Re: New archaeological evidence casts doubt on mega-tsunami theory of Minoan collapse

From: Eric Stevens (eric.stevens_at_sum.co.nz)
Date: 01/12/05 

Date: Wed, 12 Jan 2005 15:29:40 +1300

On Fri, 07 Jan 2005 08:06:45 +1300, Eric Stevens
<eric.stevens@sum.co.nz> wrote:
>"The re-analysis of
>the original tsunami hypothesis indicates that there is insufficient
>evidence to demonstrate that a large tsunami propagated throughout the
>eastern Mediterranean circa 3,500 years BP."

A subjective answer to the question of what it is that constitutes 'a
large tsunami' may be indicated from the following:

Bryant in his 'Tsunami: The underrated Hazard' writes of the Santorin
eruption on page 225:

  "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 pyroclastic 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 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. 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
    Santoririi. 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 cm2.
    Spontaneous liquefaction and flow of water-saturated muds is known
    to occur under pressure pulses of 280.

   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 rng H. In comparison, the highest measured sediment
   concentrations on the ocean seabed and in muddy tidal estuaries
   rarely exceed 12 rng l~ and 300 mg H 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 rn or more thick.
   Hormgenites 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.

   Four types of homogenites can be differentiated. In the Western
   Mediterranean, on the lonian Abyssal Plain, a 10- to 20-rn-thick
   deposit, with an estimated volume of 11 km3, was laid down on the
   seabed over an area of 1,100 km2. It appears that the tsunami wave
   slammed into the continental shelf of North Africa and either
   directly or indirectly triggered a mega-turbidity current. This
   current carried terrigenous and shelf sediment into the deep
   Mediterranean Sea, eroding flanks of undersea ridges and
   depositing homogenites with an erosional base on upsiopes. In one
   location this turbidity current rode up a ridge 223 m above the
   abyssal plain and deposited sediment. In the eastern part of the
   Mediterranean, bottom velocities and the related powerful pressure
   pulse liquefied sand into depressions, forming uniform deposits
   several metres thick with a sandy base overlying an erosional
   contact. These deposits form in cobblestone-shaped basins with a
   vertical relief of 200 m. Finally, in the Bannock Basin, the
   passage of the wave destabilised evaporites. The resulting deposits
   are 12 m thick and consist of 3 m of sand overlain by 9 m of graded
   mud deposited from suspension in highdensity brines trapped at the
   bottom of 100-m-deep depressions in the seabed. All of the
   homogenites found in the Mediterranean are derived from a single
   event and date around the time of the Santorini eruption.
   Homogenites are not found in the Eastern Mediterranean Sea, where
   tsunami wave heights were insufficient to cause resuspension or
   liquefaction of bottom sediment."

I was familiar with this text as the discussions of the last few days
had caused me to reread it. I particularly noted the depths of the
turbidity deposits etc desceiped in the last two paragraphs. What
caused me to post it on this occasion was an interview on the local TV
yesterday evening with an Indian naval officer. He said the Indian
navy has discovered that Aceh earthquake and tsunami have made drastic
changes in the shape of the sea bed over very large areas. He
mentioned on area where the depth was previously 4000' and now is only
100'! Such a change is very unlikely to have been caused by crustal
movement and can only be ascribed to turbidities etc. Clearly the
volume of water displaced in the Aceh tsunami vastly exceeds that
displaced by Santorini, yet the Santorini 'run ups' seem to vastly
exceed those of Aceh.

This then raises the question of what is a 'large tsunami'.
Dominey-Howes may well be correct when he says that there is
insufficient evidence to demonstrate that a large tsunami (in the Aceh
sense) propagated throughout the eastern Mediterranean circa 3,500
years BP, yet the evidence of tsunami deposits on land cited by Bryant
suggest that there was a very considerable wave series, nevertheless.

Eric Stevens

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