LITHOSPHERE

On the Earth, the lithosphere includes the crust and the uppermost mantle which is joined to the crust across the Mohorovičić discontinuity. The lithosphere is underlain by the asthenosphere, the weaker, hotter, and deeper part of the upper mantle. The boundary between the lithosphere and the underlying asthenosphere is defined by a difference in response to stress: the lithosphere remains rigid for long periods of geologic time, whereas the asthenosphere flows much more readily. As the conductively cooling surface layer of the Earth's convection system, the lithosphere thickens over time. It is fragmented into tectonic plates , which move independently relative to one another. This movement of lithospheric plates is described as plate tectonics. This is when plates move horizontally across the Earth's surface and the continents change their relative positions.

The concept of the lithosphere as Earth’s strong outer layer was developed by Barrell, who wrote a series of papers introducing the concept . The concept was based on the presence of significant gravity anomalies over continental crust, from which he inferred that there must exist a strong upper layer above a weaker layer which could flow . These ideas were enlarged by Daly , and have been broadly accepted by geologists and geophysicists. Although these ideas about lithosphere and asthenosphere were developed long before plate tectonic theory was articulated in the 1960s, the concepts that strong lithosphere exists and that this rests on weak asthenosphere are essential to that theory.