Re: New archaeological evidence casts doubt on mega-tsunami theory of Minoan collapse
From: grapheus (grapheus_at_www.com)
Date: 01/12/05
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Date: 12 Jan 2005 05:35:36 -0800
Eric Stevens wrote:
> 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.
Or, as advocated by J. Faucounau in his book (in French) on "Les
Peuples de la Mer" (Paris, 2003), that the "third phase" (= the
COLLAPSE of the caldera) occurred in TWO PHASES, one at the time of the
ERUPTION, the second some 400 years later, i.e. c. 1200BC.
grapheus
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