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.
This Martian sample measures 1.3 cm on its longest side. The light crystals are pyroxenes; their light colour shows that they have a low iron content; their average size is 0.4 mm. The very dark areas are maskelynites (amorphised feldspars). The sample is therefore a gabbro with light pyroxenes and dark feldspars, and is rather remarkable. © Pierre Thomas-ENS Lyon