Old Greywacke - geograph.org.uk - 1159126

Autor:

Ashley Dace

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https://czwiki.cz/b/19639

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Old Greywacke This is a Greywacke on the shore of Lochranza dating from the Lower Cambrian. It was deposited on the bottom of a deep Ocean along with black muds and shales. They form when a large underwater sediment built up then collapsed forcing sediment of varying sizes out into the Abyssal plane. The sediments settle out heaviest first causing fining up sequences, once all is calm there is usual sedimentation again for a few thousand years then it all happens again, causing turbidite sequences. They were laid down when the land mass was at a latitude of 27oN. When looking at geological maps of the island it can be seen that the Dalradian deep marine sediments occur in the northern part. After their deposition as marine turbiditic (or Greywackes), the rocks have undergone metamorphism to produce metamorphosed greywackes and schists. The regional metamorphism was caused from large earth movements and events in particular the Caledonian Orogeny (collision of Laurentia, Baltica and Avalonia with the closure of the Iapetus Ocean). The rocks underwent slight chemical changes with Chlorine giving then a green tint, the Chlorite grade rocks are at the outer edge of the Barrovian Zones. They were heated and compressed up to roughly 400 degrees temperature and 500 kilobars pressure. Around 60 Ma the Atlantic formed, magma was forced to the surface like blood when skin tears (America split from Europe). Then like a massive spot a Granite Pluton pushed upwards bending the surface rocks and folding and heating them again. The granite core can still be seen today.

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CC BY-SA 2.0

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From geograph.org.uk

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