how does igneous rock form
â Presents a âmapâ of the basic processes, the transformation of rock from one form to another. Many variations!! â These rocks are classified according to their origins, or processes by which they formed: 1. igneous rocks: formed by the cooling and hardening of magma (molten rock) from inside Earth. 2. sedimentary rocks: formed by the compaction and cementing of layers of sediment (rock fragments, plant and animal remains, minerals from water) 3. metamorphic rocks: formed by the effect of heat and pressure on other rocks. *Earthâs crust is made-up primarily of igneous rocks.