Rhyolite Rock
"Rhyolite is a
relatively rare rock."
It is an extrusive igneous rock, which is formed when extremely explosive volcanoes erupt.
Its lava is so viscous, sticky and so much lacks volatiles, that it piles up and builds lava domes instead of flowing away when it does reach ground.
It often has a porphyritic texture, with phenocrysts (large crystals) that formed underground, in intrusive conditions, and a fine-grained matrix that formed from lava once the volcano had erupted.
It can also have a banded texture which lacks phenocrysts.
Common minerals are quartz, feldspar, pyroxene, amphibole and biotite.
If a magma with the same composition would be formed completely underground, this rock would be granite.
Mount St Helens is a rhyolitic volcano. By Bmaas
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