Lesson 2 Three Classes of Rock: We learned that rocks are classified not only by how they form, but also by texture and composition. Igneous rocks are extrusive because they form when magma reaches Earth’s surface. If it cools quickly there is no time for crystallization and volcanic glass forms. Intrusive igneous rocks form when magma cools and solidifies beneath Earth’s surface. Sedimentary rock is compressed and cemented into layers formed from clay, silt, sand, and gravel as well as organic remains. Metamorphic rock is formed from large increases in temperature or pressure. Metamorphism causes changes in mineralogy, texture, and chemical composition. Foliated metamorphic rock results from extensive deformation by regional metamorphism. They contain aligned, planar arrangements of mineral grains. Non-foliated metamorphic rocks such as marble result from minimal deformation of rocks containing one or a few minerals.