Re: Effects of Thera explosion revised upward

On Sun, 27 Aug 2006 02:20:08 GMT, prd <X_header@xxxxxxxxxxx> wrote:

http://www.uri.edu/news/releases/?id=3654

From "Tsunami - The Underrated Hazard" Edward Bryant, Cambridge
University Press 2001, ISBN 0 521 77244 3 and 0 521 77599 X

- based on [Yokohama 1978: Pichlet and Fridrich, 1980; Kastens and
Cita, 1981; Bryant 1991; LaMoreaux, 1995; Cita et al, 1996; Johnstone,
1997; Pararas-Carayannis, 1998c

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"The range of dates may not be contradictory because there is evidence
that Thera may have erupted several times over a time span of 200
years.
The eruption around 1470 B.C. had four distinct phases. The first was
a Plinian phase with massive pumice falls. This was followed by a
series of basal surges producing profuse quantities of pumice up to 30
m thick on Santorini. The third phase was associated with the collapse
of the caldera and production of pyroclas-tic flows. About 4.5 km3 of
dense magma was ejected from the volcano, producing 10 km3 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 Mediterranean. 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 km2 and to a depth of between 600 and 800 m. According to the
Krakatau model, this final event produced the largest tsunami,
directing most of its energy westwards (Figure 7.4). 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 minutes.
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. 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
minutes, with a height of approximately 11 m. Refraction focussed wave
energy on the northeast corner of Crete, where run-up heightsreached
40 m above sea level. In the region of Knossos, the tsunami swept
across a 3-km-wide coastal plain, reaching the mountains behind. The
backwash concentrated in valleys and watercourses, and was highly
erosive. 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. 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 San-torini. Based upon linear wave theory,
the wave in the central Mediterranean Sea was 17 m high, while 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~: - 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 kdyne cm"2.
Spontaneous liquefaction and flow of water-saturated muds is known to
occur under pressure pulses of 280 kdyne cm~2and 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 - labelled homogenites -
formed in the deep ocean as the result of settling from suspension of
densely concentrated, fine-grained sediment. This process produced
homogeneous units up to 25 m thick with a sharp basal contact.
Homogenites can be linked hydrodynamically 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
turbulent clouds of sediment. It is estimated that the slurries
exceeded concentrations of 16,000 mg I"1. In comparison, the highest
measured sediment concentrations on the ocean seabed and in muddy
tidal estuaries rarely exceed 12 mg I"1 and 300 mg I"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 centimetres to several metres. 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 Chapter 3 by their
greater thickness, lack of laminations, and undifferentiated particle
size. Homogenites differ from debris flows by the absence of large
clasts or rock pieces derived from continental sediments.

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It was a BIG bang.



Eric Stevens
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