The solid Earth (the mantle and crust) is made of rock. You may have noticed that there are many kinds of rocks, from the soft sandy rocks that form the cliffs at Scripps beach to the hard rocks that form the mountains to the East of San Diego. Geologists have developed a way of classifying the various rocks and understand fairly well where they come from and where they go. There are three general types of rocks, those that form from melt (igneous rocks), those that are deposited from air or water (sedimentary rocks), and those that have formed by "cooking" or otherwise altering another rock (metamorphic rocks). Sedimentary rocks form by breaking down other kinds of rocks into small particles and washing or blowing them away; metamorphic rocks form from other rocks and igneous rocks form by melting other rocks. Thus rocks are always changing form and are as part of a giant cycle of renewal. This cycle is called the Rock Cycle.
