characteristics
of the andesite. These features provide m a j o r constraints on the
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GROWTH PATTERNS AND DOME EMPLACEMENT
137
Fig. 22. (a) and (b) Maps illustrating the dome configuration in March/April 1997 showing how extrusion of the 27 March 1997 lobe affected the morphology
of the dome. Legend as in Figure 5. (c) 6 April 1997. Close-up view of the 27 March 1997 lobe taken from point X on (b). Note the smooth, upper,
semi-cylindrical surface of this lobe (BW), and the prominent striations trending parallel to the direction of shear movement, with deep, cross-cutting fractures
also evident. Visual observations showed this feature to be sliding out in a stick-slip manner at an estimated 25-30 m day -1 . This lobe was c. 200m long
(from left to right) and c. 150 m wide with a e. 150 m high, steep headwall. Rockfalls were continually spalling off the near-vertical headwall (H) at the leading
edge of the lobe (in the left foreground) as a result of intermittent endogenous/exogenous activity. This lobe represents the only example throughout the
g1995-1998 period where the smooth, upper surface remained intact during growth, rather than breaking up into large, curving slabs. JL is stagnated lava from
the 21 January 1997 lobe.
rheological variations and on the principal factors that control
dome extrusion.
Throughout the 1995-1999 period, the dome andesite was
sampled by collection of blocks from dome-collapse and fountaincollapse pyroclastic flow deposits. Petrological work (e.g. Devine
et al. 1998; Murphy et al. 2000) highlighted only a minor variation in
bulk composition (SiO2 58.5-62.4%) and phenocryst/microphenocryst content (55-65%). The eruption during 1995-1999 is interpreted to have been driven by an open-system magma body fuelled
by the influx of mafic magma into a very crystal-rich magma body
resident at shallow crustal levels. Both the thermal input and
pressurization due to mafic magma influx (reflected in the 1992-1995
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138
R.B. WATTS E T AL.
Fig. 23. (a) 6 April 1997. View looking NW from White River valley area highlighting the destruction of Galway's Wall by growth of the 27 March 1997 lobe.
EL marks the steep headwall (c. 150 m high) of the lobe in which arcuate cooling fractures and zones of intensely sheared lava are evident. WF is the western
dome flanks and S the highest point on the dome (c. 970 m a.s.1.), composed of lava extruded in February and March 1997 (21 January lobe) and now inactive.
Note the deep erosional gully in Galway's Wall formed by numerous rockfalls and small pyroclastic flows sourced from the 27 March 1997 lobe. (b) Same view
as (a) but taken in mid-May 1997, showing the construction of a broad talus of lava blocks, slabs and small spines that has completely buried the former
Galway's Wall and the early plug-type extrusion feature of the 27 March 1997 lobe seen in (a).
seismic crisis and seismic crises in the previous 100 years) was
sufficient to remobilize the source region to form a crystal-rich
andesite magma. The samples contain 35-45 vol% phenocrysts and
c. 20 vol% microphenocrysts within a microlite-bearing high-silicarhyolitic glass matrix. The main difference between samples has been
the degree of crystallinity and texture of the groundmass, both
factors affected by the magma discharge rate. Between November
1995 and mid-February 1996, the erupted lava had a highly crystalline groundmass with only 5-15% residual rhyolitic glass and
typically extruded as large spines. This early phase of activity is
interpreted to be the extrusion of degassed lava infilling the conduit
from previous injections of magma that triggered the seismic crises.
This is supported by the identification of amphiboles with heterogeneous hydrogen isotope compositions in samples from this period
(Harford & Sparks 2001). Samples from periods of rapid dome
growth from August 1996 to March 1998 have tended to include
higher glass contents (up to 30%), although the glass content range
is wide (5-30%). The major pyroclastic flows sampled the deep
interior of the dome to depths of 100-200m, well away from the
influence of surface cooling. These samples still exhibit extensive
groundmass crystallization that are attributed to degassing rather
than cooling (Sparks 1997).
The ascent of andesitic magma from the chamber to the nearsurface environment has been modelled (e.g. Sparks 1997; Melnik
& Sparks 1999, 2002) and changes in rheological properties are
attributed to two intimately linked mechanisms. Gas exsolution due
to decompression during slow ascent causes a large increase in the
melt phase viscosity (Dingwell et al. 1996). Degassing also triggers
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GROWTH PATTERNS AND DOME EMPLACEMENT
139
Fig. 24. Schematic diagram illustrating the dramatic switch in activity that was experienced in mid-May 1997. Sketches on the left represent a N-S cross-section
through the dome whilst sketches on the right represent plan views of the dome at the same time.
crystallization in response to the consequent undercooling of the
melt phase and this process also increases magma viscosity. The
efficacy of both mechanisms increases during ascent and reaches a
peak in the uppermost several hundred metres of the conduit, where
Melnik & Sparks (2002) predict a zone of large overpressures.
Degassing-induced crystallization has already been noted by many
workers (e.g. Sparks & Pinkerton 1978; Lipman et al. 1985; Wolf &
Eichelberger 1997; Gardner et al. 1998; Blundy & Cashman 2001)
and is invoked for the Soufri~re Hills magma (Sparks et al. 2000).
However, the potency of this mechanism in andesitic dome-forming
eruptions had not been fully appreciated. Throughout most stages
of dome growth in the Soufri~re Hills eruption, a persistent gas
plume was seen emerging from around the dome summit, indicating
effective degassing of the magmatic system. The consequence of
degassing was to cause a profound rheological stiffening of the
magma so that the lava was better characterized as hot, crystalline
material with considerable strength (Sparks et al. 2000).
The rheological properties of the Soufri6re Hills andesitic
magma can be constrained from petrological observations and
uniaxial loading experiments on dome samples at high temperature
(Sparks et al. 2000). Petrological estimates of magma properties in
the magma chamber suggest temperatures of c. 860~ with c. 5%
H20 dissolved in the rhyolitic melt phase; thus a viscosity of
7 x 106 Pa s is estimated, based on experimental results (Dingwell
et al. 1996; Pinkerton & Stevenson 1992). Fully degassed and
highly crystalline dome samples show highly non-linear deformation behaviour under uniaxial loadings of 9-26 MPa at temperatures of c. 990~ (Sparks et al. 2000). Viscosities in an initial period
of steady deformation are of the order of 1014 Pa s, but apparent
viscosities decrease rapidly to c. 1011 Pa s just prior to failure along
shear zones.
The Soufri6re Hills andesitic magma was already rich in
phenocrysts and microphenocrysts in the magma chamber prior
to eruption. During periods of slow ascent, degassing-induced
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140
R. B. WATTS E T AL.
81400
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D o m e V o l u m e = - 8 0 . 0 x 108 m 3
1997
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~1
Extrusion Rate = ~4-6 m s
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31 AUGUST 1997
Dome Volume = -82.0 x 106
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Extrusion R a t e = - 4 - 6 m s
-1
Fig. 25. Sequence of maps illustrating the growth of the dome in the aftermath of the 25 June 1997 collapse and highlighting the development and subsequent
infilling of the explosion crater throughout August 1997. Legend as in Figure 5.
crystallization of m a g m a in the upper conduit reached the threshold
when crystals formed a touching framework. At this stage the
m a g m a transformed from a N e w t o n i a n fluid to a m u c h more viscous
n o n - N e w t o n i a n fluid with mechanical strength (Lejeune & Richet
1995). In contrast, a sufficiently fast m a g m a ascent rate reduced the
time for microlite crystallization to the extent that the critical
threshold was not attained and m a g m a could extrude in a more fluidlike manner. Thus, during a period of fluctuating discharge rates, the
flow o f crystalline m a g m a may switch between a fluid nature and
that of a near-solid, n o n - N e w t o n i a n nature. Such an oscillating
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GROWTH PATTERNS AND DOME EMPLACEMENT
141
Fig. 26. Maps showing the configuration of the 21 September 1997 collapse crater and subsequent growth of the 22 October 1997 lobe from 22 October to
3 November 1997. Legend as in Figure 5.
state is predicted by models of the non-linear dynamics of conduit
flow (Denlinger & Hoblitt 1999; Melnik & Sparks 1999, 2002;
Voight et al. 1999; Wylie et al. 1999). A consequence of this variation is heterogeneous deformation during lava emplacement, with
the formation of spines and megaspines at slow rates (for nonNewtonian lava) and the development of shear lobes emplacing
more ductile lava as large blocks and smooth slabs or pancakeforms at faster rates.
Morphologic variation
A spectrum of extrusive features was observed throughout the
1995-1998 episode of dome growth, and together these structures
can be considered to represent a morphologic continuum (Fig. 33).
Each structure was essentially composed of the same components
(i.e. a smooth, semi-cylindrical backwall and a steep, blocky headwall), and their growth bounded along shear faults was accommodated in the same manner, with the exception of pancake forms
at more vigorous rates. Periods of growth varied widely, however,
with spine and megaspine formation occurring in a few days, whilst
individual shear lobe evolution operated over several weeks and
months. The main difference between these structures related to
their size and there appeared to be a link between the formation
of each structure and the level of eruptive activity (Fig. 34). Only
occasionally did the lava morphology and behaviour suggest a
more fluid-like emplacement, with axisymmetric lateral spreading
of pancake lobes at the summit area during periods of more
vigorous discharge rates (Fig. 33). The activity of late December
1996 provided the best example of this, with emplacement of the
25 December 1996 lobe occurring at a time when seismic tremor
was commonly experienced. A similar relationship, linking growth
behaviour to discharge rate, has also been determined in experiments using a Bingham plastic analogue (Griffiths & Fink 1997;
Fink & Griffiths 1998). These laboratory experiments reproduced
many of the structures observed in the Soufri~re Hills eruption,
although their formation was attributed primarily to variations in
viscosity and the thickness of the cooled dome carapace, whereas
here we relate the variations to the combined effects of discharge
rate, cooling and degassing and related changes in the rheological properties of the magma.
On considering the morphologic continuum, a gradual transition from highly crystalline structures (at low discharge rates) to
those exhibiting more fluid-like features (at higher discharge rates)
exists. By far the most predominant style of activity was the
formation of shear lobes at moderate discharge rates, although two
distinct classifications have been observed. In Type 1 shear lobes
(Fig. 33), growth develops a stable structure that may grow over
many weeks and months, constructing a lobe with a core of massive
lava and steep talus flanks from semi-continuous rockfall activity.
Such activity is prevalent during periods of long-term average extrusion rates of 2-5 m 3 s -1. In contrast, during periods of fluctuating
extrusion rates from <1 m 3 s -1 to > 5 m 3 s -1 (e.g. July to August
1996), emplacement of megaspines would be rapidly followed by
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142
R.B. WATTS E T AL.
Fig. 27. (a) 28 September 1997. View looking south from the north (near Harris Village), showing amphitheatre in the dome following the 21 September 1997
collapse. Breach in the dome faces into the head of Tuitt's Ghaut (T). Y represents the headwall of the stagnated 17 May 1997 lobe and Z the headwall of the
stagnated 13 August 1997 lobe (see Fig. 26a). The main crater is c. 300 m in diameter. (b) 6 November 1997. High aerial view looking towards southwest at the
northern flanks of the dome showing the 22 October 1997 lobe, with headwall of massive lava sitting within the breached part of the 21 September 1997 crater.
X marks the smooth, curving upper surface of this lobe. T, Y and Z represent the same features marked in (a). Activity had completely stagnated on the
northern flanks, with major collapses affecting the southern flanks of the dome.
Type 2 shear lobes of blocky lava (Fig. 33). At such periods, the
rapid sequential emplacement of these very different structures
commonly triggered dome collapses with major pyroclastic flow
activity. As the eruption progressed, a steadier discharge rate
became established, promoting the formation of Type 1 shear lobes.
As a result, long periods of lobe construction, with associated
accumulation of rockfall debris, would occur with only minor
pyroclastic flow activity (e.g. January to March 1998). This latter
style of dome growth has been particularly predominant in the
second phase of dome growth that commenced in November 1999.
Another factor to consider when explaining the difference in
activity between the earlier and later stages of the eruption may be
the former presence of the Castle Peak dome and its gradual
destruction and complete burial by early February 1997. In the
early eruptive stages, emplacement was partly controlled by the
topography of Castle Peak. The directed explosion of 17 September
1996 was also an important moment in the eruption. Not only was
it the first magmatic explosive activity, it completely exposed the
head of the main conduit to surface conditions, as well as widening
the uppermost parts of the conduit. Prior to this event, magma had
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GROWTH PATTERNS AND DOME EMPLACEMENT
143
Fig. 28. Schematic diagram illustrating
the hypothesized growth of the dome
during extrusion of the 22 October 1997
lobe and the 4 November 1997 lobe. The
cross-sectional view is along the line X-Y
on Figure 26a. (a) 23 October 1997
(see Fig. 26a). Renewed slow dome
growth in the central crater following a
month-long period of Vulcanian
explosions. (b) 29 October 1997. Gradual
extrusion of 22 October lobe, initially
guided vertically by backwall of crater and
subsequently projecting to the north,
infilling the breached part of the crater
and directing rockfalls down Tuitt's
Ghaut. As the 22 October 1997 lobe
extrudes subhorizontally away from the
crater, two small hummocks of lava
extrude at the central summit area (see
Fig. 26b). (e) 4 November 1997. Growth
of the 22 October 1997 lobe and central
hummocks has stopped while the southern
dome flanks are destabilized by intrusion
of the 4 November 1997 lobe generating a
dome collapse down the White River
valley. (d) 6 November 1997. The southern
dome flanks have been completely
destroyed, whilst all the other flanks of the
dome are unaffected. As the 4 November
1997 lobe continues to extrude along a
southerly directed shear fault, fresh lava
breaks off the lobe's leading edge causing
a further collapse directed down the White
River valley. (e) 21 December 1997.
Growth of the 4 November 1997 lobe has
continued for almost two months,
constructing a large peak which now
straddles the former Galway's Wall.
to force its way through the fractured body of the Castle Peak
dome before extruding.
Structural control on emplacement
The dominant mode of emplacement following 17 September
1996 was through the formation of shear lobes bounded by large
arcuate faults. During emplacement, the lava extruded sporadically
in a stick-slip manner along curved fault structures, which are
interpreted as shear faults that are sourced from the sides of the
conduit. Similar features have been produced in analogue experiments by Donnadieu & Merle (1998) investigating the deformation
of a volcanic edifice through forced intrusion of viscous magma.
As indentation proceeded, asymmetric deformation generated a
curved shear fault from the base to the outer edge of the edifice,
with the fault controlling the directed emplacement of the magma
analogue. This mechanism was postulated by Donnadieu & Merle
(1998) to explain the cryptodome intrusion in the build-up to the
Plinian eruption of Mount St Helens in M a y 1980.
Observations of the Soufri6re Hills dome indicate that a similar
mechanism may be responsible for the growth and development
of individual lobes during construction and destruction of the
dome. Shear faults have not been recognized in lava domes before.
An explanation for this may be the poor preservation potential
of the shear surfaces. In the Soufri+re Hills dome, a lobe was commonly broken up in the late stages of emplacement or buried by
blocks from a later eruptive episode. The structure of a shear lobe is
sometimes preserved and, at the time of writing (October 2000),
the current configuration of the Soufri~re Hills dome still exhibits
part of the smooth backwall of the 4 November 1997 lobe extruded
in October 1997. Domes at other volcanoes also have preserved
examples, such as a 55-m-long, 25-m-wide striated lobe emplaced
near the summit of the Chinois Dome that formed in the 1929-1932
eruption of Mont Pel~e, Martinique. In some cases, a single shear
lobe may form the entire preserved part of a dome; the 34-ka
Perches dome in the Soufri~re Hills complex on Montserrat and
the Gros Piton on St. Lucia are both examples of near-vertical
shear lobes.
Growth stages and cycles
During the eruption the dome has increased in volume, notwithstanding the counteracting effects of collapses. Growth has also
been characterized by cyclic patterns, with repeated switches in the
direction of lobe extrusion and pulsations in discharge rate. Here
we discuss the interplay of individual pulses of lobe extrusion with
the overall construction of the dome and the nature of the cyclic
growth patterns.
The early stages of the first episode of dome growth involved
the gradual destruction and burial of the previous construct, the
Castle Peak dome. As eruptive vigour increased, the directed extrusion of megaspines and subhorizontal shear lobes occurred in a
non-systematic radial pattern around the central conduit. These
structures armoured the lower dome flanks, acting as a foundation
to the large subvertical shear lobes that dominated extrusion in the
later eruptive stages. U p o n stagnation, the broad headwall of each
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144
R. B. WATTS E T AL.
Fig. 29. Maps illustrating the collapse resulting from growth of the 4 November 1997 lobe and the extent of regrowth followingthis event.-Legendas in Figure 5.
lobe would form the steep upper ramparts on a sector of the volcano. This process of shear-lobe growth and stagnation continued
around all sectors of the dome, so as eventually to construct a central depression, which was further modified by explosive activity in
August, September and October 1997. This depression developed as
a result of this repeated lobe formation, with each lobe directed
away from the central conduit. In effect, the remnants of the smooth
outer lobe surfaces had stagnated and merged together to form
the inner walls of the central depression. In association with this
process, each of the headwalls of the lobes stagnated and merged
together to form the steep upper flanks of the dome. As extrusion of
individual lobes constructed steep flanks, extrusion would either
continue in the same direction, partly stagnate and widen the headwall to one side of the lobe, or completely stagnate and switch the
focus of activity to another sector.
Major switches of dome growth direction can, in several cases,
be clearly linked to cyclic behaviour of the volcano. Throughout
most of the eruption, and particularly in the latter half of the 19951998 period of dome growth, a distinct five to seven-week cycle
of activity was recognized, which was intimately linked to hybrid
seismicity and a pronounced increase and decrease in the period of
tilt cycles (Voight et al. 1999). Typically, the onset of a cycle was
marked by a period of intense hybrid seismicity that commonly
resulted in a dome collapse event and a pronounced increase in
discharge rate. In the weeks that followed, the amplitude and period
of tilt cycles decreased and increased respectively and seismicity
declined. Aseismic growth of the active lobe infilled the collapse
scar, initially as rapidly emplaced blocky lava and small spines that
developed into a broad shear lobe with oversteepened flanks. As predominantly aseismic growth progressed, the discharge rate would
gradually wane over several weeks, until dropping below a critical threshold. At this point, degassing-induced crystallization could
operate more effectively to form a near-solid plug in the upper
conduit, contributing to the retardation of surface extrusion. The
onset of a new cycle is then attributed to a pulse of fresh, less viscous
magma, rising up from the deep source. The new magma would
reach the base of the consolidated plug and the resistance to upward
flow marked the onset of hybrid seismicity. Pressure build-up
beneath the plug would continue over a few hours to several days,
as highlighted by intensifying swarms of hybrid earthquakes until
reaching a critical limit that pushed the plug into the dome. The
discharge rate rapidly increased as less viscous magma filled the
conduit and the plug material was pushed out of the way. The
rapid increase in flow rate and pressurized conditions in the upper
conduit were, in many cases, enough to rapidly destabilize a lobe
headwall and trigger a major dome collapse. The fatal 25 June 1997
dome collapse illustrates a typical growth cycle and, in the weeks
leading up to the event, aseismic growth of a northerly directed
lobe had primed the northern flanks for a collapse (Fig. 35a).
In the weeks following the collapse, rapid redevelopment of a
fresh northerly lobe was observed (Fig. 35b), with hybrid seismicity
disappearing, thus initiating the next cycle (Fig. 35c). In this interpretation, the fluctuations in discharge rate observed during each
cycle were a consequence of pulsations of fresh, low-viscosity and
probably gas-rich magma released from the deep source with
hybrid seismicity being a signature for the extrusion of a near-solid
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GROWTH PATTERNS AND DOME EMPLACEMENT
145
Fig. 30. (a) 8 November 1997. View looking north directly at the headwall (NL) of the 4 November 1997 lobe (c. 150 m high). Dotted line marks the original
southern dome flanks immediately prior to extrusion of the 4 November 1997 lobe. Note also its curved upper edge (BW) and the vigorous ash-venting
emanating from the rear of this lobe. (b) 27 February 1998. This excellent view of the southern flanks, taken high above the White River valley, highlights the
dome configuration almost as it was immediately prior to the 26 December 1997 collapse. This shot was taken many weeks following this collapse; however,
subsequent growth infilled the collapse scar (BD) and rebuilt the southern flanks in a near-identical manner. Note the characteristic rockfall chutes leading
down from the blocky summit and eroding into the talus. Pyroclastic flow deposits from the 21 September 1997 collapse can be seen in the background, with
Chances Peak (ChP) and Galway's Mountain (GM) dome also in view.
plug into the dome, possibly by stick-slip movement along the walls
of the upper conduit (Denlinger & Hoblitt 1999; Voight et al. 1999;
Wylie et al. 1999).
M o d e l s o f dome growth
The observations of the growth structures and morphological
development of the Soufri6re Hills lava dome are now discussed in
the context of concepts and models of dome growth. Two main
concepts have been developed to explain dome growth. First, the
role of surface cooling with formation of a resisting crust have been
explored by Fink & Griffiths (1992) in a series of laboratory
experiments and by scaling analyses of force balances (summarized
in Griffiths 2000). Second, the linked roles of gas exsolution and
degassing-induced crystallization have been considered in textural
studies (Cashman 1992; Cashman & Blundy 2000; H a m m e r et al.
2000) and theoretical models of conduit flow during dome extrusion
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146
R. B. WATTS E T AL.
Fig. 31. Maps illustrating the dome configuration in the aftermath of the 26 December 1997 collapse and the focus of regrowth following this event until the
cessation of growth around l0 March 1998. Legend as in Figure 5.
(Sparks 1997; Melnik & Sparks 2002). In both concepts, rheological
stiffening and the onset of non-Newtonian rheology with the
development of a yield strength are important. Here we discuss the
relative roles of cooling and degassing-induced crystallization in
controlling the dynamics of the Soufri6re Hills lava dome and
morphological features. This discussion develops that by Sparks
et al. (2000).
In Griffiths (2000) similar phenomena and range of dome
morphologies to those reported here are attributed to development
of a cooled crust. Fink & Griffiths (1998), for example, demonstrate
a similar spectrum of structures to that observed at Soufri&e Hills,
from spiny lobate domes to smooth-spreading pancake morphologies, by changing a dimensionless parameter 9 which reflects the
relative importance of heat advection (and therefore discharge rate)
and heat loss through a cooling crust. Here we invoke a different
view that this morphological spectrum is a consequence of different amounts of both degassing-induced crystallization and cooling.
For lava extruded at the lowest rates, the slowly rising magma had
time to lose more gas and crystallize efficiently, such that solidification was largely completed (90-95%) within the upper conduit,
resulting in the emplacement of a spine or a megaspine. Cooling
played a negligible role in this case. For faster-rising magma, lesser
amounts of microlite crystallization occurred during ascent, promoting the formation of shear lobes. At the fastest ascent rates,
lava extruded in a more fluid-like manner and a pancake-type
morphology resulted. In this case, emplacement could be controlled
both by degassing-induced crystallization and external cooling.
Another argument for the importance of a cooling crust put
forward by Griffiths (2000) is that many lava domes show an
increase in height with time, as in the case of Soufribre Hills (Fig.
3a). This is attributed to growth of a crust of increasing thickness.
By comparing the forces driving lateral spreading of a dome with the
yield strength of a cooled crust, a scaling result is obtained whereby
the height is proportional to time to the power of 0.25. Griffiths
(2000) shows that several domes indeed exhibit a power-law dependence, with a power approaching 0.25. Such an analysis is problematic for Soufri6re Hills because the dome collapsed many times
(see Fig. 3a). It is more meaningful to take a single episode of
growth. For the period mid-February to August 1996, height data
yield a best-fit power law with the exponent of 0.36. In another
example, the period 1 October 1996 to 5 November 1996, the height
data give a power exponent of 0.44. In neither case are these values
close to that predicted by the cooling-crust model of Griffiths (2000).
There are several problems with applying the cooling-crust
model to Soufri6re Hills. Firstly, the overall increase in height
with time does not result from the growth of a single entity with
thickening and strengthening of a cooled crust. The dome collapsed
and new lobes extruded many times, so the scaling analysis and the
growth of a single, cooling dome structure cannot be applied to the
whole dome growth of 1995-1998. Secondly, the cooling-crust
model is a static one, whereas Stasiuk & Jaupart (1997) and Melnik
& Sparks (2002) show that there are important dynamic controls on
dome height. The increase in height with time is attributed to
increasing magma chamber pressure by Melnik & Sparks (2002),
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GROWTH PATTERNS AND DOME EMPLACEMENT
147
Fig. 32. (a) May 1998. View looking NW from South Soufri+re Hills at the southern flanks of the dome, composed entirely of the 27 December 1997 lobe.
Galway's Spine (GS) on top of the blocky flanks is 50 m high and 45 m wide and its extrusion in early March signalled the cessation of dome growth.
Chances Peak (ChP) is to the left, and eastern flanks of the dome are on the far right. The scar rim of the 26 December 1997 collapse is marked BD.
(b) View looking NW from above the southern rim of English's Crater in early February 1999. The entire SE dome flanks have collapsed away exposing a
vertical section (c. 200 m high) through the hummocks extruded as part of the 22 October 1997 lobe (OL). BD marks the same location as in (a), NL marks the
remaining part of the 27 December 1997 lobe and the summit of the new dome at 977m a.s.1.
who consider it to be unrelated to surface cooling. Thirdly, consideration of height versus time for the 1 October 1996 lobe gives a
power law with an exponent of 0.44, significantly larger than the 0.25
value expected by a cooling model. Likewise, the height versus time
data can also be analysed by a dynamical model (Melnik & Sparks
2002) without invoking cooling. There is a more general problem
in that several different models can give quite similar power-law
behaviour, so that finding a power law exponent of 0.25 is not
sufficient to demonstrate that cooling is the dominant effect. Indeed
a simple dynamical model, where discharge rate is a linear function
of dome height, gives a height versus time relationship quite close to
a power law with exponent of 0.25. Fourthly, the cooling model
requires very high crustal strengths (> 108 Pa; Griffiths 2000), which
are hard to reconcile with laboratory data on rock strengths. For
example, geotechnical measurements (Voight et al. 2002) and experiments simulating explosions (Alidibirov & Dingwell 2000) indicate
strengths of about 106 Pa for dome samples. Fracturing of the
cooled crust and development of tensional cooling stresses further
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148
R. B. WATTS E T AL.
EMPLACEMENT
+ ASSOCIATED
FEATURE
SEISMIClTY
AVERAGE DISCHARGE
R A T E , m ~ S "1
A. NEAR-VERTICAL SPINE
-growth over 2-3 days
-coincident with periodic hybrid
earthquake swarms
I
I
I
I
0.5
B. WHALEBACK STRUCTURES
-growth over 4-5 days
-coincident with repetitive hybrid
earthquake swarms
C. MEGASPINE
-emplaced over 2-3 days
20m;
FZ
1.0
Om$ 7,
iii
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(.9
Z
rY
a
-generally aseismic growth
occasionally with
hybrid seismicity
lz
I
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D. SHEAR LOBE-TYPE 1
2.0
-growth (broad spines) over many
weeks to months, in form of
intermittent endogenous +
exogenous pulses
0
a
>,
I
(.9
Z
-generally aseismic growth, often
intense hybrid earthquake
swarms prior to lobe collapse
(.3
i
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--";Om
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-~ I
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-growth (blocky lava) over days
to weeks, also with endogenous
pulses
-growth and collapse often
coincident with repetitive hybrid
earthquake swarms + tremor
~@,'.~
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/
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-growth over 4-6 days
<
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G. EXPLOSIONS
I
9.0
LU
~
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-commonly occur following rapid,
large dome collapses
-see Druitt et al. (2002)
tll~/
Fig. 33. Variation in the type of structure emplaced in relation to the average discharge rate, and the relative roles of degassing-induced crystallization and
cooling. Note that the boundaries between eruption rates are arbitrarily defined.
Downloaded from http://mem.lyellcollection.org/ at University of Bristol Library on December 19, 2012
GROWTH PATTERNS AND DOME EMPLACEMENT
14
'7
03
12
E
10
0~
8
t~
K,..
I~
149
Type 1 shear lobes + blocky lava
pancake lobe
near-verticalspines : ~ - megaspines +
Type 1 + 2 shear lodes
4 November
1997 lobe
6
t~
4
whaleback
structures
1997 lobe
o0
~
Fig. 34. Graph showing the structures
extruded throughout the eruption. This
highlights a relationship between the type
of structure extruded and average
discharge rate suggesting that changes
in dome morphology may be a crude
proxy for estimating discharge rate
during the eruption.
s
\
13 December
K,..
rr
9
explosions
9
2
9
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$
9
weaken a cooled crust, an effect inconsistent with the high strength
indicated from the cooling-crust model. While we concur that cooling can have a major role in many instances of lava emplacement,
we suggest that it only becomes a factor during the most rapid
periods of dome growth at Soufri6re Hills.
Comparison with other documented dome eruptions
The growth of the Mount St Helens dacitic dome between 1980 and
1986 allowed the first intense scientific monitoring campaign of a
lava dome this century. Swanson & Holcomb (1990) detailed the
complex growth history of the dome and described distinct patterns
throughout its duration. Episodic lobe extrusion generally lasting
several days was the predominant style; however, a one-year period
of continuous endogenous growth was also observed. Three distinct
periods of episodic growth showed a linear pattern in relation to the
long-term growth rate. Each extrusion was generally preceded by a
one to three week period of endogenous activity that often faulted
and fractured the dome carapace and caused deformation of the
crater floor. Between growth episodes, the dome would slowly
spread laterally and subside as its hot, ductile core deformed due to
gravitationally induced stress. The generally consistent shape of the
dome for most of the eruption suggested a possible controlling
mechanism. Iverson (1990) modelled the dome morphology as a
core of relatively low strength enclosed by a strong, brittle carapace. This cooled, outer surface skin and the net effective viscosity
of the lava were believed to play an important role in determining
the location and manner of lobe emplacement on the dome.
Variations in the surface texture of the lava throughout the
1980-1986 eruption also highlighted a relationship between the
underlying slope and the water content of the lava (Anderson &
Fink 1990). Several types of lobes were distinguished, each type
characterized by varying degrees of smooth and scoriaceous surface
textures. Growth was often focused along a large, smooth fracture
known as a crease structure that formed predominantly on shallow
slopes. A progressive increase in the formation of smooth-surfaced
lava observed during the eruption was attributed to more effective degassing of the magma during ascent and emplacement. The
features formed on the Mount St Helens dacite dome and mechanisms responsible for their formation show only limited similarity to
those observed at the Soufri6re Hills andesite dome.
The more recent 1991-1995 dome-forming eruption of Mount
Unzen, Japan, provides another comparison (Nakada et al. 1999).
This dome grew on a steep and unstable slope such that the dome
sporadically partially collapsed generating pyroclastic flows. During
this period of dome growth, magma extrusion was near-continuous,
involving the exogenous extrusion of 13 distinct lobes. Initial growth
**
" "9 *
9
0 0r
.
Galway's Spine +
growth cessation
I~
*
22 October 199 7"lobe
* * * * ~ " ~ " 2 7 March 1997 lobe
*t
!
4oo
200
1996
600
I 800 '
1997
Time (days since start of eruption)
1000
1998
involved spine formation and subsequent collapse into blocks; however, later lobes commonly exhibited crease structures indicating
plastic deformation. Endogenous growth was prevalent at periods
of low discharge rate, notably in the later stages of the eruption,
and the style of endogenous growth was compared to the formation of basaltic lava pillows on a much larger scale (Nakada et al.
1995). The overall discharge rate followed a gradually declining
trend with time, overprinting a pattern dominated by two distinct
pulses (Nakada & Motomura 1999). The style of growth, switching
between exogenous and endogenous activity, broadly paralleled this
trend. At times of relative quiescence, a viscous plug would develop
in the upper conduit, probably in response to shallow degassing and
microlite crystallization. Groundmass crystallization of the magma
below this plug raised the overpressure beneath the plug to a critical
threshold and extruded the viscous plug, marking the onset of
a fresh pulse in activity (Nakada & Motomura 1999). A similar
mechanism was invoked to explain the extrusion of a large spine
onto the endogenous dome summit of Mount Unzen in December 1994, an event that signalled the end of the eruption. Thus
degassing, groundmass crystallization and consequent rheological
stiffening were also key influences at Mount Unzen, as proposed
here for the Soufri6re Hills lava dome.
The short eruption in 1989-1990 of Redoubt Volcano, Alaska,
involved the rapid growth and destruction of 13 silicic-andesite
domes (Miller 1994). The remote and hazardous location of this
volcano, perched precariously within a steep amphitheatre, hampered visual observations. However, the blocky nature of the domes,
and vigorous eruptive degassing, indicate that similar ascent and
emplacement mechanisms to those detailed here may have been
operating at Redoubt.
The 1951-1952 dome-forming eruption of Mount Lamington in
Papua New Guinea involved the development of structures similar
to those documented in the Soufri6re Hills eruption. After an initial
paroxysmal explosion, dome growth involved the rapid extrusion
of near-solid lava constructing a broad dome (Taylor 1958). In the
later eruptive stages, localized extrusions took place in a piecemeal fashion, forming multiple peaks on the dome. This process of
directed extrusion, and the late-stage formation of large 'hogs-back'
features, bears a distinct resemblance to the growth mechanisms
described here.
Observations by Perret (1935) throughout the 1929-1932
eruption of Mont Pel6e (Martinique) also highlights similarities
to the formation of the Soufri6re Hills dome. Perret documented
the gradual construction of a broad lobe at the head of the Rivi6re
S6che through a combination of direct observation and photography. From close proximity, he was able to describe the growth
of individual spines and detail their variable nature, with solid outer
faces and viscous interiors. Eruptive activity did not involve any
Downloaded from http://mem.lyellcollection.org/ at University of Bristol Library on December 19, 2012
150
R. B. WATTS E T AL.
(a)
(b)
400
Retarded extrusion of
350
Ill
(C)
17May1997lobe ~11~
"=>, 300
250
~"
200
"O
150
100
s0
,%~ebn
IIi
........
o~nll~al~1~;Vle;~P
JI/
l~ / 7
25Junecollapse+ rapid extrusion
o, 17May1997lobe
Developmentof
June1997obe
..........~.hA,~~'~.,L17
I~il
May1997 ....I V ~ ....il;.,=nof
;r)~11111111_.
j
0
1 May
May
June
July
31 July
Date (1997)
large dome collapses or switches in activity, as have characterized Soufri~re Hills. However, the directed extrusion of crystal-rich
andesitic spines and broad lobes on the southern flanks of Mont
Pel6e suggest similar controlling mechanisms on dome growth.
Perhaps the closest analogy to Soufri~re Hills is one of the most
active lava domes in recent history, namely Mount Merapi on the
island of Java, Indonesia. Throughout the twentieth century, intermittent activity involved the gradual effusive growth and partial
collapse of the summit lava dome, occasionally causing significant
loss of life. A wealth of descriptive data exists for its previous
eruptive activity, as summarized in Voight et al. (2000). Merapi is
noted for the formation of individual lobes, variable scales of dome
collapse, and its ability to switch the focus of activity to a different sector of the edifice. Observations at the summit indicate the
presence of large, elliptical-shaped lobes of crystal-rich andesitic
Fig. 35. (a) Development of the dome in
the build-up to the 25 June 1997 dome
collapse. This schematic view is from high
above the eastern dome flanks looking
westwards. This period of activity was
marked by poor visibility and only limited
views of the summit area were possible.
Shaded area is the active 17 May 1997
lobe. (b) Following the dome collapse on
25 June 1997, rapid development of the
27 June 1997 lobe (shaded) was apparent
infilling the collapse amphitheatre. The
steep flanks on the northern side had been
reconstructed by 7 July 1997, as seen
during a brief window of good visibility
on that day. (c) Graph highlighting the
anti-correlation between periods of rapid
lobe growth with the frequency of hybrid
earthquakes for the period May to July
1997. Note that the 25 June 1997 dome
collapse occurred during a period of
intense hybrid earthquake activity such
that individual hybrid events were
merging together and could not be
recorded separately. For this reason, the
collapse event does not appear to occur at
a period of peak earthquake activity.
lava nestled within horseshoe-shaped collapse scars. All of these
features have been documented during the Soufri6re Hills eruption.
Conclusions
This eruption provided invaluable improvements to our understanding of the processes operating during the ascent and emplacement of crystal-rich intermediate lavas. It has also highlighted
the need for further research to elucidate the links between dome
growth and associated seismic manifestations (e.g. Hidayat et al.
2000), that may result in even better real-time diagnostic capability.
The recognition of directed extrusion along shear zones helps to
explain the common occurrence of dome collapses and debrisavalanche-forming sector collapses that are a prominent feature
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GROWTH PATTERNS AND DOME EMPLACEMENT
of andesitic volcanism. Indeed, theories described here have also
advanced our understanding of the mechanisms that control dome
collapse and the triggering of dome-collapse pyroclastic flows. The
most significant observations are summarized below.
151
D. Swanson and T. Druitt for their constructive reviews. This paper is
dedicated to the remarkable spirit and resilience of the Montserratian
population and to the memory of the nineteen people who lost their lives on
25 June 1997. R.S.J.S. acknowledges a NERC Professorship and R.B.W. a
University of Bristol studentship. Published by permission of the Director of
the British Geological Survey.
Controls on ascent a n d e m p l a c e m e n t o f andesitic
lava-degassing versus cooling
The evidence from the Soufribre Hills eruption indicates that the
processes operating during shallow magma ascent are so effective
that the Soufri6re Hills andesite extrudes as one of the most viscous
forms of lava on the Earth's surface. Degassing and subsequent
microlite crystallization of the crystal-rich andesitic magma feeding
the dome operate such that significant rheologic stiffening of the
magma occurs. Emplacement switches between hot, crystalline
material with considerable strength (at slow ascent rates) and a
viscous Newtonian fluid (at faster ascent rates). Cooling plays only
a minor role in the emplacement of crystal-rich andesitic lava.
M o r p h o l o g i c variation during e m p l a c e m e n t
The growth of the dome was predominantly governed by extrusions
of near-solid spines at low discharge rates and shear lobes at
moderate discharge rates. On emplacement, these structures were
bounded by ductile shear faults that possibly correspond to the
sidewalls of the main conduit. Lava was emplaced at the dome
summit in the form of these structures, which sporadically broke up
into blocky areas during growth or at the post-emplacement stage.
Infrequently, some lateral spreading of more mobile, fresh magma
was evident, forming a pancake-like, smooth-surfaced lobe, such as
the 25 December 1996 lobe. The higher gas content and lower
crystal content of the magma at the head of a fresh pulse could be
important factors in explaining more fluid-like behaviour.
S t r u c t u r a l controls on e m p l a c e m e n t
The observations described support the stick-slip mechanisms of
dome growth proposed by Denlinger & Hoblitt (1999), Voight et al.
(1999) and Wylie et al. (1999). In essence, the shallow ascent of
magma involves it being pushed to the surface in a piston-like
manner by magmatic pressure. Surface extrusion of the lava was
accommodated along shear faults, either in a vertical manner (as
spines) or subhorizontally away from the central conduit (as shear
lobes) or as blocky lava often during growth spurts. Mapping of
these structures highlighted no clear pattern as to the direction in
which emplacement occurred. A simple theory is that the extrusion
takes place along the path of least resistance (e.g. emplacement of
the 22 October 1997 lobe into the open breach of the crater). The
orientation of the curved faults is governed by stress distributions,
as proposed by Donnadieu & Merle (1998). A new direction of
faulting developed when a pulse of fresh, less viscous magma was
impeded by a plug of crystalline, stagnated lava. The imposing
size of these structures, the complex manner in which they are
emplaced, and their ability to stagnate and switch the focus of
extrusion pose problems in forecasting hazards during the active
stages of such eruptions. The destructive potential of shear lobes (as
demonstrated by growth of the 27 March 1997 lobe and its
demolition of the southern dome flanks) deserves consideration in
monitoring future dome-forming eruptions.
The authors would like to thank all staff of the Montserrat Volcano
Observatory, especially members of Team Volume and Team Seismic
throughout the eruption. Special mention goes to B. Darroux of the
Montserrat Police Force who is responsible for the impressive photographic collection, particularly in the early eruptive stages. Thanks also to
G. Skerritt of the Surveys Department who collected most of the theodolite
data listed in Table 3, and to S. Powell at the Imaging Unit, Department
of Earth Sciences, University of Bristol. We also acknowledge J. Fink,
References
ALIDIBIROV, M. & DINGWELL, D. B. 2000. Three fragmentation mechanisms for highly viscous magma under rapid decompression. Journal of
Volcanology and Geothermal Research, 100, 413-421.
ANDERSON, S. W. & FINK, J. H. 1990. The development and distribution
of surface textures at the Mount St. Helens dome. In: FINK, J. H.
(ed.) Lava Flows and Domes, IA VCEI Proceedings in Volcanology, 2.
Springer-Verlag, New York, 25-46.
ANDERSON, S. W. & F~NK, J. H. 1992. Crease structures indicators of
emplacement rates and surface stress regimes of lava flows. Geological
Society of America Bulletin, 104, 615-625.
BLUNDY, J. & CASHMAN, K. 2001. Ascent-driven crystallisation of dacite
magmas at Mount St. Helens 1980-1986. Contributions to Mineralogy
and Petrology, 140, 631-650.
CALDER, E. S., LUCKETT, R., SPARKS, R. S. J. & VOIGHT, B. 2002. Mechanisms of lava dome instability and generation of rockfalls and pyroclastic
flows at Soufri6re Hills Volcano, Montserrat. In: DRUITT,T. H. & KOKElaar, B. P. (eds) The Eruption of SoufriOre Hills' Volcano, Montserrat,
from 1995 to 1999. Geological Society, London, Memoirs, 21, 173-190.
CASHMAN, K. V. 1992. Groundmass crystallization of Mount St. Helens
dacite 1980-1986 - a tool for interpreting shallow magmatic processes.
Contributions to Mineralogy and Petrology, 109, 431-449.
CASHMAN, K. & BLUNDY, J. 2000. Degassing and crystallization of ascending andesite and dacite. Philosophical Transactions of The Royal Society,
358, 1487-1513.
COLE, P. D., CALDER, E. S., SPARKS, R. S. J. Er AL. 2002. Deposits from
dome-collapse and fountain-collapse pyroclastic flows at Soufri~re Hills
Volcano, Montserrat. In: DRUITT, T. H. & KOKELAAR,B. P. (eds) The
Eruption of SoufriOre Hills Volcano, Montserrat, from 1995 to 1999.
Geological Society, London, Memoirs, 21, 231-262.
DAAG, A. S., DOLAN, M. T., LAGUERTA,E., MEEKER,G. P., NEWHALL,C. G.,
PALLISTER, J. S. & SOLIDUM, R. 1996. Growth of a postclimactic lava
dome at Pinatubo Volcano, July-October 1992. In: NEWHALL, C. &
PUNONGBAYAN, R. (eds) Fire and Mud." Eruptions and Lahars of Mount
Pinatubo, Philippines. University of Washington Press, Seattle, 647-664.
DENLINGER, R. P. & HOBLITT, R. P. 1999. Cyclic eruptive behaviour of
silicic volcanoes. Geology, 27, 459-462.
DEVlNE, J. D., MURPHY, M. D., RUTHERFORD, M. J. ETAL. 1998. Petrologic
evidence for pre-eruptive pressure-temperature conditions, and recent
reheating, of andesitic magma erupting at the SoufriSre Hills Volcano,
Montserrat, West Indies. Geophysical Research Letters, 25, 3669-3672.
DINGWELL,D. B., ROMANO,C. & HESS,K.-U. 1996. The effect of water on the
viscosity of a haplogranitic melt under P-T-X conditions relevant to
silicic volcanism. Contributions to Mineralogy and Petrology, 124, 19-28.
DONNADIEU, D. & MERLE, O. 1998. Experiments on the indentation process
during cryptodome intrusions: New insights into Mount St. Helens
deformation. Geology, 26, 79-82.
DRUITT, T. H., YOUNG, S. R., BAPTIE, B. ET AL. 2002. Episodes of cyclic
Vulcanian explosive activity with fountain collapse at Soufribre Hills
Volcano, Montserrat. In: DRUITT, T. H. & KOKELAAR,B. P. (eds) The
Eruption of SoufriOre Hills" Volcano, Montserrat, from 1995 to 1999.
Geological Society, London, Memoirs, 21, 281-306.
FINK, J. H. & GRIFnTHS, R. W. 1992. A laboratory analog study of the
morphology of lava flows extruded from point and line sources. Journal
of Volcanology and Geothermal Research, 54, 19-32.
FINK, J. H. & GRIFFITHS, R. W. 1998. Morphology, eruption rates, and
rheology of lava domes: Insights from laboratory models. Journal of
Geophysical Research, 103, 527-545.
GARDNER, C. A., CASHMAN,K. V. & NEAL, C. A. 1998. Tephra-fall deposits
from the 1992 eruption of Crater Peak, Alaska: implications of clast
textures for eruptive processes. Bulletin of Volcanology, 59, 537-555.
GRIFFITHS, R. W. 2000. The dynamics of lava flows. Annual Review of Fluid
Mechanics, 32, 477-518.
GRIFFITHS, R. W. & FINK, J. H. 1997. Solidifying Bingham extrusions:
a model for the growth of silicic lava domes. Journal of Fluid Mechanics, 347, 13-36.
Downloaded from http://mem.lyellcollection.org/ at University of Bristol Library on December 19, 2012
152
R. B. WATTS E T AL.
HAMMER, J. E., CASHMAN, K. V. & VOIGHT, B. 2000. Magmatic processes
revealed by textural and compositional trends in Merapi dome lavas.
Journal of Volcanology and Geothermal Research, 100, 165-192.
HARFORD, C. L. & SPARKS, R. S. J. 2001. Recent remobilisation of shallowlevel intrusions on Montserrat revealed by hydrogen isotope composition of amphiboles. Earth and Planetary Sciences Letters, 185,285-297.
HARFORD, C. L., PRINGLE, M. S., SPARKS, R. S. J. & YOUNG, S. R. 2002. The
volcanic evolution of Montserrat using 4~
geochronology. In:
DRUITT, T. H. & KOKELAAR, B. P. (eds) The Eruption of SoufriOre Hills
Volcano, Montserrat, from 1995 to 1999. Geological Society, London,
Memoirs, 21, 93-113.
HIDAYAT, D., VOIGHT, B., LANGSTON,C., RATDOMOPURBO,A. & EBELING, C.
2000. Broadband seismic experiment at Merapi Volcano, Java, Indonesia: very-long-period pulses embedded in multiphase earthquakes.
Journal of Volcanology and Geothermal Research, 100, 215-231.
IVERSON, R. M. 1990. Lava domes modeled as brittle shells that enclose
pressurized magma, with application to Mount St Helens. In: FINK, J. H.
(ed.) Lava Flows and Domes, IAVCEI Proceedings in Volcanology.
Springer-Verlag, New York, Vol. 2, 47-69.
LEJEUNE, A.-M. & RICHET, P. 1995. Rheology of crystal-bearing silicate
melts: An experimental study at high viscosities. Journal of Geophysical
Research, 100, 4215-4229.
LIPMAN, P. W., BANKS, N. G. & RHODES, J. M. 1985. Degassing-induced
crystallization of basaltic magma and effects on lava rheology. Nature,
317, 604-607.
LOUGHLIN, S. C., CALDER, E. S., CLARKE, A. B. ET AL. 2002. Pyroclastic
flows and surges generated by the 25 June 1997 dome collapse,
SoufriGre Hills Volcano, Montserrat. In: DRUITT, T. H. & KOKELAAR,
B. P. (eds) The Eruption of Soufrikre Hills Volcano, Montserrat, from
1995 to 1999. Geological Society, London, Memoirs, 21, 191-209.
MELNIK, O. & SPARKS, R. S. J. 1999. Nonlinear dynamics of lava dome
extrusion. Nature, 402, 37-41.
MELNIK, O. & SPARKS, R. S. J. 2002. Dynamics of magma ascent and lava
extrusion at SoufriGre Hills Volcano, Montserrat. In: DRUITT, T. H. &
KOKELAAR, B. P. (eds) The Eruption of Soufridre Hills Volcano. Montserrat, from 1995 to 1999. Geological Society, London, Memoirs, 21,
153-171.
MILLER, A. D., STEWART, R. C., WHITE, R. A. ET AL. 1998. Seismicity
associated with dome growth and collapse at the Soufri&e Hills
Volcano, Montserrat. Geophysical Research Letters, 25, 3401-3404.
MILLER, T. P. 1994. Dome growth and destruction during the 1989-1990
eruption of Redoubt Volcano. Journal of Volcanology and Geothermal
Research, 62, 197-212.
MURPHY, M. D., SPARKS, R. S. J., BARCLAY, J., CARROLL, M. R. &
BREWER, T. S. 2000. Remobilization of andesite magma by intrusion of
mafic magma at the SoufriOre Hills Volcano, Montserrat, West Indies.
Journal of Petrology, 41, 21-42.
NAKADA, S. & MOTOMURA, Y. 1999. Petrology of the 1991-1995 eruption at
Unzen: effusion pulsation and groundmass crystallization. Journal of
Volcanology and Geothermal Research, 89, 173-196.
NAKADA, N., MIYAKE, Y., SATO, H., OSHIMA, 0. & FUJINAWA, A. 1995.
Endogenous growth of the dacite dome at Unzen Volcano, Japan
1993-1994. Geology, 23, 157-160.
NAKADA, S., SHIMIZU, H. & OHTA, K. 1999. Overview of the 1990-1995
eruption at Unzen Volcano. Journal o[" Volcanology and Geothermal
Research, 89, 1-22.
NEWHALL, C. G. & MELSON, W. G. 1983. Explosive activity associated with
the growth of volcanic domes. Journal o[ Volcanology and Geothermal
Research, 17, 111 131.
NORTON, G. E., WATTS, R. B., VOIGHT, B. ETAL. 2002. Pyroclastic flow and
explosive activity at SoufriGre Hills Volcano, Montserrat, during a
period of virtually no magma extrusion (March 1998 to November
1999). In: DRUITT, T. H. & KOKELAAR, B. P. (eds) The Eruption of
Soufridre Hills Volcano, Montserrat, from 1995 to 1999. Geological
Society, London, Memoirs, 21, 467-481.
PERRET, F. A. 1935. The Eruption of Mt. PelOe 1929-1932. Carnegie
Institution of Washington, Baltimore.
PINKERTON, H. & STEVENSON, R. J. 1992. Methods of determining the
rheological properties of magmas at sub-liquidus temperatures. Journal
of Volcanology and Geothermal Research, 53, 47-66.
View publication stats
ROBERTSON, R. E. A., COLE, P. D., SPARKS, R. S. J. ET AL. 1998. The
explosive eruption of Soufri&e Hills Volcano, Montserrat, West Indies,
September 17, 1996, Geophysical Research Letters, 25, 3249-3432.
ROBERTSON, R. E. A., ASPINALL, W. P., HERD, R. A., NORTON, G. E.,
SPARKS, R. S. J. & YOUNG, S. R. 2000. The 1995-1998 eruption of the
SoufriGre Hills Volcano, Montserrat, WI. Philosophical Transactions of
the Royal Society, 358, 1619-1637.
ROOBOL, M. J. & SMITH, A. L. 1998. Pyroclastic stratigraphy of the SoufriGre Hills Volcano, Montserrat: implications for the present eruption.
Geophysical Research Letters, 25, 3392-3396.
SPARKS, R. S. J. 1997. Causes and consequences of pressurisation in lava
dome eruptions. Earth and Planetary Science Letters, 150, 177-189.
SPARKS, R. S. J. & PINKERTON, H. 1978. Effects of degassing on rheology of
basaltic lava. Nature, 276, 385-386.
SPARKS, R. S. J., YOUNG, S. R., BARCLAY, J. ET AL. 1998. Magma
production and growth of the lava dome of the Soufri6re Hills Volcano, Montserrat, West Indies: November 1995 to December 1997.
Geophysical Research Letters, 25, 3421-3424.
SPARKS, R. S. J., MURPHY, M. D., LEJEUNE, A. M., WATTS, R. B., BARCLAY, J. & YOUNG, S. R. 2000. Control on the emplacement of the
andesite lava dome of the Soufri&e Hills volcano, Montserrat by
degassing-induced crystallization. Terra Nova, 12, 14-20.
SPARKS, R. S. J., BARCLAY, J., CALDER, E. S. ET AL. 2002. Generation
of a debris avalanche and violent pyroclastic density current on 26
December (Boxing Day) 1997 at SoufriGre Hills Volcano, Montserrat.
In: DRUITT, T. H. & KOKELAAR, B. P. (eds) The Eruption of SoufriOre
Hills Volcano, Montserrat, from 1995 to 1999. Geological Society,
London, Memoirs, 21, 409-434.
STASIUK, M. V. & JAUPART, C. 1997. Lava flow shapes and dimensions as
reflections of magma system conditions. Journal of Volcanology and
Geothermal Research, 78, 31-50.
SWANSON, R. A. & HOLCOMB, R. T. 1990. Regularities in growth of the
Mount St Helens dacite dome 1980-1986. In: FINK, J. H. (ed.) Lava
Flows and Domes. IA VCEI Proceedings in Volcanology. Springer-Verlag,
New York, Vol. 2, 3-24.
SWANSON,D. A., DZURISIN, D., HOLCOMB, R. T. ET AL. 1987. Growth of
the lava dome at Mount St Helens, Washington, (USA) 1981-1983.
In: FINK, J. H. (ed.) The Emplacement of Silicic Domes and Lava Flows.
Geological Society of America, Boulder, Special Paper, 212, 1-16.
TAYLOR, G. A. M. 1958. The 1951 eruption of Mount Lamington, Papua.
Commonwealth of Australia, Department of National Development,
Bureau of Mineral Resources, Geology and Geophysics, Canberra.
VOIGHT, B., SPARKS, R. S. J., MILLER, A. D. ET AL. 1999. Magma flow
instability and cyclic activity at Soufri+re Hills Volcano, Montserrat,
British West Indies. Science, 283, 1138-1142.
VOIGHT, B., CONSTANTINE, E. K., SISWOWIDJOYO, S. & TORLEY, R.
2000. Historical eruptions of Merapi Volcano, Central Java, Indonesia,
1768-1998. Journal of Volcanology and Geothermal Research, 100,
69-138.
VOIGHT, B., KOMOROWSKI.J.-C., NORTON, G. E. ETAL. 2002. The 26 December (Boxing Day) 1997 sector collapse and debris avalanche at
SoufriGre Hills Volcano, Montserrat. In: DRUITT, T. H. & KOKELAAR,
B. P. (eds) The Eruption of Soufrigre Hills Volcano, Montserrat, from
1995 to 1999. Geological Society, London, Memoirs, 21,363-407.
WOLF, K. J. & EICHELBERGER, J. C. 1997. Syneruptive mixing, degassing, and crystallization at Redoubt Volcano, eruption of December
1989 to May 1990. Journal of Volcanology and Geothermal Research,
75, 19-37.
WYLIE, J. J., VOIGHT, B. & WHITEHEAD, J. A. 1999. Instability of magma
flow from volatile-dependent viscosity. Science, 285, 1883-1885.
YOUNG, S. R., SPARKS, R. S. J., ASP1NALL,W. P., LLYNCH, L. L., MILLER,
A. D., ROBERTSON, R. E. A. & SHEPHERD, J. B. 1998. Overview of the
eruption of Soufri&e Hills Volcano, Montserrat, July 18 1995, to
December 1997. Geophysical Research Letters, 25, 3389-3392.
YOUNG, S. R., VOIGHT, B., BARCLAY,J. ETAL. 2002. Hazard implications of
small-scale edifice instability and sector collapse: a case history from
SoufriGre Hills Volcano, Montserrat. In: DRU~TT, T. H. & KOKELAAR,
B. P. (eds) The Eruption of Soufridre Hills' Voh'ano, Montserrat, from
1995 to 1999. Geological Society, London, Memoirs, 21, 349-361.