Rock (geology) kind of rock may thus be traced gradually merging into those of another. Hence the definitions adopted in rock names simply correspond to selected points in a continuously graduated series. Igneous rock (derived from the Latin word "igneus," meaning "of fire," from "ignis" meaning "fire)" is formed through the cooling and solidification of magma or lava. This magma may be derived from partial melts of pre-existing rocks in either a planet's mantle or crust. Typically, the melting of rocks is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in
Magma (the extrusive equivalents of gabbro, diorite and granite, respectively). During a volcanic eruption the magma that leaves the underground is called lava. Lava cools and solidifies relatively quickly compared to underground bodies of magma. This fast cooling does not allow crystals to grow large, and a part of the melt does not crystallize at all, becoming glass. Rocks largely composed of volcanic glass include obsidian, scoria and pumice. Before and during volcanic eruptions, volatiles such as CO and HO partially leave the melt through a process known as exsolution. Magma with low water content becomes increasingly viscous. If massive exsolution