There are three main rock groups.  The sedimentary, igneous, and metamorphic rocks are the types of rocks found on Earth.  They are created by a rock cycle. The igneous rock is formed from magma and lava erupting from the earth’s core.  They are changed over time by weathering into sedimentary rock.   The igneous rocks often are not weathered, but they become buried under layers of other rock, or often fall into areas where there is a great deal of movement of the tectonic plates is occurring.  When this happens, the energy created is heat and pressure.  This heat and pressure alter the original rock to produce metamorphic rock. 

"Metamorphic rock is rock that has changed from one type of rock into another. The word metamorphic (from Greek) means "of changing form." Metamorphic rock is produced from either igneous rock (rock formed from the cooling and hardening of magma) or sedimentary rock (rock formed from compressed and solidified layers of organic or inorganic matter). Most of Earth's crust is made up of metamorphic rock."