LESSON H5P - What are rocks?
How are rocks classified?
Rocks are classified into three major groups according to how they form. Rocks can be studied in hand samples that can be moved from their original location. Rocks can also be studied in outcrop, exposed rock formations that are attached to the ground, at the location where they are found.
Igneous rocks form from cooling magma. Magma that erupts onto Earth’s surface is lava. The chemical composition of the magma and the rate at which it cools determine what rock forms as the minerals cool and crystallize.
Sedimentary rocks form by the compaction and cementing together of sediments, broken pieces of rock-like gravel, sand, silt, or clay. Those sediments can be formed from the weathering and erosion of preexisting rocks. Sedimentary rocks also include chemical precipitates, the solid materials left behind after a liquid evaporates.
Metamorphic rocks form when the minerals in an existing rock are changed by heat or pressure within the Earth.
