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MAJOR ROCK TYPES

The major rock types covered here are igneous (intrusive and extrusive), sedimentary, and metamorphic. Which rock type forms depends on the environment under which it forms. This is called the Rock Cycle. When sediment (e.g., clay, sand, gravel, and shells) solidifies (or solids precipitate out of water) it becomes sedimentary rock (e.g., shale, sandstone, conglomerate, and limestone). When any rock type is subjected to heat and pressure (enough to make the rock "plastic" (malleable) but not completely melted) it becomes metamorphic rock (e.g., slate, quartzite, gneiss [pronounced nice]). When any rock is completely melted and resolidified it becomes igneous. If the melted rock solidifies under ground, it becomes intrusive igneous (e.g., granite). If the melted rock solidifies above ground, it becomes extrusive igneous (e.g., rhyolite and basalt). Below are pictures of the basic rock types.

Sedimentary (sandstone)	Metamorphic (gneiss)	Intrusive Igneous (granite)	Extrusive Igneous (basalt)