Igneous rocks are created from solidified magma (rock that has been melted inside the earth), sedimentary rocks are created when smaller bits of rock or sand are cemented together, and metamorphic rock are formed when other types of rocks are subjected to heat and pressure.ntrusive rocks harden slowly beneath the surface of the earth, and often form large mineral crystals within the rock. Granite is a good example of an intrusive rock. Porphyritic intrusive rocks have large crystals embedded in a matrix of smaller crystals.