How are igneous rock classified?

Answer 1

Igneous rocks are generally classified first on the basis of their formation (and therefore their texture) as intrusive or extrusive. Intrusive rocks are solidified from magma under the surface resulting in a visible crystalline texture, and extrusive igneous rocks are solidified from lava at or near the surface, resulting in a small or nearly invisible crystalline texture.

Igneous rock can additionally be classified by general chemistry composition as felsic, intermediate, mafic, and ultramafic. Felsic rocks are higher in silica and aluminum. Mafic rocks are higher in magnesium and iron.