Volcanic rock

Volcanic rock is igneous rock which comes to the surface during an eruption and which, on contact with the atmosphere or water cools very rapidly.

Classification of volcanic rocks

Volcanic rocks are classified according to several criteria. First of all, they are observed with the naked eye, and they were then classified on the basis of their appearance, and this was of course the first method used. With later progress in geology, chemistry and physics, a mineralogical classification was introduced (from minerals in volcanic products), to which was added chemical classification (from the chemistry of lava) and finally genetic classification (from the way they are deposited).

A cooling basalt flow. © USGS

Examples of volcanic rocks include basalt, with a low silica content (SiO2) and rhyolite which is a volcanic rock with a high silica content. Rhyolite has the same chemical composition as its granite equivalent (a plutonic rock) and basalt has the same chemical composition as gabbro (another plutonic rock). Intermediate volcanic rocks include andesite and dacite.

We know of Martian volcanic rocks.