Volcanic Eruptions - Tsunami - page 158

consistently spaced waves arrived over a period of 36 h.
Between 5,000 and 6,000 boats in the strait were sunk. In
total, 36,417 people died in major towns and 300 villages
were destroyed because of the tsunami.
Within 4 h of the final eruption, a 4 m high tsunami
arrived at North West Cape, West Australia 2,100 km away
(Bryant and Nott 2001 ). The wave swept through gaps in
the Ningaloo Reef and penetrated 1 km inland over sand
dunes. 9 h after the blast, 300 riverboats were swamped and
sunk at Kolkata (formerly Calcutta) on the Ganges River
3,800 km away (Myles 1985 ). The wave was measured
around the Indian Ocean at Aden on the tip of the Arabian
Peninsula, Sri Lanka, Mahe in the Seychelle Islands, on the
Island of Mauritius, and at Port Elizabeth, South Africa
8,300 km away. Tsunami waves were measured over the
next 37 h on tide gauges in the English Channel, in the
Pacific Ocean, and in Lake Taupo in the center of the North
Island of New Zealand, where a 0.5 m oscillation in lake
level was observed (Choi et al. 2006 ). Around the Pacific
Ocean, tide gauges in Australia and Japan and at San
Francisco and Kodiak Island measured changes of 0.1 m up
to 20 h after the eruption (Pararas-Carayannis 1997 ).
Honolulu recorded higher oscillations of 0.24 m with a
periodicity of 30 min. Smaller, subsequent eruptions of
Krakatau generated lesser tsunami throughout the strait
until October 10th. The last tsunami was observed in
Welcome Bay, where it surged 75 m inland beyond the
high-tide mark.
The tsunami in the Pacific has been attributed to the
atmospheric pressure wave because many islands that
would have effectively dissipated long-wave energy
obscured the passage from the Sunda Strait into this ocean.
The atmospheric pressure wave also accounts for seiching
that occurred in Lake Taupo, which is not connected to the
ocean (Choi et al. 2003 ). Finally, it explains the long waves
observed along the coast of France and England when the
main tsunami had effectively dissipated its energy in the
Indian Ocean. The generation of tsunami in Sunda Strait
and the Indian Ocean has been attributed to four causes:
lateral blast, collapse of the caldera that formed on the north
side of Krakatau Island, pyroclastic flows, and a submarine
explosion (Nomanbhoy and Satake 1995 ). Lateral blasting
may have occurred to a small degree on Krakatau during the
third explosion; however, its effect on tsunami generation is
not known. During the third explosion, Krakatau collapsed
in on itself, forming a caldera about 270 m deep and with a
volume of 11.5 km 3 . However, modeling indicates that this
mechanism underestimates tsunami wave heights by a fac-
tor of three within Sunda Strait. Krakatau generated massive
pyroclastic flows (Self and Rampino 1981 ). These flows
probably generated the tsunami that preceded the final
explosion. At the time of the third eruption, ash was ejected
into the atmosphere towards the northeast. Theoretically, a

pyroclastic flow in this direction could have generated
tsunami up to 10 m in size throughout the strait; however,
the mechanism does not account for measured tsunami run-
ups of more than 15 m in height in the northern part of
Sunda Strait. The pyroclastic flow now appears to have sunk
to the bottom of the ocean and travelled 10-15 km along the
seabed before depositing two large islands of ash. The 40 m
high run-up measured near Merak to the northeast supports
this hypothesis. The tsunami's wave height corresponds
with the depth of water around Krakatau in this direction.
As well, the third explosion of Krakatau at 9:58 AM more
than likely produced a submarine explosion as ocean water
encountered the magma chamber. Van Guest's description
of the eruption—presented in Chap. 1 —indicates that the
magma chamber was visible in the strait before the third
explosion at 10 o'clock in the morning. A submarine
explosion could have generated tsunami 15 m high
throughout the Strait. If the explosion had a lateral com-
ponent northwards, as indicated by the final configuration of
Krakatau Island, then this blast, in conjunction with the
pyroclastic flow, would account for the increase in tsunami
wave heights towards the northern entrance of Sunda Strait
(Fig. 8.3 ).

8.4 Santorini, Around 1470 BC

The prehistoric eruption of Santorini around 1470 BC, off
the island of Thera in the southern Aegean Sea north of
Crete (Fig. 8.5 ), is probably the biggest volcanic explosion
witnessed by humans (LaMoreaux 1995 ; Menzies 2012 ). It
is also one of the most controversial because legend, myth,
and archaeological fact frequently are intertwined and dis-
torted in the interpretation of the sequence of events. The
eruption has been linked to the lost city of Atlantis descri-
bed by Plato in his Critias, to the destruction of Minoan
civilization on the island of Crete 120 km to the south
(Pararas-Carayannis 1998 ), and to the exodus of the Isra-
elites from Egypt in the Bible (Bryant 2005 ). Certainly,
Greek flood myths refer to this or similar events that gen-
erated tsunami in the Aegean. Plato's story of Atlantis is
based on an Egyptian story that has similarities with Car-
thaginian and Phoenician legends.
The great Minoan empire was a Bronze Age maritime
civilization centered on the island of Crete that flourished
from 3000 to 1400 BC (Menzies 2012 ). The Minoan sea-
farers dominated trade in the eastern Mediterranean, and on
this basis were able to accumulate great wealth and prevent
the development of any other maritime power that could
have threatened them. The Minoans were noted for their
cities and great palaces at Ayía, Knossos, Mallia, Phaestus,
Triáda, and Tylissos—all decorated by detailed and lively
frescoes. By far the largest and best-known palace was that