Plate tectonics is a theory proposed by H. H. Hess in 1960 to explain the mechanics of continental drift. The theory proposes that the crust of the Earth, together with the upper portion of the mantle, which forms the lithosphere, consists of rigid slabs or plates. The plates continually shift their position in relation to one another. Beneath the lithosphere is the asthenosphere, which is believed to be plastic.
Oceanic ridges, trenches and transform faults bind the plates. Oceanic ridges appear where two plates are moving apart. This leaves a gap that is continuously filled with magma, rising from the asthenosphere. When magma cools, it forms new crust, which becomes part of the moving plates. This is referred to as sea-floor spreading. Although the rate of spreading is slow, it is not negligible. The Atlantic Ocean is opening up at a rate of 0.75 inches per year. The fastest rate observed is at the Pacific Rim, which creates four inches of new crust each year.
At points where plates converge, trenches are formed. One plate slides under the other and enters the mantle. Therefore, plate edges are destroyed at trenches. Since the volume of the earth is constant, a balance is maintained between the amount of crust created at the ridges and that destroyed at the trenches.
The leading edges of the colliding plates may be oceanic crust, or one may be oceanic while the other is continental. The oceanic will be the sinking plate. Both plates may also be continental. In the last two cases the sedimentary cover is crumpled and injected with material melted by heat generated by the collision. This produces mountains such as the Andes and the Himalayas.
Transform faults separate two crustal plates where they slide past one another. A transform fault links two segments of a ridge, and the ridge offset gives an apparent motion opposite to the real movement. They are active only between the ridge crests where opposite crustal spreading occurs. It is only a dead scar where both sides move together at the same rate. They sometimes slice through continents, for example, the San Andreas Fault, which runs from the Gulf of California through San Francisco.
One of the first theories explaining continental drift came about in 1927, when British geologist Arthur Holmes suggested convection currents in the mantle may be responsible. Heat differences generate convection currents. Global tectonics theory suggests that convection currents exist in the asthenosphere and possibly in the lower mantle. These currents form convection cells that rise under ridges and descend under trenches. Measurements of heat radiated from the Earth that depicts high values along ridges and low values among trenches tend to support this theory.
The hypothesis of sea-floor spreading became further established in 1966. Independent oceanographic data involving microfossils and sediment thickness, showing it is thicker on older crusts where they have had more time to accumulate, provided additional evidence. Measurements of heat flow from the Earth’s interior as well as paleomagnetic and seismological studies gave it further credence. The term “global tectonics” came to be used in 1968 to explain the relationship between spreading ridges, transform faults, sinking trenches, drifting continents and mountain building.