Rocks and the Rock Cycle
The Rock Cycle is a cycle of formation and destruction in which rocks are lithified, compacted, cemented together by chemical and physical forces, crushed, folded, melted and recrystallized by processes related to those that shape the large-scale features of the crust. Igneous rocks are formed by direct crystallization of minerals from magma, which is a hot, doughy material mixed with gases and steam. ... If it solidifies before it reaches the surface it forms an intrusive igneous rock and if it is thrown out by volcanic action, it forms an extrusive igneous rock.