A lithosphere is the rigid, outermost shell on Earth. It is composed of the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater. The outermost shell of a rocky planet, the crust, is defined on the basis of its chemistry and mineralogy. This includes the crust and the uppermost mantle, which constitute the hard and rigid outer layer of the Earth. The uppermost part of the lithosphere that chemically reacts to the atmosphere, hydrosphere and biosphere through the soil forming process is called the pedosphere.