CAUSES OF PLATE MOTION

There are various forces that act upon the plates to make them move. Three possible mechanism of plate motion –

Convectional Currents :-

• Convection involving the whole mantle. There is a large amount of radiogenic heat concentrated in the mantle. This leads to the formation of convection currents. As the current ascends from below, they diverge and spread laterally. Convection would cause the lithosphere to split and as the plates may move laterally, the currents carry the overlying slab of lithosphere with them. This marks the site of ocean ridges. The currents encounter a similar current from the opposite parts of the mantle and drag the lithosphere down into the mantle at trenches. The high heat flow at oceanic ridges is an evidence to this process. There are, however, two major problems –
  • The mantle is layered and the sort of convection would preclude such layering.
  • below 700km mantle is too resistant to allow penetration of subduction plates
• Mantle Plume : Another variety of mantle convection involves jet-like plumes of low density material from the core mantle boundary. As the plume reaches the lithosphere, it spreads out laterally doming surficial zones of the earth and moving them along in the direction of radial flow. The center of the Afar triangle in Ethiopia is one site of plume that flowed upward and outward carrying the Arabian, African and Somali plates. Mantle plumes are thought to be responsible for the breaking of the Pangaea.

Slab Pull :-

Slab pull is thought to operate at subduction zones where the subducting oceanic plate becomes colder and denser than the surrounding mantle (gabbro metamorphosed into eclogite) and pulls the rest of the slab along. It is believed that the absolute velocity of a plate chiefly depends on the proportion of the margin that is subducting. This subduction is aided by gravity pull, for example, plates like Pacific and Cocos, which have about 40% of their margins represented by subduction zones, have high plate velocities (5 cm/yr). Plates like the North American, which have smaller proportion of subducting margins, moves slowly (1-3 cm/yr). Hence, it can be said that the major deriving force is slab pull.

Ridge Push :-

Ridge push results from the fact that –

• Spreading centres stand high on the ocean floor. This result in gravitational sliding of the lithospheric slab away from the oceanic ridge raised by rising material in the asthnosphere. It has been estimated that a surface slope of only 1:3000 would produce a movement of the lithosphere of 4cm/yr, over the asthnosphere
• Magma rising along the axis of the zones of spreading from wedges of new lithosphere on either side of trailing edge of the plate, thus the plates are pushed apart.