Igneous Rocks: Intrusive and Extrusive
Igneous rocks are the product of the consolidation of magma. Extrusive igneous rocks form when magma pours out on the surface of the earth as lava flows or is blown into the air and then settles on the surface to form beds of pyroclastic rocks. Intrusive igneous rocks form when magma consolidates beneath the surface of the earth.
Bibliography Billings, M. (1942). Structural Geology. Englewoods Cliffs, NJ: Prentice-Hall