4.2. Movement of Plates

These tectonic plates float on and travel independently over the asthenosphere, which lies over the mantle. Much of the earth's seismic activities occur at the boundaries of these plates. It is a relatively slow movement, driven by thermal convection currents and other geological activities originating deep within the earth's mantle. Plates have moved horizontally over the asthenosphere as rigid units.

The movement of a plate is defined by the position of its pole of rotation and its angle of rotation about the rotation axis; its rate of movement varying with distance from the pole of rotation, being nil at the pole and reaching a maximum at the equator relative to the pole of rotation. The strips of normal and reverse magnetic field that parallel the mid-oceanic ridges help scientists determine the rates of plate movement. These rates vary considerably.

The arctic ridge has the slowest rate(less than 2 cm per year), and the East Pacific Rise near Easter Island in the South Pacific has the fastest rate (more than 15 cm per year). The eastern part (Australia) is moving northward at the rate of 5.6 cm per year while the western part (India) is moving only at the rate of 3.7 cm per year due to impediment by Himalayas. This differential movement is resulting in the compression of the plate near its center at Sumatra and a potential division into Indian and Australian Plates. The rate of spreading at the Mid- Atlantic Ridge near Iceland is relatively slow, about 2 cm per year.