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Fig. 7.20 Imbricated boulder
train at the eastern end of
Dunraven Beach, south Wales
Searle 1991 ). All three of these islands are bound by near-
vertical cliffs 150-500 m high. In places, these cliffs form
amphitheaters that appear to be the headwalls of former
landslides. The largest of these occurs on the northwest side
of Tristan da Cunha itself and is associated with a debris
avalanche more than 100 m thick, covering an area of about
1,200 km 2
mechanisms reducing high volcanic islands to guyots.
Mapping of submerged guyots on the Hawaiian Ridge
shows the same density of landslides as present around the
main islands. There are approximately a thousand sea-
mounts higher than 1,000 m in the Pacific Ocean (Holcomb
and Searle 1991 ). Over 300 of these seamounts lie along the
Mariana Island arc in the west Pacific (Fig. 7.2 ). Many
seamounts show extensive turbidites on their flanks and
have the potential to generate landslides 20-50 km 3 in size.
The perceived view of an uneroded circular or elliptical
atoll is also illusionary. For example, on Johnston Atoll in
the Line Islands south of Hawaii, one or more major land-
slides have removed much of the southern margin. Blocks
of carbonate up to a kilometer in size have been detected on
the adjacent seabed, which in places has been infilled to a
depth of 1,500 m. Ninety-five percent of atolls are in fact
polygonal in shape, with deep embayments cut into at least
one seaward flank (Stoddart 1965 ). If the aprons around
these features are signs of past landslides, then such
deposits cover 10 % of the ocean. Whenever there has been
a failure, there has been the potential for a tsunami. Whelan
and Kelletat ( 2003 ) estimate that over the past 2 million
years that there have been at least 100 mega-tsunami gen-
erated by landslides off volcanoes.
and having an estimated volume of 150 km 3 .
The
slide
has
been
tentatively
dated
as
younger
than
100,000 years.
If amphitheater forms in cliffs or a stellate-shape island
are the signatures of former landslides (Whelan and Kelletat
2003 ), then this process could have removed between 10
and 50 % of the exposed portion of most volcanic islands.
For example, in the Pacific Ocean such features appear on
American and Western Samoa, Tahiti, the Society chain,
and the Marquesas Group (Keating 1998 ; Holcomb and
Searle 1991 ) (Fig. 7.2 ). On these islands, the highest sea
cliffs do not face the prevailing winds or swell but are
protected from marine erosion by reefs. On the island of
Tutuila in the Samoan Islands and on the islands of the
Manua Group up to half of the volcanic complex is missing.
On the Samoan islands, SeaMARC II side-scan sonar
reveals profuse slump blocks, chaotic slumps, landslide
sheet flows, turbidites, and avalanche debris flows. Land-
slides have also removed large portions of the upper parts of
Guam in the Mariana Islands and of Rarotonga, Mangaia,
and Aiutaki in the Cook Islands. As well, the western half of
Volcán Ecuador in the Galápagos Islands is missing. In the
Atlantic Ocean, the Cape Verde group and the Island of St.
Helena also have marked sea cliffs, while in the Caribbean
Sea large headwall scars are evident on the westward sides
of Dominca, St. Lucia, and St. Vincent Islands in the Lesser
Antilles. The Azores Islands are also faceted with amphi-
theater scars. In the Indian Ocean, large landslides can be
inferred from Gough, Marion, Prince Edward, Amsterdam,
St. Paul, Bouvetoya, Possession, and Peter I Islands.
Nor do volcanoes have to emerge above sea level to have
undergone failure. The seas are pockmarked by numerous
atolls, guyots (eroded volcanic islands), and seamounts
evincing amphitheater and stellate forms similar to the
above (Fairbridge 1950 ). Mass wasting is one of the major
7.7.2
Other Topography
Other topography besides volcanoes can also produce sub-
marine landslides. These include river deltas, passive con-
tinental margins, submarine canyons, deep-sea fans, the
walls of deep trenches near subduction zones, and the slopes
of mid-ocean ridges (Moore 1978 ). Some of the sites where
slides have been identified as originating from these types
of topography are mapped in Fig. 7.2 . Major river deltas are
prone to landslides because of the volume of sediment
continually being built up on their submerged distal ends.
The rivers with the largest sediment loads—the Amazon,
Mississippi, Nile, and Indus—have built up relatively steep
fans more than 10 km thick (Masson 1996 ; Masson et al.
1996 ). The Amazon Fan consists of several major slide
 
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