This rock is a “welded tuff”. Volcanoes explode when gases trapped in bubbles inside of molten rock build pressure and explode, tearing the molten rock apart. That explosion forms fine-grained particles called ash and larger chunks that still hold the shape of the bubbles called pumice.

Those particles fly through the air but they’re still molten rock, they’re still hot. If they land in a big enough pile, the heat in the ash grains can cause them to reshape. The pile of hot ash will literally turn itself back into a rock that we call a welded tuff.

Larger chunks of pumice also get incorporated into these tuffs as they weld. A piece of pumice is mostly open space with a tiny bit of rock in-between. Heat that pumice up and it loses all strength – the weight of the rock on top of it will squeeze a piece of pumice down into the flat layers you see here. Those flattened pumice are called fiamme. Here, the fiamme are a little weaker than the surrounding tuff, so they’re also eroding out and leaving open spaces.


Image credit: http://bit.ly/1AVB9iR

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