Beginning of Essay
The land below us is always in motion. Plate tectonics studies these restless effects to give us a better understanding of the Earth and its past. New molten rocks are poured out in the form of magma from the mid-ocean ridges. The rock is recycled and re-entered ....

Middle of Essay
.... the solid crust-mantle. This process has been occurring for roughly about 3.8 billion years. The mantle is about 2900 km. thick, which lies above a layer of molten magma that still exists today. The immense heat from the magma (approximately 2700(C) causes convection in the mantle (Figure 1). Convection is caused by non-uniform temperature in a fluid and density differences. This continuous convection is the cause of plate movement. Each complete cycle, called a convection cell, drives the plate in the direction of the cell. How does a 'solid' mantle move? The mantle may be solid but, as with most solids, it will deform if long term stress is ....