Bull Volcanol(2003)65:165-181
DOl 1O.1007/s00445-002-0251-0
,.,
RES
I~ ,\ R C II
\ R
ric
L I:
M. G. Petterson
S.
J.
Cronin·
P. W. Taylor· D. Tolia .
A. Papabatu
. T. Toba . C. Qopoto
The eruptive history and volcanic hazards of
Savo,
Solomon Islands
Received:16August
20011
Accepted:II September
20021
Publishedonline:23 November2002
©
Springer-Verlag2002
Abstract
Savo Island is the 6-km-diameter emergent
that similar deposits formed from dome explosions or
summit of an' andesitic-dacitic stratovolcano, rising from
collapses of eruption columns during later eruptions
the Iron Bottom Sound, 35 km NW of Honiara, Solomon
(1830-1840 a.d.). The high-sodium magmas (ca. 5-7 wt%
Islands. Savo has erupted at least three times within
Na20) apparently crystallised and strongly degassed prior
recorded history and the 3,000 inhabitants
maintain
to eruption. Shallow explosions were possibly caused by
extensive oral traditions of past events. Through descrip-
entrapment of magmatic gases beneath a dome or conduit
tion and interpretation of the volcaniclastic sequences on
plug of highly crystalline, near solid magma. Repeated
the island, in conjunction with' historical accounts and oral
sealing of the vent may have been due to inward collapse
traditions, we reconstruct the eruptive processes on Savo.
of the highly altered rocks of the surrounding hydrother-
Block-and-ash flow (BAF) deposits are volumetrically
mal system; these rocks probably were saturated due to
dominant on the island within three main depositional
contemporaneous
high intensity rainfall events. BAFs
environments: near-vent sequences, thick medial channel
were hot enough to char vegetation and attain aligned
sequences and distal fan sequences. The deposits com-
clast TRM (thermal remnant magnetism) up to 3 km from
prise universally non-vesicular
and highly porphyritic
the vent, many being accompanied by ash-cloud surges.
(40-70% phenocrysts), high-silica andesite and dacite
Changes with distance in the BAF deposits appear mostly
clasts. These appear to have been derived from collapsing
dependent on flow confinement and are limited to an
lava domes during an 1560-1570 a.d. eruption. However,
overall decrease in thickness and maximum clast size, and
eyewitness descriptions and crater morphology suggest
an increased definition of weak planar fabrics. In distal
fan sequences, there is strong evidence for syn- and post-
Editorialresponsibility:1. McPhie
eruptive redeposition of primary deposits. Si~ce t~e Savo
------------.:.'----------
population is concentrated on coastal volcaniclastic fans,
M.G.Petterson
we consider the greatest volcanic risk to life is from
BritishGeologicalSurvey,Keyworth,Nottingham.NGI2 5GG.
BAFs, associated ash-cloud surges and lahars. Hence, the
UK
main channels and fans are designated as the highest of
SJ. Cronin() )
three relative hazard zones on a simple map prepared to
Instituteof NaturalResources,MasseyUniversity,
aid local education and planning initiatives on Savo.
PrivateBag 11222 PalmestonNorth.NewZealand
e-ma~l:sc.ronin~geomar.de
Keywords Block-and-ash flows· Lahars· Lava domes·
e-mail:s.J.cromn@massey.ac.nz
Solomon Islands. Vulcanian explosions
Tel.: +49-431-6002139
Fax: +49-431-6002924
P.w. Taylor
Introduction
AustralianVolcanologicalInvestigations,P.O. Box 291,
PymbleNSW2073,Australia
.
Savo Island is the emergent upper 485 m of an andesitic
D. Tolia . A. Papabatu. T. Toba . C. Qopoto
to dacitic stratovolcano, rising 1,400 m from Iron Bottom
Ministryof EnergyWaterand MineralResources,
Sound, 35 km NW of the Solomon Islands capital Honiara
Minesand MineralsDivision,P.O. Box G37 Honiara,
(Fig. 1). Up to 3,000 inhabitants coexist with the volcano,
SolomonIslands
which has erupted explosively several times, as recorded
Presentaddress:,
in written and oral history. Inhabitants are dispersed
SJ. Cronin,GE?MAR Forsch~ngsz~ntrum,
.
among small villages and settlements along most of the
Abt.VulkanologieundPetrologie,Wischhofstrasse1-3,24148Kiel, coastal fringe of the island. Eruptive products from the
Germany
166
Fig. 1 Solomon Islands in rela-
tion to the Miocene Vitiaz
Trench System (hatched) and
the active New Hebrides-San
Cristobal trench. Nearby major
features include the Ontong
Java plateau (with crosses).
north of the Solomon block.
Note the highly oblique con-
vergence of the Pacific and
Australian plates. Figure adapt-
ed from Petterson et al. (1999)
central vent are unusually well exposed (for a moist
the Vitiaz Trench System north of the Solomon Islands
tropical environment) within several radiating drainages.
occurred up to 12 Ma (Yan and Kroenke 1993). Since that
Superficially. the ~~ttsteristics
of, the depos!ts suggest
time, ~ere
has been. ~orthwar,d. ~u.bduction ~f the
Savo has parallels wIth several recently active, dome-
Austrahan Plate along tffe!'Ne\\f·Ol~-San
Cnstobal
forming
volcanoes
including
Unzen,
Redoubt
and
Trench (Fig. I), although there may have been further
Montserrat, although the vertical relief of Savo is not
intermittent and local activity along the Vitiaz Trench
quite as dramatic. Like these three other volcanoes
System (Petterson et al. 1997, 1999). The Ontong Java
(Gardner et al. 1994; Cole et al.
1998; Miyabuchi
Plateau collided with the Solomon Arc from ca. 12-9 Ma;
1999), the exclusively non-vesicular clasts within the
uplifted portions of the plateau form some of the larger
volcaniclastic
deposits
on Savo
may
be related
to
islands (Coleman
1966; Petterson et al. 1999). Other
disruption of lava domes. However, present crater mor-
islands are a combination
of ocean-floor and plateau
phology, plus eyewitness accounts, suggest that external
lithologies, constructed during the two major arc-building
water within a shallow hydrothermal system and tempo-
periods (stage 1, Eocene-Lower Miocene; stage 2, Upper
rary crater lakes played an important role in recent Savo
Miocene to present day). The present-day arc comprises
eruptions.
the New Georgia
Group, Russell Islands and Savo,
Through analysis of the sedimentary
features and
together with many tens of submarine volcanoes south
lithology
of the
deposits,
coupled
with
eyewitness
of the New Britain-San
Cristobal Trench (Crook and
accounts and legends, we interpret eruptive processes at
Taylor 1994; Kroenke 1995).
Savo. In addition, we outline the changes in deposit
Savo is a single stratovolcano constructed on unknown
character with distance from source on this small volcanic
basement above ca. 14-km-thick crust (Furumoto et al.
island, and implications for downstream flow transfor-
1976), and represents the easternmost limit of the New
mation within block-and-ash flows (BAFs). Using the
Georgia terrain (Petterson et al. 1999). It is a modern,
interpreted volcanic history of Savo as a guide, we
northern extension of the Mio-Pliocene to Recent Gallego
produce a simple volcanic hazard map for the island, to be
Volcanic Field of NW Guadalcanal (Hackman 1980). A
used within local risk management initiatives.
number of variably eroded centres in the Gallego field
resemble Savo in size, geochemistry (Stanton 1994) and
form, with andesitic-dacitic
dome complexes and block-
Tectonic and geological setting
and-ash flow fans dominant.
Savo is a slightly elliptical 6x7-km-diameter
island
The Solomon Islands are part of the Greater Melanesian
(Fig. 2), with a basal diameter of 9 km at 900-1,000 m
Arc, formed at the boundary of the Australian and Pacific
below sea level and an estimated volume of 10 krrr'. A
Plates. Southward subduction of the Pacific Plate along
1.5xl-km crater (long axis oriented NE-SW) dominates
Fig. 2 Topography
of Savo Is-
land. showing the main stream
channels. crater. coastal village
names and the extent of
presently active geothermal
fields (stippled). Topographic
contours shown at 40. 120 200.
280. 360 and 440 m elevation
the central summit area, the floor of which is mostly about
are probably
controlled
more by local groundwater
80 m lower than the lip (Fig. 2). A small, forested dome
circulation than regional heat-flow changes. Hence. it
occupies part of the crater floor, whereas a second dome
appears that the near-surface environment has been in
straddles the north-eastern crater rim and upper Mbazo
thermal equilibrium with underlying deep-seated magma
valley (Fig. 3). Larger, older domes and exhumed
for at least the last 40 years.
cryptodomes, many of which reach 240 m high. are
The
unvegetated
hydrothermal
fields
are
mostly
mostly located in the southern to south-western sector of
formed in highly altered volcaniclastic deposits. The
the island. A drainage network radiates outward from the
area, depth and degree of alteration are high in near-
crater area. with well-developed coastal volcaniclastic
crater,
upper
flank
locations
such
as
the
upper
fans formed at each one of the several channels. The
Poghorovuraghala
valley (Fig. 2), where frequent and
steeply dipping medial flanks of the volcano are highly
large-scale collapses occur, generating locally destructive
dissected, in places forming a series of radial steep-sided
lahars. such as in 1998 (Cronin et al. 2000b).
ridges and deep intervening valleys. Primary and second-
Intermittent seismic recording on Savo has detected
ary rain forest cloaks much of Savo. Unforested areas are
several tectonic earthquakes, including three events in
related only to hydrothermal activity and. near the coast,
1985 located close to the eastern and south-eastern Savo
agriculture.
coast and at <1 km focal depth. These may relate to a
A large number of hydrothermal areas are located on
faulted south-eastern margin of the stratovolcano. During
Savo and offshore, including features such as fumaroles,
an in-crater field experiment in 1994 involving M. Lardy
small geysers, hot-water springs and areas of steaming
(ORSTOM, Vanuatu),
M. Larue (SOPAC, Fiji), M.
ground (Fig. 2). They are mainly concentrated around the
Petterson and T. Toba (Mines and Minerals Division,
central crater and south-eastern sector of the island (Toba
Solomon Islands),
volcano-tectonic
earthquakes
were
1995), although inhabitants in other sectors have reported
noted during 2 h of recording with a portable seismome-
additional newly formed areas, including offshore hot
ter.
springs (Cronin et al. 2000b). Ad hoc monitoring of some
of the hydrothermal fields over the last 40 years shows
essentially constant temperatures, ranging mostly between
92 and 103°C
(Toba 1995). Temporal variations in
temperature are not synchronous at separate fields, and
women and children were clubbed in the struggle, and
hands clinging in desperation to the gunwales were
chopped off without mercy" (p. 107, Grover 1958). The
eruption felled trees with a "glowing cloud" in the south-
western sector, and fallen trees pointed seaward. Grover
(unpublished record
1961; reproduced in Toba 1993)
recorded an account of one of the climactic episodes of
the 1830s-1840s eruptions from the perspective of a
Guadalcanal inhabitant, related by the son of the eyewit-
ness. The episode occurred before Europeans settled in
the Solomon Islands, at a time of year around June/July
and it was associated with heavy rain and earthquakes. It
started in the afternoon in a small way, but by midnight
there were sounds like the continued hooting of a steamer,
and also like a river in flood rolling large boulders. Many
repeated muffled explosions occurred, along with earth-
quakes at a rate of about 120 per minute. During the
cataclysmic
outburst
that
followed, the Guadalcanal
Mountains and people standing outside their houses
watching, were all lit up, as though by daylight. Millions
of "great fiery rocks" were thrown into the sky above the
fire on Savo, falling into the sea. A thin layer of ash
covered the north-western
coast of Guadalcanal. The
explosions continued through the night and next day
before dying out on the following night. Many Savo
refugees arrived on Gaudalcanal by morning, and stayed
for up to 10 years because the populated southern sectors
of Savo had been rendered completely desolate (Fig. 4B).
Oral traditions, or kastom stories, as they are known in
the Solomon Islands, are the main method of historical
recording by Melanesian and Polynesian people (Bascom
1965). In many South Pacific locations, momentous or
frightening events, such as volcanic eruptions are well
recorded by oral traditions (e.g. Papua New Guinea,
Blong 1982; Tonga, Taylor 1995; and Fiji, Cronin and
Neall 2000).
Toba (1993) documented
oral traditions on Savo
through a series of villager interviews. Legends were
consistent around the island, and all described local
perspectives of two main eruptions.
An early event, termed by local inhabitants as the
toghavitu eruption (meaning 7000 or 1007), was preceded
by the filling of the crater with water, and then a period of
increasingly violent earthquakes and tsunamis. The height
of the eruption was marked by thunder, lightning, high
seas and gale-force winds. All versions of the legend
insist that no one was left alive on the island afterward.
Processes described included fall of large blocks all over
the island, and red-hot gaseous material flowing down
valleys, "emitting foam". Later, mud and debris flowed
down valleys like water into the sea. This process
apparently built up the island to be much larger than its
pre-eruption dimensions (Fig. 4A). Many people were
killed by falling debris, being engulfed by both hot and
cold flows and sinking into deposits. The "7000" or
"1007" figure, depending on translation, represents the
death toll. Almost the entire island was affected (Fig. 4A),
leaving only a dusty grey remnant. Only a few survived
by escaping to neighbouring islands, especially Guadal-
Fig. 3 Geological map of Savo, showing the four mapped
volcaniclastic facies associations (Table 2) as well older domes
and lavas. Locations of sections presented in Figs. 5. 7 and 9 are
numbered. and stars denote locations of flux-gate magnetometer
measurements
Historic eruption records
The first written record of a Savo eruption was made in
the journal of Spanish mariner, Alvaro del Mendaiia, who
observed Savo erupting in 1568 (Guppy 1887; Amherst
and Thomson 1901). The log records volcanic plumes,
falling ash and the presence of white "roads" cutting
through the jungle from the centre of the island to the
northern shore (Amherst and Thomson 1901).
Guppy (1887) recorded a second protracted period of
activity in the 1830s to mid 1840s, with intermittent
(phreatic?) activity continuing until the 1880s. Local
descriptions of the events included: "erupted materials
rushed down deep channels", to form smooth surfaces that
"look like roads from the sea"; "large quantities of water,
dust and ash were ejected"; "day and night were the
same"; "dust affected adjacent islands"; "several natives
were killed". Guppy (1887) also noted that, up until 1877,
volcaniclastic
deposits
were unvegetated,
resembling
roads, which afforded easy access to the summit.
Grover (1958) also provided an account of the 1830s-
1840s eruptions, as related by Bishop J. M. Aubin, who,
after arrival in 1906, noted local descriptions of the
eruption. Mass evacuations took place during the height
of eruptions - "stories are told of efforts to get away by
canoes of which there were not sufficient for all: how
Fig. 4 Areas
(stippled)
affected
by eruptions
as described
in
apparent
"growth"
of the island
during
this eruption.
B 1830-
Kastom
legends
from
Savo
as collected
by Toba
(1993).
A
1840 a.d. event. C 1953 a.d. lahar inundation
Toghavitu
event;
un stippled
area
in the north
represents
the
canal. Resettlement of Savo was apparently by Savo
correspond well with the long period of activity in the
people who returned from Guadalcanal.
1830s-1840s recorded by Guppy (1887) and Grover
The second, later event, was preceded by similar signs
(1958). Despite there being no specific age ascribed to the
as the toghavitu eruption, including filling of the crater
earlier toghavitu event, it was reportedly a long time
with water, but it was smaller and some sectors in the
(several generations) before the second eruption and a
south-west and east were not affected (Fig. 4B). Fewer
long time before the settlement of Europeans. The event
people were killed (tens to hundreds) and many evacuated
may relate to activity observed by Mendafia in 1568.
to safer places on Savo and to neighbouring islands,
However, Mendafia, also described there being many
including Guadalcanal. These accounts correspond specif-
inhabitants on the island and bountiful gardens there
ically to the 1830s-1840s eruptions described by Guppy
(Amherst and Thomson 1901). These factors suggest that
(1887) and Grover (1958). Apparently a long period had
he arrived either some time before or after the catas-
elapsed since the toghavitu eruption, although the mem-
trophic phase of activity as described in the toghavitu
ories of that event were still strong, reinforced perhaps by
event.
the reputed occurrence of several other smaller eruptions
A
radiocarbon
date
of
270±45
years
b.p.
in the interim period. Processes described included ash
(NOAAI5325,
Dr G.S. Burr, University of Arizona,
falls, which spread as far as the north-western coast of
AMS facility) was obtained on a wood sample below 6 m
Guadalcanal. Local hazards included explosions, hot gas
of volcaniclastic deposits in the Ndavalaka fan of NW
clouds, and flows within the valleys. Two types of flow
Savo (Fig. 3). The volcaniclastic deposits appear to reflect
were distinguished: those that burned people (pyroclastic
a single eruptive sequence, with no palaeosols or other
flow or surge), and those into which people sank and
evidence of long-term quiescence observed. The Oxcal
drowned (lahars). There were periods of total darkness
3.5 calibration program (Bronk-Ramsay 2002) and the
over the island and eruptions continued over a long
calibration curve of Stuiver et al. (1998), indicate ages of
period, impacting on different areas at different times. An
1520-1560 a.d. (P=0.42) and 1630-1670 a.d. (P=0.58)
additional
account
of this second
major period
of
within 1s error, and 1480-1680 a .d. (P=0.90), 1770-
eruptions (Grover 1958) recorded how a woman escaped
1810 a.d. (P=0.09) and 1930-1950 a.d. (P=O.OI) within
death by sheltering in a cave on the lee side of one of the
2s error. The most probable intercept predicted by Oxcal
south-western sector domes,
3.5 and the CALIB4.3 program is 1645 a.d. If the most
A third oral tradition, related by some of the villagers
probable age-range of 1630-1670
a.d. is correct, the
(Toba 1993), describes mudflows that originated from the
wood was contained either in the deposits of the 1830s-
crater area, possibly following a period of heavy rain in
1840s eruption and had a ca. 200-year in-built age (i.e.
1953. These inundated the main river channels of the
age of the tree), or it was in the deposits from an earlier
island (Fig. 4C).
eruption, but subsequent to the pre-1568 event. Unfortu-
nately, no further datable samples were collected during
the initial stages of survey (1992-1995).
Samples col-
Eruption ages and timing
lected
in
1999 from
the
uppermost
units in
the
Poghorovuraghala fan were formed by the 1953 lahars
Both written accounts and oral traditions point to there
and the 1830s-1840s activity and are too young for the
being two major eruptions, as well as several other less
14Cmethod.
catastrophic events. The legends of the second event
.•
171
Table 2 Lithofacies
descriptors
used for volcaniclastic
associations
on Savo
Associations
Code
Lithofacies"
Transport
processes
Block-and-ash
flow fan association
FBA
Dm(m) >Dpw(m) »Sp,
Ppwtc),
Block-and-ash
flows, debris flows,
So/Me >Pc(c), Cc(c)
tephra falls, surges, fluvial
Medial block-and-ash
flow association
MBA
Dm(m) »Dpw(m)
c-Spw,
Block-and-ash
flows, debris flows,
ppw(c)
e-Sc
tephra falls, surges
Proximal crater and flank association
PCA
Sp, Spw, Ppwtc), Dm(m),
Tephra falls, block-and-ash
flows, surges
Dpw(m) >Pc(m), Sc
Fluvial fan association
FFA
Pc(c), Sc, Dpw(m)
Fluvial, debris flows
"Xy(z): X dominant grain size, y sedimentary structures, z matrix or clast support, where applicable; X: D diamicton (here used to describe a
poorly sorted unit with a sand matrix containing distinctly coarser pebbles, cobbles or boulders); C cobble (64-256
rnrn); P pebble (2-
64 mm); S sand (0.063-2 mm); M mud «0.063
mm); y: m massive; pw weak planar fabric; p planar bedded; c cross bedded; (z): m matrix-
support; c clast-support
Further evidence comes from the relative degrees of
lithofacies
and lithofacies
associations
on Savo
dissection and drainage densities on the volcaniclastic
fans that encircle Savo. Aerial photographic interpretation
The complex volcanic sequence exposed on Savo is
suggests that the Rembokola and Poghorovuraghala fans
simplified by its classification
into four lithofacies
are the least dissected, the Ndavalaka fan (where the
associations (Table 2), denoted by their main constituent
radiocarbon date comes from) is intermediate, and the
facies. These recent volcaniclastic
deposits on Savo
Kuila, Mbazo and Megapode Field fans are significantly
overlie an apparent basement of basaltic andesite lavas,
more dissected, with a much higher drainage density.
domes, and gabbroic-dioritic shallow intrusions (Fig. 3).
Hence, the Poghorovuraghala and Rembokola fans could
relate to the 1830s-1840s eruptions, the Ndavalaka fan to
an eruption some time between 1630 and 1670 a.d., and
Block-and-ash flow fan association (FBA)
the Kuila, Mbazo and Megapode Field fans to the earlier
toghavitu event.
Six discrete areas of FBA are present (Fig. 3), five in the
The cryptodomes exposed in the south-western sector
northern
half
of
Savo,
and
one
in
the
south
of Savo (Fig. 3) are probably the oldest units, but are too
(Poghorovuraghala). Each of these represent a fan, with
young for the K-Ar dating method attempted, that is
an associated shallow offshore portion that is most likely
<100,000
years old
(David Rex, Leeds University,
composed of submarine volcanic mass-flow and turbidite
personal communication).
deposits. The low-lying FBA fans have resulted in a
significant enlargement of the aerial extent of the island
and are economically important, being prime agricultural
Composition and petrography
and habitation sites. The gross morphology of these fans
comprises an upper valley-confined section and lower
The volcaniclastic products of recent eruptions on Savo
broader coastal section. The coastal fans are flat to gently
are volumetrically
dominated
by andesitic to dacitic
undulating with a gentle seaward dip. Coastal erosion has
compositions. Typical clasts in the youngest volcanic las-
trimmed these fans into small cliffs (1-6 m high), which
tic deposits, within the Poghorovuraghala and Rembokola
afford good transverse-fan exposure.
fans, are all very similar and are the most silicic of all
Most sections show little or no evidence for long
samples analysed from Savo (62-66 wt% Si0
2;
Table 1).
hiatuses and they probably represent deposition from
The clast compositions of the older Ndavalaka fan are
single eruptive episodes.
However,
dark brown and
possibly less evolved, or at least at the low-silica end of
reddish brown palaeosols are found in some exposures,
the range within the younger fans. The most basic rocks
along with major cut-and-fill structures in others, indi-
(43-50 wt% Si0
2)
include hornblendite or pyroxenite
eating that these fans are composite structures formed
accessory clasts in the recent volcaniclastic deposits, and
from the deposits of a number of rapid-deposition periods,
basalt and gabbro within domes and exhumed crypto-
interrupted by significant periods of slow deposition,
domes on the lower island flanks (Table 1).
stability or erosion.
The most common phenocryst assemblage includes
The FBA is characterised
by extreme variability
hornblende
+
plagioclase
+
alkali feldspar
+
magnetite
±
(Figs. 5 and 6), vertically in exposures up to 9 m thick,
pyroxene
±
biotite, with accessory sphene, apatite and
and laterally over tens to hundreds of metres in sections at
zircon. Hornblende is commonly rimmed by magnetite.
various angles transverse to palaeoflow directions.
Almost all samples have high phenocryst contents (40-
The most abundant lithofacies [Dm(m); Table 2] is a
70 vol%) and a mostly cryptocrystalline groundmass;
massive 'diamicton';
this term is here used to denote a
microcrystalline groundmass textures also occur. Com-
very poorly sorted deposit with a sand matrix that either
mon gabbroic enclaves consist of hornblende
+
pyroxene
supports or fills interstices between clasts of pebble to
+
plagioclase
+
magnetite.
boulder size (commonly <0.5 m diameter; Fig. 6a). Single
172
Fig. 5 Logged sections and fa-
cies interpretations
within the
FBA. Facies codes are given in
Table 2. Numbered
section lo-
cations are given on Fig. 3.
Scale at column bases indicates
modal grain size, estimated
by
point counting
(50-100
clasts at
each site), and field textural
estimates for finer units
Fig. 6a,b Photographs
of lithofacies in the FBA. a Typical Dm(m)
deposits exposed in the Ndavalaka
fan, with coarse-tail
reversely
unit in the Rembokola
fan. Note thin Sp unit preserved at top, and
graded Dm(m)
(and intercalated
Sp) overlying
massive
Dm(m),
underlying
Sp that grades
downward
into Dm(m).
b Stacked
which in tum overlies Sc and Pc(m)
I
2
3
4
Kula
Fan
NdavaliQ Fan
RemlJokola
Fan
PogholOvll"aghaia
Fan
7m-
Sm-
5m-
Sm-
llm(m)otlpw(m)
llm(m)
Spw
Sp
~m~m)
llm(m)
llm(m)otlpw(m)
~m~m)
Sc
~m~m)
llm(m)
::<m)
2.5
m _
IDpw(m)
llm(m)
::::~::
Sp
llm(m)
3.5
m-
Sp
.--_.-
Sp
Sp
llm(m)
llm(m)
Sp
4m-
4m-
llm(m)
l
~m~m)
~m)
i~~;
llm(m)/Sp
Sc
1116
2
54mm
Sp
.
llm(m)
~
Sp
~m)
EmoianII...-
llm(my
~m)
--.0
W~~,>t=-'
1/16
2
54
mm
Sc
Poortydollnod-.o
Sc
<-.::.,...=-iru-..,
llm(m)
~g-; -~
'.'1>
0
llQ.
~mJ
1116 2
64mm
••• ~
::--pollbIo-
1116 2
54mm
p-@~
Fig. 7 Logged sections and fa-
cies interpretations
from the
MBA. Facies codes are given in
Table 2. Numbered
section lo-
cations are given on Fig. 3.
Scale at column bases indicates
modal grain size estimated by
point counting
(50-tOO
clasts at
each site), and field textural
estimates for finer uni ts
depositional units range between 0.5 and >8 m thick.
two Dpw(m) units in the Poghorovuraghala catchment
Clasts comprise 20-40 vol% of the deposits, increasing to
showed random NRM orientations.
>60% in clast-concentration
zones and in coarser de-
Less common lithofacies, Sp and Ppw(c), consist of
posits. Clasts are uniformly non-vesicular, angular to
moderately well-sorted, horizontally bedded or laminated
subrounded, high-Si0
2
andesite to dacite, whereas the
sand and pebble layers, 2-3 em to I m thick. Some units
poorly sorted, fine-coarse sand matrix is pale brown to
contain charred or unaltered wood fragments and/or tree
white, crystal and crystal-fragment rich; sand grains are
stumps in growth position.
highly
angular.
Subordinate
clast lithologies
include
Another minor lithofacies (Sc and Me) comprises
hornblende gabbro, basaltic andesite, gabbroic diorite,
centimetre- to l-m-thick, poorly sorted, fine sand-dom-
hornblendite and red and white hydrothermally altered
inated beds containing pebbles and rare cobbles (Fig. 6b).
clasts. The latter are conspicuous and make up 5-10% of
These units are internally thinly bedded or laminated with
the clasts, based on point counts made at most described
low-angle cross-bedding in some locations (Fig. 5). Beds
sections. Deposit fabric, where present, comprises indis-
are distinguished by alternating fines-poor and fines-rich
tinct, horizontal,
laterally discontinuous,
matrix-poor,
layers. Some units contain charcoal fragments.
pebble- or cobble/boulder-concentration
zones, sand lay-
Further lithofacies, Pc(c) and Cc(c), are commonly
ers, clast strings and alternating matrix-
and
clast-rich
channel-filling, and are either cross bedded on moderate-
layers. Both normal and reverse coarse-tail grading occur.
high angles, or planar bedded, with alternating layers of
Charcoal fragments are common within some deposits.
medium-coarse sand, fine sand and pebble or cobble-rich
At several sites within the Poghorovuraghala FBA
deposits. Single beds are well sorted and the coarser
(Fig. 3), in deposits correlated with the 1830s-1840s
deposits are clast-supported. Bed thickness ranges be-
eruptions and the 1953 lahars, natural remnant magnetism
tween centimetre-
and metre-scale,
and the coarsest
(NRM) of 5-15 em-diameter clasts was recorded using an
deposits contain clasts up to 0.7 m diameter. Pc(c) and
FG Instruments BR-2 portable magnetometer (ten clasts
Cc(c) show the widest range of clast lithologies, com-
measured at each site). This technique is a fairly reliable
monly containing clasts of dacite, basalt and andesite as
alternative to laboratory studies of remnant magnetism
well as rare hydrothermally altered clasts.
(Hoblitt and Kellog 1979). In the upper Poghorovuraghala
valley, uniformly aligned NRM was found for clasts in
Dm(m) units, which, in conjunction with the charcoal
Medial block-and-ash flow association (MBA)
present, we interpret as thermal remnant magnetism,
implying an emplacement temperature in excess of Curie
Up to 50% of the area of Savo is mapped as MBA (Fig. 3),
Point (ca. 350 "C), In two further sites near the coast
which probably makes up the greatest portion of the cone
(Fig. 3), Dm(m) clasts had random NRM orientations.
volume. Sections >40 m thick through these deposits are
Subordinate but still common lithofacies [Dpw(m),
found within the mid-upper parts of the main river valleys
Fig. 5] are characterised by weak planar fabric, defined by
(Figs. 7 and 8). Palaeosols are noted in some exposures,
discontinuous
layers of sand or pebbles, and/or clast
along with eroded contacts where coarse Dm(m) units are
strings. NRM of coarse pebble- to cobble-sized clasts in
cut into weakly bedded Spw, Ppw(c) and Sc layers.
5
6
7
8
Ndavalaka
Fan
Poghorovuraghala
Fan
Ndavalaka
Fan
UpperNdavalaka
Fan
7m-
3.5 -
173
diameters and d-max measurements (the average of the
maximum diameters measured from the five largest clasts
observed
in any given outcrop),
a weak trend of
. decreasing maximum clast size with increasing distance
from the crater is noted. Clast content varies between 10
and 60% by volume (modal range 20-40%). These units
are dominated by andesitic to dacitic clasts (generally
uniform in composition within any given stratigraphic
. unit), with 5-10%
gabbro, basalt, basaltic
andesite,
hornblendite and hydrothermally altered clasts. Charcoal
fragments are commonly found within Drn(m) units.
NRM measurements on clasts at one location in the
Poghorovuraghala
Valley (Fig. 3) showed uniformly
oriented magnetism.
The less common Dpw(m) lithofacies is similar to the
MBA Dm(m), but contains weak fabrics defined by
discontinuous, horizontal, sand- or pebble-rich layers or
clast strings. NRM measurements on clasts at one location
in the Poghorovuraghala Valley, where charcoal was also
found (Fig. 3), showed uniformly oriented clast magne-
tism, whereas another showed random clast orientation.
Spw, Ppw(c) and Sc are minor lithofacies of MBA.
These are pale brown, shower-bedded, laminated or low-
angle cross-bedded units between a few centimetres and
I m in thickness. Some of these units clearly cap Dm(m)
units, whereas others may be related to the units directly
above.
Proximal crater and flank association (PCA)
The PCA is distributed concentrically within 0.5-1 km of
the summit crater, forming the steep inner and outer crater
walls (Fig. 3). The upper tributaries of the major rivers
originate on the lower slopes of the outer crater-wall
Fig. 8 The basal portion of a typical Dm(m) within the MBA. in
the middle reaches
ofPoghorovuraghala
catchment.
Deposits of
this uniform character
dominate the mapped extent of the MBA.
The camera case is ca. 25 em high
':fence, these sections probably include deposits from
several eruptive episodes separated by significant time
breaks. Post-depositional
hydrothermal
alteration has
cemented small portions of some deposits, with local
development of fines-poor fossil fumarole vents.
The dominant lithofacies present is Dm(m) (Figs. 7
and 8), in beds 0.5 to >40 m thick, with most falling in the
range of 2-10 m. The MBA is homogeneous in character,
both vertically and laterally. The non-welded, very poorly
sorted deposits contain clasts up to 2.5 m diameter (modal
range of 0.3-0.7 m diameter based on point counting of
4-5 sections within each main catchment), supported
within a matrix of fine sand to fine pebbly sand. Clasts are
dominantly subangular-angular, and uniformly dense with
no vesicular clasts found. Based on point counts of clast
Fig. 9 Logged sections and fa-
cies interpretations
from the
PCA. Facies codes are given in
Table 2. Numbered
section 10-
cations are given on Fig. 3.
Scale at column bases indicates
modal grain size, estimated by
point counting
(50-100
clasts at
each site), and field textural
estimates for finer units. Note
scale for column 9 is different
from that for the other three
columns
175
Fluvial fan association (FFA)
Three small triangular FFA areas occur along the lower
reaches of minor and ephemeral stream systems (Fig. 3).
They are sourced on the medial flanks rather than the
crater
area and have a limited
inland
extent. The
representative Siata fan dips seaward more steeply than
FBA
areas and dominantly
contains
Pc(c)
and Sc
lithofacies. These units are moderately well sorted, thinly
bedded (ern- to -on-scale), in both planar and cross beds,
and contain clasts mostly <1 em diameter. Clasts are
dominantly andesite-dacite, although most other litholo-
gies found on the island are also present as clasts, both
unaltered and hydrothermally altered.
Minor
volumes of Dpw(m)
units are noted, and
dominate a small number of outcrops. These beds contain
coarse, angular clasts up to 0.3 m diameter, within fine-
medium sand matrices. Clast lithologies are similar to the
interbedded Pc(c) and Sc facies.
Domes and lava units
Crater domes
Two recent domes occur within the present crater area
(Fig. 3). The smooth, central crater dome is ca. 500 m in
diameter with a maximum height of ca. 30 m. Its summit
is ca. 50 m lower than the lowermost portion of the crater
rim. It is cloaked in dense forest except in areas with
vigorous hydrothermal activity. Much of the surface rock
is hydrothermally altered, but some areas preserve flow-
fold structures, and fresh samples are silicic-andesite to
dacite, medium to coarsely porphyritic, with hornblende
and plagioclase phenocrysts dominant. Mafic and ultra-
mafic, hornblende-dominated enclaves are present.
The second dome is 800x500 m in basal dimensions
,
and located on the north-eastern crater margin within the
upper Mbazo catchment. It comprises coarsely porphyritic
I
pale-coloured silicic andesite to dacite, similar to the
I
central crater dome. Based on geomorphic relationships
I
and similarities in composition,
this feature appears
I
genetically related to the Mbazo and Megapode Field
;
volcaniclastic fans, and hence is slightly older than the
I
central crater dome.
l
Older flank domes
,
Six steep-sided mounds, ranging between 400 and 1,500 m
I
diameter, are present on the lower island flanks (Fig. 3),
with five of these located in the south-western quadrant.
l
These features rise up to 240 m from their surrounds. The
I
central parts of these mounds are not exposed, but based
;
on their form and the composition of apron sediments
l
they are assumed to be domes or exhumed cryptodomes.
The poorly exposed surrounding aprons comprise mas-
sive, cemented, coarse-block-bearing breccias, with larger
clasts being subrounded-rounded, Clast lithologies in-
Fig. 10
PeA
exposure in the present crater walls, including stackec
Sp and Ppw(c) and intercalated
Sc, overlain
by
massive Dm(m)
deposits. The crater rim ranges between 485 (islanc
summit) and 280 m in altitude within a breach on the
eastern side (Rembokola catchment), whereas the crate]
floor lies at ca. 220 m. Extensive hydrothermal alteratior
occurs in many parts of the mapped extent of PCA. The
PCA is dominated by Sp/Spw, Ppw(c) and Dm(m),
Dpw(m) lithofacies, with lesser but important volumes
0:
Pc(m) and Sc (Fig. 9).
The mantling and shower-bedded Sp, Spw and Ppw(c
(Fig. 10) are well sorted, commonly with alternating SI
and Ppw(c) beds in composite units up to 15 m in tota
thickness. The Ppw(c) ranges between 0.5 and 1 m ir
thickness, containing dense, subangular, andesitic clast:
up to 6 em in diameter. Sp ranges between 0.05 and 0.2 rr
thick, and contains rare outsized clasts up to 3 em.
Dm(m) and Dpw(m) lithofacies are unwelded am
maximum clast diameters are up to 0.6 m (modal range 7-
15 em diameter). Normal grading is common.
Andesitii
to dacitic clasts dominate,
with ca. 10% subsidiarj
lithologies, as described for medial and distal Dm(m
units.
Sc and Pc(m) facies are intercalated with Sp ant
Ppwtc)
units in beds <0.5 m in thickness, distinguished b:
their poorly sorted character, presence of outsized clast
and cross bedding. These beds are discontinuous am
highly variable in thickness.
176
.•
elude gabbro, gneissic gabbro, gabbroic diorite, micro-
lived Vulcanian eruption columns formed via interaction
gabbro and basalt, with enclaves and clasts derived from
of meteoric water with magma at shallow depths (e.g.
enclaves of ultramafic hornblendite. The cemented sand-
Nairn and Self 1978).
grade matrix contains
lithic
clasts,
hornblende
and
The non-vesicular, highly crystalline clast lithologies
plagioclase crystals and stands proud of clasts in weath-
in the Savo deposits suggest a degassed, shallow intrusive
ered exposures. The degree of weathering of these units
or dome origin. In the latter case, BAF generation may
implies a significantly
greater age than that of the
have involved Merapi-style mechanisms.
Merapi-style
volcaniclastic fans emplaced around and between the
BAFs are commonly confined to single catchments or
mounds.
single sectors of a cone, and occur particularly where a
dome stands proud of a crater or is emplaced on the outer
flanks of a volcano (Gardner et al. 1994; Ui et al 1999;
Lavas
Abdurachman et al. 20(0). These conditions are met by
the older Mbazo dome (Fig. 3), which appears the most
Minor basaltic, basaltic andesite and silicic andesite lavas
likely
source for BAF deposits
in the Mbazo
and
are locally interbedded within medial flank sequences,
Megapode Field fans. These BAF deposits were probably
and are exposed usually only for 100's of metres in the
emplaced soon before and after the 1568 a.d. passage of
upper parts of stream valleys, beneath volcaniclastic
the Mendaiia expedition. Mendaiia described constant ash
successions (Fig. 3). These are mostly well jointed,
cloud production from the centre of Savo, and at least one
aphyric to porphyritic, hornblende- and plagioclase-bear-
white "road" descending from the island's summit to its
ing lavas, with at least one unit containing pyroxene and
northern coast (Amherst and Thomson 1901), matching
olivine. The distribution
of the lavas is poorly con-
the location of the Mbazo and Megapode Field fans.
strained, but single flow unit volumes are estimated to be
These are the oldest of the three generations of volcani-
<
I
x 10
7
rrr'.
clastic fans on the island, based on their overall drainage
density and degree of incision, and appear the most likely
correlative of the oldest toghavitu event. It is also possible
Dscussion
that the Kuila fan and underlying parts of the Ndavalaka
fan also formed during this eruption, correlating with the
Volcanic processes during recent eruptions
legend of how the island grew considerably during the
toghavitu event.
In three of the major volcaniclastic associations, Dm(m)
Dm(m)
units
forming
the upper
portion
of the
lithofacies dominates, and some units contain clasts that
Ndavalaka fan (Fig. 3), with an intermediate
drainage
preserve evidence of hot emplacement
(aligned clast
density and degree of incision, appear to be BAF deposits
TRMINRM). Hot emplacement, coupled with the exclu-
of an eruption sometime between 1630-1670 a.d., based
sively non-vesicular character
and relatively uniform
on the radiocarbon age of buried wood.
composition of the clasts, indicate that small-volume
pyroclastic flows, probably block-and-ash flows (BAFs)
are a dominant eruptive product on Savo. This inference
1830s-1840s a.d. eruptions
is corroborated by historic accounts of glowing clouds
descending the volcano flanks and kastom stories of red-
Dm(m) units from the 1830s-1840s eruptions appear to
hot gaseous flows. Within all sequences, interbedded
be concentrated within two main valleys in different
Dpw(m) facies that are lithologically similar to Dm(m),
sectors of the cone (Rembokola and Poghorovuraghala
are interpreted
as both
lahar/debris
flow and BAF
fans, Fig. 3), although legends suggest that most catch-
deposits. The reports of lahars and the presence of their
ments of the island were affected in some way (Fig. 4b).
deposits indicate that water was plentiful, resulting in syn-
Eyewitness accounts and legends described how, before
eruptive or rapid post-eruptive remobilisation of primary
the eruptions, the crater filled with water, and hydrother-
deposits.
mal systems increased their activity. During the eruptions,
Such dense-rock types of pyroclastic flow are typically
large amounts of water were ejected in addition to dust,
derived from the gravitational collapse and mass-wasting
and rapidly repeated muffled booming noises occurred
of lava domes ("Merapi-style"
BAFs; Neumann van
throughout the climactic phases. Both hot gas and ash
Padang 1933; Boudon et al.I993).
Explosive dome
flows (BAFs) and cool, watery flows (that is debris flows)
destruction through
gas overpressures
("Pelean-style"
were observed. Climactic phases, especially
those ~t
BAFs; Lacroix 1904; Robertson et al. 1998) produces
night, involved incandescent ejecta being thrown vern-
deposits
that contain
some
vesicular
clasts. Dome
cally into the air, followed by glowing
avalanches
destruction may also be followed by scoria-and-ash flows
descending the slopes, and widespread ash falls that
derived from collapse of Plinian or Vulcanian eruption
affected neighbouring islands. No reports refer to persis-
columns ("St Vincent-style"; Anderson and Flett 1903;
tent lava domes or any raising of the central part of the
Miller 1994). In the absence of domes, block-and-ash or
island, as might accompany dome growth.
scoria-and-ash flows typically result from more open-vent
The Rembokola
and Poghorovuraghala
catchme.nts
gas-driven eruptions, or from collapse of dense, short-
coincide with the lowest points in the present crater nm,
177
ca. 50 and ca. 90 m, respectively, above the top of the
Although strongly hydrothermally altered, the Sc and
central crater dome, respectively. According to eyewit-
Pc(c) units that are common throughout PCA sequences
ness reports and legends, a deep crater has existed
are interpreted as near-vent surge deposits, which may
continually on Savo. The present central crater dome has
relate
to
either
of the two
explosion
mechanisms
a pristine morphology and does not resemble the remnants
suggested above. Other surge deposits in MBA and
of a much larger structure. Gravitational dome collapse
FBA were probably generated by segregation of ash-
may not have generated the BAFs during this latest
cloud surges from BAFs (e.g., Fisher 1979; Fujii and
eruption because that process would require a dome to
Nakada 1999).
stand proud of the crater, or be emplaced on its rim or
The presence of Sp, Spw and Ppw(c) units in all facies
outer flanks. The alternative
is that the BAFs were
associations is interpreted to represent significant tephra-
produced by crater-centred blasts or collapse of crater-
fall, which is not typical of Merapi-style eruptions (e.g.,
centred dense eruptive columns, with pyroclasts prefer-
Watanabe et al. 1999), and only significant in Pelean
entially channelled into the lowest points of the crater
eruptions when subsequent Vulcanian or Plinian eruptions
rim.
occur (e.g., Robertson et al. 1998). The dense non-
Based
on
available
data,
the
Rembokola
and
vesicular clasts within Sp, Spw and Ppw(c) appear to
Poghorovuraghala
BAF deposits are the most silicic
reflect the highly crystalline and degassed state of the
erupted from Savo (Table I). They are homogeneous,
erupted magma.
with *90 vol% of pyroclasts being a uniform and highly
The exclusively non-vesicular, crystalline nature of
crystalline hi~h-Si02 andesite to dacite and matrices
juvenile
clasts in the Dm(m), Sp, Spw and ppw(c)
dominated by crystal fragments. These compositions are
lithofacies is unusual for an interpreted explosive origin
consistent with derivation from a degassed dome, or
(cf. Nairn and Self 1978), and implies efficient magma
cryptodome, or other shallow intrusion body. The pres-
degassing at shallow depths before eruptions. The Savo
ence of up to 10% non-juvenile clast lithologies (horn-
andesite and dacite are extremely sodic (Table I), as is the
blende gabbro, basalt, basaltic andesite, hornblendite and
nearby Gallego field on Guadalcanal (Hackman 1980;
hydrothermally
altered
clasts)
in
the Dm(m)
units
Stanton 1994). High Na20 may result in lower magma
suggests that explosions excavated vent and conduit wall
viscosities at super liquidus temperatures (e.g., Mysen et
rock in addition to the essentially solid juvenile magma.
al. 1982) and facilitate gas escape. Similar dense-rock
The 1830s-1840s explosions on Savo could have been
BAFs have been formed by early vent-clearing explo-
driven by the entrapment of crystallisation-derived mag-
sions, for example at Galeras, where only a minor
matic gases beneath a dome, resulting in Pelean-type
juvenile component is present in deposits (Calvache and
blasts (e.g., Boudier et al. 1989; Robertson et al. 1998;
Williams 1992). At Galeras (Stix et al. 1997) pressure rise
Voight and Elsworth 2000). Although such blasts are
beneath solidifying magma in the conduit, coupled with
commonly laterally directed, the trajectory could have
hydrothermal sealing' of the upper edifice, set up condi-
been controlled by the crater rim topography. Pelean-style
tions for explosive evacuation of the upper-vent blockage.
blasts are typically single events that lead to rapid
The 1982 EI Chichon eruption also generated early phase,
decompression
and a phase
during which vesicular
dense-rock, proximal BAFs; in this case, repeated influx
pyroclasts are erupted. Neither of these characteristics
of external water led to phreatomagmatic blasts (Macfas
appear to be the case on Savo. Instead, eyewitnesses
et al. 1997).
reported short-lived repeated explosions, most with a
Regardless of how explosions were generated on Savo,
strong vertical component. That all ejecta appear uni-
at climactic phases, ejection of incandescent pyroclasts
formly highly crystalline may imply shallow-level explo-
formed dense eruption columns that collapsed to form
sions that did not reach underlying, presumably more
BAFs. In some cases, the BAFs were hot enough to form
fluid, magmas.
aligned clast TRM in the deposits and to char incorpo-
Another possible mechanism involves building gas
rated vegetation.
pressures from crystallising
shallow magma, possibly
augmented by interaction with water of the hydrothermal
system, beneath a sealed vent, leading to shallow-level,
Proximal to distal variations
vertical Vulcanian blasts. Repeated sealing of the vent
may have been due to inward collapse of highly altered
PCA sequences contain volumetrically important fall,
and saturated rocks of the hydrothermal system. Steam
surge, BAF and lahar deposits. MBA sequences are the
generated from external water may have also contributed
most uniform, being overwhelmingly dominated by very
togas overpressures. A. factor that may have contributed
thick BAF deposits, with minor fall and surge units. FBA
to the saturation of the upper edifice is rainfall. One of the
sequences are the most diverse, with all of the primary
eyewitnesses reported heavy rainfall during the events;
deposit types present, along with laharic and fluvially
high-intensity rainfalls are common to this area during
reworked deposits.
cyclonic storms, and rainfall is high year-round (3,000-
Many BAFs travelled up to 3 km before entering the
5,000 mm/year; Solomon Islands Meteorological Service
sea. Minimum vertical heightlrunoutdistance
ratios (H/L)
2002).
measured from the crater rim are between 0.09 and 0.16.
These values suggest either unusually high-energy flows
178
(cf. Yamamoto et al. 1993) or that, more likely, the true H
[Pc(c), Sc] and BAF deposits. The FBA areas are
value is considerably greater because many BAFs may
envisaged to have been temporary broad fans of anasto-
have resulted from collapsing, vertical or subvertical
mosing braided channels, while rainfall induced high
eruption columns.
sedimentation rates through syn- and post-eruptive col-
With distance from source, a major reduction in bed
lapse and channelling of Dm(m) and Dpw(m) units, and
thickness (from >40 m in MBA to <2 m in FBA) is the
subsequent localised deposition of debris flow [Dpw(m)]
main change observed in the Dm(m). The thick Dm(m) in . and fluvial sediments [Sc, Pc(c)].
MBA sequences may be accretions from several closely
Surge beds [Sc, Pc(m)] are found within all three main
spaced BAF events. Successive
volcanic
debris-flow
lithofacies associations, although they are best represent-
deposits can accrete with no boundaries preserved be-
ed in PCA. The units within MBA and FBA are thin, and
tween single units (e.g., Cronin et al. 2000a). However,
dominantly fine-grained. Some may be distal equivalents
this process is aided by the saturated nature of debris flows
of the PCA surge deposits and most probably relate to
and post-depositional settling. That unweathered contacts
ash-cloud surges associated with the BAFs (e.g., Fisher
are seen between internally homogeneous units in MBA
1979; Fujii and Nakada 1999). Ash cloud surges are also
sequences suggests each unit represents a single block-
indicated by historical reports of hot gas and dust flows
and-ash flow. The thickness changes record the transition
that were emitted from the main valleys (Toba 1993;
from strongly valley-confined
(MBA)
to unconfined
Cronin et al. 2000b).
(FBA) flow conditions. Evidence of erosion in MBA
Some minor catchments on the lower volcano slopes
sequences suggests that rapidly moving BAFs swept
were not directly affected by BAFs. Instead, reworking of
through these channels, removing thin fine-grained de-
thin marginal BAF deposits, surge deposits and tephra
posits along the flow axis while plastering thick deposits
falls occurred through fluvial and debris-flow processes,
along channel margins. Slowing of the flows and depo-
building up the FFA (Fig. 3) mostly with Pc(c) and Sc .
sition probably accompanied their spread onto FBA fans,
facies.
where their capacity for erosion was greatly reduced.
Based on point counting data at most described
sections, overall changes in Dm(m) texture from the
Volcanic hazards
crater to the coast are slight. Maximum
clast size
decreases between MBA and FBA, but modal clast size
An interpretation of the volcanic hazards for Savo hinges
is similar, as are the volumetric clast content, lithology
on the eruption mechanism determined for the latest
and matrix nature. NRM!fRM
measurements
indicate
events and the resulting BAF generation. During the
that many clasts remained above Curie Point temperatures
toghavitu event, at about ca. 1560-1570 a.d., the Mbazo
for the full 3 km, whereas others either cooled during this
and Megapode Field fans probably formed from BAFs
time or never reached such temperatures. Sedimentary
generated by Merapi-style collapses of the dome located
structures, where present, are limited to weak planar
in the upper Mbazo valley, or possibly a series of domes
fabric, clast strings and both normal and reverse coarse-
that preceded it. However, during the latest eruptions,
tail grading. These
all imply laminar
shear in the
apparently ca. 1830-1840 a.d., the upper deposits of
depositional boundary layer; deposition possibly occurred
Rembokola and Poghorovuraghala
fans were probably
via progressive accretion from the base or lateral margins
emplaced by BAFs generated through the explosive
of a dense flow, such as is typical for debris flows (e.g.,
disruption of crater domes or from collapses of short-
Major 1997). The weak planar fabric is more commonly
lived, dense, vertical eruption columns, and channelled
displayed in BAF deposits in FBA sequences than MBA,
favourably into breaches in the crater rim. Given the
probably because the latter comprise deposits plastered
predominance of highly crystalline, apparently degassed
onto channel margins and, hence, most exposures are
magma erupted in recent events, and the presence of an
subparallel to depositional boundary layers rather than
extensive hydrothermal system on Savo, we would expect
normal to them.
.'
similar eruption processes in future, including:
The similarity in lithology plus flow and depositional
processes between the Savo BAFs and debris flows makes
1. Initial eruption phase: near-vent phreatic blasts, lahars,
distinguishing the origin of the deposits difficult at many
landslides from hydrothermally altered central areas,
outcrops. However, charcoal within BAFs [Dm(m)] is not
phreatomagmatic
blasts and surges, possible dome
as finely disseminated as charcoal within debris-flow
growth.
units, and the former may have uniform clast NRMffRM
2. Climactic phase(s): either dome-centred blasts or vent-
and degassing pipes. In addition, Dm(m) is commonly
centred explosions and dense vertical eruption col-
covered by thin Sp and Spw facies, which appear to
umns, near-vent surges, column-collapse BAFs and
represent tephra falls composed of fines elutriated from
associated ash-cloud surges, ballistic fallout, tephra
the BAF, or co-eruptive ash clouds. Dpw(m) is interpreted
falls and lahars.
as either BAF or debris-flow deposits (depending on other
3. Waning and post-eruptive phase: secondary lahars,
factors at specific sites) and most commonly occurs in
secondary
phreatic
explosions,
near-vent
phreatic
FBA. FBA depositional packages separated by palaeosols
blasts, exogenous dome formation, landslide-induced
commonly
contain
interbedded
debris-flow,
fluvial
debris flows.
179
Of these processes, we consider the BAFs to pose the
greatest risk to life, because they impact on the coastal
areas and the lower slopes where the population is
concentrated, and may also generate laterally spreading
ash-cloud surges. Risk due to lahars may also be high,
given their rapid onset, flow and propensity to spread
widely once on coastal fans. Large surges could affect the
whole island, but eyewitness reports and legends mostly
describe flows and gas clouds being derived from the
main channels. Ballistic fallout during climactic phases of
eruptions could affect large portions
of the island.
According to legends, falling blocks apparently killed
people. Presently no inhabitants are located within 1.5 km
of the crater, and most are >2 km distant.
A simple three-zone map (Fig. IIA) shows areas of
relative susceptibility to ground-hugging hazards includ-
ing surges, BAFs and lahars. The highest hazard zone (A)
is centred on the crater, main channels and FBA fans
occupied during the most recent eruptions. This also takes
the present crater rim geometry into account.
The intermediate hazard zone (B) encompasses catch-
ments less likely to be affected by ground-hugging flows,
as indicated by past deposition and catchment morphol-
ogy, plus interfluve areas on the middle slopes. Interfluve
areas near the main catchments are still likely to be
subject to ash-cloud surges associated with BAFs (e.g.,
Fisher 1995). In addition, all parts of zone B may be
affected by ballistic fallout, ash falls, large vent-derived
surges and rain-triggered lahars.
The lowest hazard zone (C) comprises areas that are
well protected by topography from direct line-of-sight of
the crater. These small coastal areas are in the lee of the
large dome structures, particularly in the south-western
sector of Savo. Depositional evidence along with histor-
ical and kastom knowledge (Cronin et al. 2000b) suggests
these are the safest refuge areas on the island during
eruptions, at least temporarily. These areas may still be
subject to significant ballistic and tephra-fall hazards
during climactic eruptive phases, and access to many of
them may involve crossing through parts of zone A.
Latter (1991) prepared a first -order assessment of
tephra-fall hazards from Savo, giving predicted ash falls
for eruption volumes of I krrr' at two differing times of
year, when wind conditions vary. However, Latter (1991)
could find no evidence
of tephra
deposits on the
neighbouring Russell and Florida Islands generated by
large eruptions at Savo. The historical and depositional
evidence we have gathered suggests that Plinian or sub-
Plinian eruptions are unlikely; instead, Vulcanian-type
eruptions may generate the greatest tephra falls on Savo.
Hence, for hazard assessment, we have used the limits of
a lower-volume scenario (0.1 km
3)·
proposed by Latter
(1991), along with the local reports of ash fall on
Guadalcanal in 1830-1840 a .d., to estimate the potential
area affected by ashfall from climactic phases of activity
(Fig. lIB).
Warning signals of past eruptions have included an
increase in the surface expression and vigour of hy-
drothermal activity, die-off of vegetation around the
Fig.
II
Simple volcanic hazard assessment maps for Savo. A Flow
hazards on Savo, particularly BAFs, ash-cloud surges and lahars,
divided into three zones. B Tephra fall hazards estimated from a
0.1 krrr' eruption of Savo at two different times of the year, adapted
from Latter (1991). Inset: wind rose diagram .of surface wind
directions from Hydrographer of the Navy (1988)
crater area, filling of the crater depression with water and
pre-event earthquakes. During the last eruption, historical
records and kastom stories tell that the warning signs were
recognised early enough to allow some pre-event evac-
uation(Cronin
et al. 2000b); however, as described
above, during sudden onsets of climactic activity, mass
evacuations were chaotic (Grover 1958). Hence, in case
of future signs of reawakening, pre-evacuation of villages
in hazard zone A (Fig. llA) should be considered a high
priority.
From presently available data, we suggest that the last
three major eruptions occurred in ca. 1560-1570 a.d. (as
seen by Mendafia and probably recorded in the toghavitu
legend), sometime between 1630-1670 a.d. (the radio-
180
carbon-dated
Ndavalaka
fan event) and 1830-1840
a.d.
confined
to the few coastal
areas protected
from direct
(as recorded
by the first Europeans
in the area). These
sight of the crater by dome-cored
hills. The map has been
dates imply average
quiescent
intervals
in the order of
developed
in consultation
with local inhabitants
(Cronin
100-200
years.
Such
intervals
correspond
with
local
et
al. 2000b)
and
is in broad
agreement
with
their
traditional
views,
although
other
smaller
events
are
perceptions
of potential
refuge
areas. The simple
map,
known to have occurred
in between
and since, such as
along
with
the
revival
of relevant
traditional
kastom
the lahars in 1953. Further stratigraphic
and radiocarbon
stories,
can
be
used
to improve
local
awareness
of
studies are required to establish a longer history of events.
volcanic
hazards
on Savoand
encourage
preparation
of
risk
management
strategies.
It is also
being
used
by
government
in planning
development
of infrastructure
in
Conclusions
the wider Savo region.
The earliest
of the latest
three
major
Savo
eruptions,
Acknowledgeme~t The au~ors thanl.' .t~e ~ople
~f Sav~ and
sometime
before
1568 a.d.
involved
the formation
of a
officers ~t the Mines and Minerals DlvlSl~n In HOniara, Without
.'
.
whom this paper would not have been possible, M.G.P. thanks the
lav~ dome In the upper Mbazo catchment.
The lava dome,
UK Department for International Development (UKDFID, formerly
which
also generated
block-and-ash
flows,
formed
the
UKODA) and the British Geological Survey for their assistance and
large Mbazo and Megapode
Field BAF deposit
fans in
support. S.J.C. thanks the NZ Foundation for Research, Science and
northern
Savo.
Two
later
eruptions
(1630-1670
and
Technology and the Alexander von Humboldt Foundation (Ger-
1830-1840
d)
I
d
"1
BAF d
.
f
many). The authors also thank the Solomon Islands National
a. " a so pro. uced simi. ar
eposit
ans,
Disaster Council, UNDHA in Suva, the Australian IDNDR
but the latter apparently
involved
different
processes.
Committee, and SOPAC for their encouragement and support.
During
the 1830s-1840s
a.d.
eruptions,
a persistent
Drs G. Burr and D. Rex are thanked for their C
l4
and K-Ar
crater-hosted
vent,
the absence
of large
domes
above
de~rminations, respectively. Michel Lardy of O~STO~
and
crater rim level
and eyewitness
reports
of incandescent
MI~hel Larue .formerly of SOPAC are thanked for their asslsta~ce.
..'
.
..
This manuscnpt benefited greatly from comments by Ian Nairn,
vertical Jets and subsequent
glowing
avalanches
indicate
Russell Blong and Jocelyn McPhie.
BAF generation
was via either
dome-derived
blasts or
shallow explosions
from a repeatedly
sealed vent system.
!he
~ghly
crystalline
and
non-vesic.ular
state
of the
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