Great Landslides - Tsunami - page 137

above sea level. Because all islands are rising, the elevation
of run-up would have been lower than these values at the
time of the tsunami.
This stripping removed spherically weathered basaltic
boulders up to 0.5 m in diameter from the soil and swept
then downslope. These boulders were then re-entrained by
subsequent waves, together with coral gravel, and deposited
in three inversely graded beds up to 4 m thick. The size of
boulders and thickness of the deposit decreases upslope to a
150 m elevation. The larger particles support each other and
are imbricated upslope—facts indicative of transport in
suspension or as bedload. The voids between the large clasts
are infilled with silt and pebbles that include marine
foraminifera and sea-urchin spines. Fine calcareous material
can be found filling crevices to an altitude of 326 m on
Lanai. Mollusk shells are scattered throughout the deposit
with species derived from 20 to 80 m depth of water
characteristic of a slightly warmer environment than exists
at present. The surface of the deposit consists of a branching
network of ridges with a relief of 1 m and a spacing of
10 m. The ridges are asymmetric in profile with the steepest
end facing downslope—a fact suggesting that backwash
was the last process to mould the surface.
The hydrodynamics of the flow can be determined from
this internal fabric and morphology. The inverse grading is
suggestive of a thin, fast moving sheet of water typical of
wave run-up or backwash. With a dune spacing of 10 m and
a maximum boulder size of 1.5 m, the flow of water was
1.6 m deep—Eq. 3.3 and obtained a minimum velocity of
6.3 m s -1 —Eq. 3.1 . These conditions are favorable for
deposition of inversely graded deposits under run-up or
backwash. The boulder-sized material and high elevation of
deposition and stripping indicate that the flow velocity and
run-up limits were an order of magnitude greater than that
produced by storm waves or by tsunami generated by a
distant tectonic event. The latter have a recorded maximum
run-up height of no more than 17 m on these islands. The
presence of multiple beds indicates a wave train that
included three or four catastrophic waves, with the second
wave being most energetic and reaching a maximum ele-
vation of 365 m above sea level. If run-up height is set at
ten times the tsunami wave height approaching shore, then
the tsunami wave was 19-32 m high when it reached the
coast. This is a conservative estimate, given the steep slopes
of the islands and the fact that the swash was sedi-
ment-laden. Only waves generated by submarine landslides
or asteroid impacts with the ocean are this big. The first
wave in the tsunami wave train picked up material from the
ocean, washed over and dissected any offshore reefs, and
carried all this material in suspension, at high velocity, up
the slopes of the islands. The first wave was so powerful on
Molokai that it cracked the underlying bedrock to a depth of
10 m

subsequently infilled with fluidized sediment. The first wave
also picked up boulders from the weathered surface as it
swept upslope. These were then mixed with the marine
debris in the backwash and laid down by subsequent waves
into graded beds. The last backwash molded the surface of
the deposits into dune bedforms.
The asymmetry in the elevation of maximum stripping
around Lanai and Kahoolawe suggests that the source of the
tsunami had to be from the southeast. Uranium-thorium
dates from the coral on Lanai show that the event occurred
during the Last Interglacial around 105,000 years ago
(Moore and Moore 1988 ). However, the deposit on Molokai
represents an older event that occurred at the peak of the
Penultimate Interglacial 200,000-240,000 years ago
(Moore et al. 1994a ). The most likely source of the tsunami
was one or more of the Alika submarine debris avalanches
off the west coast of the main island of Hawaii (Moore and
Moore 1988 ). Slides to the southwest of Lanai cannot be
ruled out, but they are older than the age of the deposits. If
this latter source caused the tsunami, then the wave must
have been generated by water backfilling the depression left
in the ocean. This mechanism is similar to that proposed for
the tsunami that swept the Burin Peninsula on November
18, 1929. The tsunami could also have been generated by an
asteroid impact in the Pacific Ocean off the southeast coast
of Hawaii. Asteroids as agents of tsunami generation will be
discussed in Chap. 9 . Unfortunately, the Hulopoe Gravel on
Lanai has been questioned as a tsunami deposit (Keating
and Helsley 2002 ). Some locations covered in Hulopoe
Gravel also contain soil profiles between layers, and it is
difficult to envisage deposition up gullies that should have
then been scoured by backwash. Finally, the imprint of
human activity cannot be ignored in the deposition of either
some of the boulder piles or molluscs on Lanai.

7.5.2 The Canary Islands

The Canary Islands have formed over the last 20 million
years as shield volcanoes that rise from depths of
3,000-4,000 m in the ocean to heights of 1,000 m above sea
level (Carracedo et al. 1998 ). The islands are similar to the
Hawaiian Islands in that their origin has been linked to hot-
spot activity. Because of structural control and scarring by at
least 19 landslides, the islands have taken on Mercedes star
outlines (Fig. 7.9 ) (Carracedo et al. 1999 ). The scars consist
of large amphitheaters or depressions that are backed by
high cliffs. The most prominent of these lies on the island of
El Hierro where landslides have left a Matterhorn-like peak.
As well, Tenerife has been shaped by multiple landslides
(Masson et al. 2006 ). Giant landslides occur more frequently
on the younger islands of Tenerife, La Palma and El Hierro
to the west that are less than 2 million years old. On average, (Moore et al. 1994a ). These crevices were then