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LET’S ROCK The rock cycle shows the continuous changing of one rock type to another. Rocks are a mixture of minerals, mineraloids, glass or organic matter. Igneous rocks form from the cooling of magma or lava. They are the most abundant type of rock. Basaltic igneous rock is from the cooling of lava. It has small crystals due to rapid cooling at the earth’s surface. It makes up the oceanic crust. It is dense, heavy, contains Fe and Mg, and is usually dark colored. Granitic igneous rock is from the cooling of magma. It has larger crystals due to the slower cooling deep within the earth. It makes up the continental crust. It is light colored, low density, and contains Si and O. Metamorphic rocks are “changed rocks”. They are formed from heat and pressure (enough to change the rock, but not enough to melt it!). Foliated metamorphic rocks have minerals in flat parallel bands, often alternating light and dark minerals. The heat and pressure occurs in one direction. Nonfoliated metamorphic rocks do not have bands. The heat and pressure is in all directions. Sedimentary rocks are formed from compaction and cementation of other particles that have been eroded and deposited. Clastic sedimentary rocks are formed from other rock particles. Chemical sedimentary rocks are formed from dissolved minerals. Organic sedimentary rocks are formed from one living organisms.