BBC: Sumatra coastal cave records stunning tsunami history

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BBC: Sumatra coastal cave records stunning tsunami history

Postby driggs » Dec 12, 2013 10:49 am

Very cool intersection of speleosciences: scientists in Sumatra are able to date the timelines of major tsunamis over thousands of years by investigating cave sediments interspersed with bat guano deposits.

http://www.bbc.co.uk/news/science-environment-25269698

A cave on the northwestern coast of Sumatra holds a remarkable record of big tsunamis in the Indian Ocean.

The limestone opening, close to Banda Aceh, retains the sandy deposits washed ashore by huge, earthquake-induced waves over thousands of years.

Scientists are using the site to help determine the frequency of catastrophes like the event of 26 December 2004.

This is being done by dating the cave's tsunami-borne sediments, which are easy to see between layers of bat droppings.
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Re: BBC: Sumatra coastal cave records stunning tsunami histo

Postby Bob Thrun » Dec 12, 2013 9:14 pm

The story says each tsunami deposited a layer of sediments on top of a layer of guano. The guano was deposited by bats between tsunamis. Wouldn't the tsunami also wash away some of the previous deposits?
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Re: BBC: Sumatra coastal cave records stunning tsunami histo

Postby paul » Dec 13, 2013 7:15 am

Bob Thrun wrote:The story says each tsunami deposited a layer of sediments on top of a layer of guano. The guano was deposited by bats between tsunamis. Wouldn't the tsunami also wash away some of the previous deposits?


Presumably because there was such a long time between successive tsunamis, the sediment layers had settled and solidified enough to prevent them being washed away?
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Re: BBC: Sumatra coastal cave records stunning tsunami histo

Postby Squirrel Girl » Dec 13, 2013 7:51 am

Hypothetically speaking, there are quite a few reasons why this can happen. In this particular situation, I have no idea. But to start with, many tsunamis are a giant wall of water (though the Japanese and Indonesian ones in recent years were dramatic). Tsunamis are a raising of the water level, not so much a wall of water coming in. It depends. Another consideration is that if 20 cm of guano were deposited, maybe the top 10 cm were eroded away. It depends. Alternatively, a vaguely uniform layer of guano was deposited. When the tsunami came in, it might have eroded, completely, the front (seaward) edge of the layer of guano, but left the layer farther back, largely untouched, but buried. It depends.

Yet if they found the layers, it kinda shows that such a series of events can happen.
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