Rocks are divided into three main categories according to how they were formed: igneous, sedimentary or metamorphic processes. Rocks differ in their texture, mineral and chemical composition, and bedding characteristics depending on which of these three processes that formed them. Igneous rocks, which are also referred to as magmatic rocks, are formed from materials that come from the depths of the Earth. There are two categories of igneous rocks: intrusive (or plutonic) rocks and volcanic rocks. Sedimentary rocks are the product of the consolidation of loose sediment that has accumulated in beds. A metamorphic rock is derived from a pre-existing igneous or sedimentary rock that is transformed, to a greater or lesser degree, by the action of physical factors such as temperature and pressure, or chemical factors that include the addition of new minerals.