Rocks come from various sources, depending on the type of rock. Igneous rocks, for example, form from the cooling and solidification of magma or lava. Sedimentary rocks, on the other hand, form from the accumulation of sediment, which can come from the erosion of other rocks, the remains of living organisms, or chemical precipitation. Finally, metamorphic rocks form from the alteration of pre-existing rocks due to heat, pressure, and/or chemical reactions.