The most common type of igneous rock, basalt, also known as malfic rock, can be found on oceanic plates at divergent plate boundaries. Intermediate and felsic igneous rocks show up along continental margins, as well. Because igneous rocks form from the cooling of lava and magma, they can be found around areas where volcanic activity is or has been present.
Igneous rocks form from magma and lava that cools and solidifies. As this happens, the magma or lava crystallizes, creating igneous rock, of which there are many variations. Igneous rocks come in two types, extrusive and intrusive. Igneous rocks are split into these two categories based upon how they cool. Extrusive rocks form form lava on the surface that crystallizes, forming finely grained crystals, but intrusive rocks instead cool and crystallize below the surface of the earth from magma. They cool at a slower rate, forming large, coarsely-grained crystals. There is an unwritten third type of igneous rock that is technically extrusive, but it differs from other extrusive rocks because the lava cools instantly, forming a glass-like texture.
There are many ways scientists describe igneous rocks, which have four different types of mineral composition: felsic, intermediate, mafic and ultramafic. Texture is also an important characteristic that classifies igneous rocks and is determined by the size of its grains or crystals. These grain types are listed as pegmatitic, phaneritic, porphyritic, aphanitic, glassy, vesicular, frothy and pyroclastic.