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Rocks provide the primary record of geological history. We classify rocks into three categories – igneous, sedimentary, and metamorphic. These classifications help us to understand how each rock was formed. These three basic categories of rock form the rock cycle, a series of processes that create and transform the types of rocks on Earth.
Each type was formed through some kind of physical change, whether it was melting, cooling, eroding, compacting, or deforming.