Geochemistry, Geophysics, Geosystems
Supporting Information for
New insights into landslide processes around volcanic islands from Remotely
Operated Vehicle (ROV) observations offshore Montserrat
Watt, S.F.L.1, Jutzeler, M.2, Talling, P.J.2, Carey, S.N.3, Sparks, R.S.J.4, Tucker, M.4,
Stinton, A.J.5, Fisher, J.K.6, Wall-Palmer, D.6, Hühnerbach, V.2, Moreton, S.G.7
1
School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham B15 2TT,
UK, 2 National Oceanography Center, Southampton SO14 3ZH, UK, 3 Graduate School of Oceanography,
University of Rhode Island, RI 02881, USA, 4 School of Earth Sciences, University of Bristol, Bristol BS8
1RJ, UK, 5 Montserrat Volcano Observatory, Flemmings, Montserrat, West Indies, 6 School of Geography,
Earth and Environmental Sciences, Plymouth University, Plymouth PL4 8AA, UK, 7 NERC Radiocarbon
Facility (East Kilbride), Scottish Enterprise Technology Park, East Kilbride G75 0QF, UK
Contents of this file
Table S1
Movie S1 to S10
Introduction
This supporting information contains a single table that provides descriptions and locality
information for all samples discussed in the text, and captions to ten video clips (Movie
S1 to S10) provided as separate .mp4 files.
Table S1. Sample location and description for selected samples collected during NA037.
Sample
Location
description
Dive
Lat.
(N)
Long.
(W)
Depth
(m)
NA037001
SW corner
of Wembley
block
H1308
16.718
716
62.054
955
970
H1308
16.718
702
62.055
097
975
H1308
16.719
041
62.054
653
942
NA037002
NA037005
SW corner
of Wembley
block
SW corner
of Wembley
block
Sample description
Dense grey andesite, sparsely vesicular. Phenocryst
phases dominated by coarse plagioclase and equant
clinopyroxene with less frequent orthopyroxene in an
altered groundmass.
Highly indurated coralgal limestone consisting of a
mixture of cm-sized rhodoliths, benthic forams
(notably Amphistegina and peneroplids) and other
bioclasts (including gastropods, bivalves, echinoids
and calcareous red algal fragments) within a matrix of
micrite. Microbialite-micritic filaments and areas of
peloids occur, representing probably in-situ bacterial
precipitate. Numerous cavities of probable spongeboring origin are filled by calcite cement and some
bioclasts have textures indicating replacement of
original aragonite by neomorphic calcite. The
characteristics of this clast suggests formation in a
shallow-moderate depth environment, while the
replacement of aragonite by calcite suggests
diagenesis in its current deep water environment.
A weakly indurated micritic limestone with planktonic
forams (Globorotalia, Orbicula), pteropods, planktonic
gastropods, with minor fragments of shallow-water
1
NA037011
Hummock
near margin
of Deposit 1
W end of
hummock at
margin of
Deposit 1
NA037025
Near SW
corner of
Deposit 5
H1309
16.657
829
62.285
106
806
NA037026
On SW
margin of
Deposit 5
H1309
16.658
019
62.289
854
823
H1310
16.610
819
62.186
474
836
bioclasts (bivalve, foram, echinoid), and silt-sized
volcanic crystals set in a micrite matrix with
conspicuous (mm-sized) burrow fills. The exterior has
some tubeworm clasts and small coral fragments. The
mixture of shallow and deeper water fauna with minor
volcanic fragments and a fine matrix suggests a mass
transport origin.
Dark grey dense andesite, phenocryst phase
dominated by tabular plagioclase, greenish
orthopyroxene common, and hornblende present by
almost entirely replaced by a mixed alteration
assemblage (chlorite, smectite, oxides). Some smaller
hornblende grains better preserved, with thin oxide
rims.
Crumbly, reddish orange, unconsolidated
hydrothermally altered clay minerals, replacing
primary phenocrysts and groundmass, with cm-sized
altered clasts
Fresh grey andesite, phenocryst phase includes large,
complex zoned plagioclase and hornblende replaced
by a black alteration assemblage, with less frequent
brown orthopyroxene.
A well-sorted, cemented grainstone with medium to
coarse sand grade. Predominantly bioclastic with
shallow-water forams (penerolids), calcareous algae
(branched forms), green algae (Halimeda), and minor
bivalve fragments and volcanic grains. Thin (20 to 50
μm) isopachous bladed calcite coats the grains to
provide the cement, but the rock has high porosity.
There are local areas of peloidal sediment, likely the
result of bacterial precipitation.
A dense limestone of encrusted volcanic lithics and
bioclasts, including benthic and planktonic forams,
calcareous red algae, mollusc fragments, serpulids,
sponge spicules, radiolaria, echinoid spines and
pteropods, cemented by micritic-microsparitic-sparry
calcite cement. The encrusted volcanic grains
(comparable to oncoids or rhodoliths) are inferred to
have formed by rolling in intermittent currents, and are
suggestive of shallow to moderate water
environments, consistent with the fossil assemblage.
Encrusting forams on red algal crust occur with
microbial filaments. Gastropod shells have had the
original aragonite replaced by coarse calcite, while
sponge spicules have also been replaced by calcite,
consistent with transport from a shallow- to a deepwater environment, and deep-water diagenesis.
Phosphate grains of probable microbial origin occur
within cavities (sponge borings) in calcareous algae.
Grey crystal-rich andesite with 2-cm alteration rind;
fresher internal samples have phenocryst phase
dominated by clinopyroxerne, less frequent
orthopyroxene, abundant plagioclase and rare
hornblende, as small crystals with thick oxide rims.
H1310
16.611
076
62.186
930
820
Green-grey fresh andesite, very similar to samples
037, with a crystalline, fresh groundmass.
NA037008
NA037009
NA037037
NA037042
E end of
hummock at
margin of
Deposit 1
E side of
hummock in
centre of
Deposit 3
Centre of
hummock
(same as
sample 037)
in Deposit 3
H1308
16.719
263
62.060
909
961
H1308
16.719
37347
62.062
086
954
H1308
16.719
479
62.063
212
987
Video files (.mp4): Movie S1 to S10
Selected short video clips highlighting features in Deposits 1 and 2 and the surrounding
seafloor that are discussed in the text. In footage from I217, the two red laser points
mark a distance of 10 cm.
2
Movie S1 (S1_mont_dep1.mp4) Hydrothermally altered lithologies exposed at the
surface of a hummock in Deposit 1, showing irregular domain boundaries within a single
debris-avalanche hummock (including still image in Figure 2c; location in Figures 1 and
2a; footage from Dive H1308). Red and yellow altered material are in direct contact with
unaltered grey breccias. A drape of gray volcaniclastic sand obscures some surfaces,
and is derived from offshore transport of sediment during the recent eruption of Soufrière
Hills volcano (principally from dome-collapse generated pyroclastic density currents).
Movie S2 (S2_mont_dep1drape1.mp4) Post-emplacement sedimentary drape exposed
on a hummock in Deposit 1 (footage from Dive H1308, eastern margin of Deposit 1;
Figure 1). The exposure comprises a poorly sorted mixture of hemipelagic mud and
outsized volcanic lithic clasts, with crude bedding. The lithic clasts may be locally derived
by mass-wasting of hummocks on the irregular surface of Deposit 1. The basal part of
the hemipelagite sequence is relatively devoid of large clasts, whereas the upper part
has lenses of abundant lithic clasts that pinch and swell along the outcrop. The
sequence is conformable with the underlying debris-avalanche deposit and forms a
mantle over the irregular surface.
Movie S3 (S3_mont_dep1drape2.mp4) Post-emplacement sedimentary drape exposed
on a hummock in the northern part of Deposit 1 (footage from Dive I219; Figure 1). The
basal unit is a gray, clast-supported volcaniclastic breccias, overlain by a poorly-sorted
unit of angular volcanic clasts embedded in fine hemipelagic sediment.
Movie S4 (S4_mont_SWWembley1.mp4) Exposure of the surface of the Wembley
block on its SW side (Dive H1308; Figure 1), showing fresh monomict lava breccias and
massive lava units. Clasts are of dense gray andesite. Gray volcaniclastic sand coats
many surfaces and is derived from the recent eruption of Soufrière Hills volcano.
Movie S5 (S5_mont_SWWembley_talus.mp4) Exposure of post-depositional clastic
drape at the base of the Wembley block on its SW side (from Dive H1308, including still
image in Figure 4d; location in Figures 1 and 2a). The basal unit is a monomict clastsupported breccias that represents the primary block surface. Unconformably overlying
this, at a high angle, is a matrix supported breccias of volcanic lithic clasts within a
hemipelagite matrix, displaying crude bedding. This is interpreted as material derived
from mass-wasting of the steep walls of the Wembley block.
Movie S6 (S6_mont_SWWembley_talus2.mp4) Exposure from the same areas as
Video S5 (Dive H1308), showing the side of the mixed carbonate-lithic drape overlying
the underlying clast-supported volcanic breccias at a high angle.
Movie S7 (S7_mont_SEWembley1.mp4) Exposure on the SE side of the Wembley
block, showing the block surface (Dive I217; Figure 1). Angular volcanic clasts are
encased in a white mud, interpreted as clayey carbonate (hemipelagic) sediment, infilling
around the clasts on all faces. A close-up image of the white material shows its typical
sculpted erosion pattern, suggesting a fine-grained, cohesive material.
Movie S8 (S8_mont_SEWembley2.mp4) A further exposure on the SE side of the
Wembley block (lower half of block, Dive I217, 5–7 m from the base of the exposure; see
Figure 5), showing volcanic brecciated clasts with a matrix of hemipelagic mud encasing
the clasts on some surfaces. The volcanic block shows jigsaw-type fracturing of a single
massive clast.
3
Movie S9 (S9_mont_SEWembley_drape.mp4) Footage moving up the top 4 metres of
post-emplacement sedimentary drape on top of the Wembley block, exposed on the SE
side of the block (Dive I217, 46–50 m; Figure 5). Flay-lying, conformably bedded
hemipelagic units crop out, with sequences in between obscured by reworked gray
volcaniclastic sand. The sequence is similar in appearance to that exposed in the
margins of seafloor depressions (Video S10) and sampled in short cores during JR123
(see text). The final part of the video shows the typical texture of the hemipelagic beds,
comparable to the material exposed on the faces of the Wembley block (e.g. Videos S7
and S8).
Movie S10 (S10_mont_seafloor_depression.mp4) A scarp in the seafloor forming the
circular wall of a depression, east of the Wembley block (Dive I217, Figure 2, from the
same area as Figure 11c). The scarp exposes the typical seafloor sequence of
hemipelagic mud interbedded with sandy volcaniclastic turbidites, as has been widely
sampled by the JR123 sediment cores [e.g. Trofimovs et al., 2013], and is similar to the
post-emplacement sequence on top of the Wembley block. The hemipelagic units do not
contain the coarse volcanic clasts than the sedimentary drape on top of the Deposit 1
hummocks (Video S1 and S2).
4