The asthenosphere is between 250 and 400 miles thick. It is part of the Earth known as the Upper Crust, which is defined as anything as deep as 416 miles.
The rocks in the asthenosphere are not rigid, but are semi-plastic in nature. They are more like clay, as the temperature in the layer is very close to their melting point.
The asthenosphere is responsible for the movement of the lithosphere, the layer of solid rock above it. The rocks float on the semi-dense layer below it. This motion causes plate tectonics: the slow movement in the earth´s crust that is the cause of earthquakes, volcanoes and continental drift.