Rhyolite specimens found in Cumbria. Rhyolite is a felsic (silica rich) rock, often formed in highly explosive volcanoes. Specimens are supplied in a polythene bag in lots of 1 kg with a label.
Rhyolites are compositionally very similar to granite. The texture can range from porphyritic to glassy. Typically containing >69% silica the rhyolite melts tend to form polymerised & highly viscous lavas. They also occur as volcanic breccias, plugs & dykes. If cooling is too rapid crystals will not develop the resultant rock is glassy & commonly known as Obsidian.