An igneous rock forms by solidification of magma. There are two main types: volcanic rocks that crystallize or solidify from erupted lava or fragmental (pyroclastic) material that reached the surface and contacted the atmo- or hydrosphere and plutonic or intrusive rocks that crystallize from magma that did not reach the surface. These two types are distinguished by their grain size: intrusive rocks are coarse-grained and typically show an interlocking fabric because they crystallize slowly in a thermally insulated setting, whereas volcanic rocks are fine-grained or even glassy because they cool rapidly following eruption at the Earth’s surface; crystals typically float in a fine-grained matrix. Common minerals in igneous rocks are olivine, pyroxene, amphibole, mica, feldspar, and quartz. Common intrusive rock types include peridotite, gabbro, granodiorite, and granite; their volcanic equivalents are komatiite, basalt, andesite, and rhyolite,...