Top stone is obsidian, below that is pumice
and in lower right hand color is rhyolite (light color)
is an igneous
, volcanic (extrusive
, of felsic composition, with aphanitic
texture. Mineral assembly is usually quartz
, alkali feldspar
(in a ratio > 1:2). Biotite
are common accessory minerals.
Rhyolite can be considered as the extrusive equivalent to the plutonic granite rock. Due to their high content of silica, rhyolite form highly viscous lavas. They can also occur as breccias or in volcanic necks and dikes. Like obsidian, rhyolites frequently show flow, spherulitic, nodular, and lithophysal structures.
See also: igneous rocks, list of rocks, granite