Why are igneous rocks crystalline?
Igneous rocks comprise the bulk of the crust of the Earth. The processes that lead to the formation of igneous rocks have brought Earth’s water to the surface (as well as the atmosphere and most of the chemicals that make up organic life), produced most of its metallic ore deposits, and are responsible for some of our most spectacular scenery and dangerous hazards. They are the principal preserved responses to tectonic plate divergence and convergence, and as such constitute a record of those activities in the past. As such, it is in our interest to learn to recognize, to interpret, and to understand them. Igneous rocks form when magma (molten rock material) freezes and solidifies. Volcanic eruptions take place when magma reaches the surface before it solidifies. The magma may flow onto the surface as fluid lava or it may erupt explosively as rapidly expanding gas propels bits of lava and rock outward. The rocks resulting from volcanic eruptions cool rapidly and are known as extrusive
