Types of Rocks: Metamorphic: formed when igneous or sedimentary rocks are put under heat and/or pressure in the Earth's crust. Igneous: Formed when crystallized through melting and cooling. Sedimentary: Formed through the accumulation of sediment. Compaction - process by which overlying pressure from rocks and soil reduces the size or volume of sediments. Rock cycle - the continental process by which rocks can be changed into different types.