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Pyroclastic Flow deposit â.. Cwm Idwal

Mike Raine (July)

This fascinating piece of brecciated tuff tells us a lot about the violent volcanic history of Snowdonia. Brecciated means broken bits whilst, tuff is a rock formed by explosive activity when volcanoes erupt violently rather than pouring out lava flows.

The rock photographed can be seen soon after crossing the stream when walking into Cwm Idwal from Ogwen Cottage. The path here was built using rocks found in the immediate vicinity whereas todayâÂ.Â.s built paths usually have rocks flown in from other localities.

When you look as this rock you can see lots of different things. There are angular fragments of different sizes, these are the remains of rock formed by the volcanoes previous eruptions, these former rocks have been blasted apart by an enormous eruption, bigger than that seen on Mount St.Helens in the eighties. There are some elongated 'fiamme' of dark rock, this would have been molten rock which has cooled, and therefore been stretched, as it flowed down the mountain within the pyroclastic flow. The whole mass is then held together in a âÂ.Â.cementâÂ.Â. of volcanic ash. So this is a pyroclastic flow deposit of the Lower Rhyolitic Tuff formation which is rock formed in a major and dramatic period of volcanic activity in Snowdonia 450 million years ago.

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