Original: Two main zones are distinguished in the upper mantle : the inner asthenosphere composed of flowing rock in the state of plasticity , about 200 km thick , and the lowermost part of the lithosphere , composed of rigid rock , about 50 to 120 km thick .
Simplified: It is called the asthenosphere , and is composed of flowing rock in the state of plasticity , about 200 km thick , and the lowermost part of the lithosphere , composed of rigid rock , about 50 to 120 km thick .