Volcanic Eruptions - Tsunami - page 160

pumice, after it was deposited, would easily have mixed
with rainwater and flowed into any stream or river, coloring
it red. The 3 days of darkness possibly refer to tephra clouds
blowing south across Egypt at the beginning of the eruption.
The darkness was described as a ''darkness, which could be
felt''. Egyptian documents around 1470 BC refer to a time
of prolonged darkness and noise, to a period of 9 days that
''were in violence and tempest: none…could see the face of
his fellow'', and to the destruction of towns and wasting of
Upper Egypt. There is also direct reference to the collapse
of trade with Crete (Keftiu). Volcanic shards have been
found in soils on the Nile delta with the same chemical
composition as tephra on the Santorini Islands. The parting
of the Red Sea most likely occurred in the marshes at the
northern end of the sea. The Bible attributes the parting to
wind (Exodus 14:21). The wind may refer to the atmo-
spheric pressure wave produced by the explosion of the
volcano. Such waves, akin to that generated by Krakatau in
1883, can generate seiching or tsunami in enclosed basins or
distant oceans.
The eruption around 1470 BC had four distinct phases
(Pichler and Friedrich 1980 ; Cita et al. 1996 ). The first was
a Plinian phase with massive pumice falls. This was fol-
lowed by a series of basal surges producing profuse quan-
tities of pumice up to 30 m thick on Santorini. The third
phase was associated with the collapse of the caldera and
production of pyroclastic flows. About 4.5 km 3 of dense
magma was ejected from the volcano, producing 10 km 3 of
ash. The volume of ejecta is similar in magnitude to that
produced by the Krakatau eruption in 1883. The ash drifted
to the east-southeast, but did not exceed 5 mm thickness in
deposits on any of the adjacent islands, including Crete. The
largest thickness of ash measured in marine cores appears to
originate from pumice that floated into the eastern Medi-
terranean. It is possible at this stage that ocean water made
contact with the magma chamber and produced large
explosions, which generated tsunami in the same way that
the eruption of Krakatau did. The final phase of the eruption
was associated with the collapse of the caldera in its
southwest corner. The volcano sunk over an area of 83 km 2
and to a depth of between 600 and 800 m. According to the
Krakatau model, this final event produced the largest tsu-
nami, directing most of its energy westwards (Fig. 8.5 ). It is
estimated that the original height of the tsunami was
46-68 m in height, and maybe as high as 90 m. The average
period between the dozen or more peaks in the wave train
was 15 min.
Evidence of the tsunami is found in deposits close to
Santorini. On the island of Anapi to the east, sea-borne
pumice was deposited to an altitude of 40-50 m above
present sea level (Yokoyama 1978 ). Considering that sea
levels at the time of the eruption may have been 10 m
lower, this represents run-up heights greater than those

produced by Krakatau in the Sunda Strait. On the Island of
Crete, the wave arrived within 30 min with a height of
approximately 11 m (Johnstone 1997 ). Refraction focused
wave energy on the northeast corner of Crete, where run-up
heights reached 40 m above sea level. In the region of
Knossos, the tsunami swept across a 3 km wide coastal
plain reaching the mountains behind. Massive dump
deposits containing imbricated cobbles were emplaced
along this coast (Bruins et al. 2008 ). The backwash con-
centrated in valleys and watercourses, and was highly ero-
sive. Evidence for the tsunami is also found in the eastern
Mediterranean on the western side of Cyprus, and further
away at Jaffa-Tel Aviv in Israel (Yokoyama 1978 ; Myles
1985 ; Cita et al. 1996 ). At the latter location, pumice has
been found on a terrace lying 7 m above sea level at the
time of the eruption. However, the tsunami wave here had
already undergone substantial defocussing because of wave
refraction as it passed between the islands of Crete and
Rhodes. The greatest tsunami wave heights occurred west
of Santorini. Based upon linear wave theory, the wave in the
central Mediterranean Sea was 17 m high (Kastens and Cita
1981 ). Closer to Italy over the submarine Calabrian Ridge,
it was 7 m high. Bottom current velocities under the wave
crest in these regions ranged between 20 and 50 cm s -1 —
great enough to entrain clay-to-gravel sized particles. The
maximum pressure pulse produced on the seabed by the
passage of the wave ranged between 350 and 850 kdy-
ne cm -2 . Spontaneous liquefaction and flow of water-sat-
urated muds is known to occur under pressure pulses of
280 kdyne cm -2 and greater.
Some of the evidence for a large tsunami comes from the
discovery of unusual deposits on the seabed of the central
Mediterranean Sea, where wave heights were highest. These
deposits—labeled homogenites—formed in the deep sea as
the result of settling from suspension of densely concen-
trated, fine-grained sediment (Kastens and Cita 1981 ). This
process produced homogeneous units up to 25 m thick with
a sharp basal contact. Homogenites can be linked hydro-
dynamically to the passage of a tsunami wave. As sediment
fails via liquefaction due to the pressure pulse, oscillatory
flow under the wave suspends finer particles, creating tur-
bulent clouds of sediment. It is estimated that the slurries
exceeded concentrations of 16,000 mg L -1 . In comparison,
the highest measured sediment concentrations on the ocean
seabed and in muddy tidal estuaries rarely exceed 12 and
300 mg L -1 respectively. Gravity sorting occurred under
this extreme concentration. Sand-sized particles settled first
to the bottom and were deposited at the erosional contact
with the seabed as a fining upward unit, whose thickness
ranged from a few centimeters to several meters. Finer clay-
sized sediment was deposited over the next few days as a
massive undifferentiated clay deposit that was up to 20 m or
more thick. Homogenites differ from turbidites described in