There are three major types of rock: igneous, sedimentary, and metamorphic.
The rock cycle illustrates the relationships among them (see diagram).
It can then be turned into a metamorphic rock by heat and pressure that change its mineral content, resulting in a characteristic fabric.
All three types may melt again, and when this happens, new magma is formed, from which an igneous rock may once more crystallize.
These studies explain the chemical changes associated with the major seismic discontinuities in the mantle and show the crystallographic structures expected in the inner core of the Earth.More recently, seismologists have been able to create detailed images of wave speeds inside the earth in the same way a doctor images a body in a CT scan.