Although the mantle is solid rock (except for partially melted zones in the asthenosphere), it is so hot that it flows slowly as a viscous fluid over geologic time. Mantle rock flows in elliptical or circular patterns. Hot rock from deep in the mantle rises to the base of the lithosphere. At the same time, cool upper mantle rock sinks. This flow of solid rock is called mantle 'convection'. Mantle convection is thought to be closely related to the movement of the lithospheric plates.
Geologists who study the earth's mantle have suggested three different models of mantle convection that might cause movement of the lithosphere. The first model involves convection of the entire mantle, with the heat source at the core mantle boundary. In the second model, convection occurs only in the upper mantle, above the 670 km seismic discontinuity. In this model, the mantle below the discontinuity has little or no movement and simply acts as the heat source to drive the convection. The third model is a combination of the first two and consists of two convection systems stacked one on top of the other. In this model, the boundary between the two convection cells is the 670 km seismic discontinuity, and the lower cells transmit heat to the upper cells. Thus the plates of the lithosphere somehow move in response to the flow in the underlying mantle.
Geologists who study the earth's mantle have suggested three different models of mantle convection that might cause movement of the lithosphere. The first model involves convection of the entire mantle, with the heat source at the core mantle boundary. In the second model, convection occurs only in the upper mantle, above the 670 km seismic discontinuity. In this model, the mantle below the discontinuity has little or no movement and simply acts as the heat source to drive the convection. The third model is a combination of the first two and consists of two convection systems stacked one on top of the other. In this model, the boundary between the two convection cells is the 670 km seismic discontinuity, and the lower cells transmit heat to the upper cells. Thus the plates of the lithosphere somehow move in response to the flow in the underlying mantle.
