Igneous rock as extrusive rocks. Igneous rock may form with crystallization to form granular, crystalline rocks, or without crystallization to form natural glasses. Igneous and metamorphic rocks make up 90–95% of the top 16 km of the Earth's crust by volume. Igneous rocks form about 15% of the Earth's current land surface. Most of the Earth's oceanic crust is made of igneous rock. Igneous rocks are also geologically important because: In terms of modes of occurrence, igneous rocks can be either intrusive (plutonic and hypabyssal) or extrusive (volcanic). Intrusive igneous rocks are formed from magma that cools and solidifies within the crust
Tholeiitic magma series distinguished from rocks in the calc-alkaline magma series by the redox state of the magma they crystallized from (tholeiitic magmas are reduced; calc-alkaline magmas are oxidized ). When the parent magmas of basalts crystallize, they preferentially crystallize the more magnesium-rich and iron-poor forms of the silicate minerals olivine and pyroxene, causing the iron content of tholeiitic magmas to increase as the melt is depleted of iron-poor crystals. However, a calc-alkaline magma is oxidized enough to precipitate significant amounts of the iron oxide magnetite, causing the iron content of the magma to remain more steady as it cools than with a