Volcanic rock  From Wikipedia, the free encyclopedia - View
original article  Jump to: navigation, search Ignimbrite is a
deposit of a pyroclastic flow.     Setting and size[edit]
Photomicrograph of a volcanic lithic fragment (sand grain);
upper picture is plane-polarized light, bottom picture is cross-
polarized light, scale box at left-center is 0.25 millimeter.
Volcanic rocks are usually fine-grained or aphanitic to glass in
texture. They often contain clasts of other rocks and
phenocrysts. Phenocrysts are crystals that are larger than the
matrix and are identifiable with the unaided eye. Rhomb porphyry
is an example with large rhomb shaped phenocrysts embedded in a
very fine grained matrix.  Volcanic rocks often have a vesicular
texture caused by voids left by volatiles trapped in the molten
lava. Pumice is a highly vesicular rock produced in explosive
volcanic eruptions.   Most modern petrologists classify igneous
rocks, including volcanic rocks, by their chemistry when dealing
with their origin. The f