Volcanic rock

Ignimbrite is a deposit of a pyroclastic flow.

Vesicular olivine basalt from La Palma (green phenocrysts are olivine).

Volcanic rock is an igneous rock of volcanic origin.

Volcanic rocks are usually fine-grained or aphanitic to glassy 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. They were created during fractional crystallization of magma before extrusion. Rhomb porphyry is an example with large rhomb shaped phenocrysts enbedded in a very fine grained matrix.

Volcanic rocks are named according to their chemical composition. Basalt is a very common volcanic rock with low silica content. Rhyolite is a volcanic rock with high silica content. Rhyolite has the same chemical composition as granite and basalt is compositionally equal to gabbro. Intermediate volcanic rocks include andesite, dacite, trachyte and latite.

Volcanic rocks often have a vesicular texture, which is the result voids left by volatiles escaping from the molten lava. Pumice is a rock, which is an example of explosive volcanic eruption. It is so vesicular that it floats in water.

Pyroclastic rocks are the product of explosive volcanism. They are usually felsic (high in silica). Examples of pyroclastic rocks are tuff and ignimbrite.

Shallow intrusions, which possess structure similar to volcanic rather than plutonic rocks are also considered to be volcanic.

See also

*Intrusion