The three forms of rocks are igneous rocks, sedimentary rocks and metamorphic rocks. Igneous rocks are formed when the molten rock that comes out of a volcano (lava) cools down. While sedimentary rocks are formed by various layers of small rocks and soil that settle down on riverbeds, seabeds and lakebeds. However metamorphic rocks are formed due to the change in heat and pressure in the minerals of igneous and sedimentary rocks.