There are three main families of rocks: 1. Igneous rocks form when magma cools, either underground to become intrusive rocks or on the surface to become extrusive rocks like lava. 2. Sedimentary rocks form from the compaction and cementation of sediments like sand and clay over time and pressure at the bottoms of bodies of water. 3. Metamorphic rocks form from the changes to existing rocks' forms and appearances caused by heat and pressure within the earth.