Continental Drift and Plate Tectonics: Exam Notes

The map of the world has not always looked as it does now. The continents have moved, and they are still moving, a few centimetres a year. The explanation grew in three steps: Alfred Wegener's continental drift theory in 1912, Harry Hess's sea floor spreading in 1961, and the theory of plate tectonics in 1967, which holds the field today. Examiners ask the names, the dates, the evidence and above all the plates and their boundaries, so this note keeps all four in view.

Wegener's continental drift theory

Alfred Wegener, a German meteorologist, put forward the drift theory in 1912 and set it out in his book The Origin of Continents and Oceans. He held that all the continents once formed a single landmass called Pangaea, surrounded by one great ocean, Panthalassa. About two hundred million years ago Pangaea began to break: the northern part, Laurasia or Angaraland, separated from the southern part, Gondwanaland, and the narrow Tethys Sea lay between them. India, Africa, South America, Australia and Antarctica were parts of Gondwanaland. Wegener named two forces for the drift: the pole-fleeing force, caused by the rotation of the earth and the bulge at the equator, and the tidal force, caused by the attraction of the moon and the sun. Both were found far too weak to move continents, and that is the standard criticism of his theory.

The evidence Wegener and others gathered

• The jig-saw fit: the coastlines of South America and Africa face each other like torn pieces of one sheet.
• Rocks of the same age across the ocean: a belt of rocks about two thousand million years old runs along the coast of Brazil and reappears in western Africa.
• Tillite, the sedimentary rock formed from glacial deposits: the Gondwana sequence of India matches those of Africa, South America, Australia and Antarctica.
• Placer deposits: rich gold placers of the Ghana coast whose source rock lies in Brazil.
• Fossils: Glossopteris, the plant of the Gondwana coal measures, and reptile fossils spread across continents now separated by wide oceans.

Convection currents and sea floor spreading

Arthur Holmes suggested in the nineteen thirties that thermal convection currents in the mantle, kept going by the heat of radioactive decay, could drag the crust. Mapping of the ocean floor after the Second World War showed mid-oceanic ridges, deep trenches and thin sediments, and in 1961 Harry Hess proposed sea floor spreading. Molten material rises along the mid-oceanic ridge, pushes the older floor aside and solidifies as new crust; where the floor meets a continent it bends down into a trench and is consumed. The proofs are striking: the rocks of the ocean floor are youngest at the ridge and grow older away from it, no part of the ocean floor is more than about two hundred million years old, the sediments on the ridge are very thin, and the stripes of normal and reversed magnetic polarity on either side of a ridge are mirror images of each other. Earthquakes and volcanoes also crowd along the ridges and the trenches, not at random.

Plate tectonics and the plates

The theory of plate tectonics was given in 1967. It says that the lithosphere, the crust together with the rigid upper mantle, is broken into plates that float on the plastic asthenosphere and move as rigid units; the continents ride on the plates and do not plough through the ocean floor as Wegener thought. The continental crust averages about thirty kilometres in thickness and the oceanic crust only about five, which is why the two behave differently when they meet. The lithosphere is divided into seven major plates and a number of minor ones.

The three kinds of plate boundary

At a divergent or constructive boundary two plates move apart and new crust is formed; the Mid-Atlantic Ridge, which runs through Iceland, is the standard example, and the East African Rift Valley shows the same process on land. At a convergent or destructive boundary one plate goes under the other in a subduction zone. Three cases are asked: ocean meeting continent, which raises fold mountains such as the Andes; ocean meeting ocean, which builds island arcs such as Japan and the Aleutians; and continent meeting continent, which crumples the sediments between them into the highest mountains, as the Indian plate did against the Eurasian plate to raise the Himalaya. At a transform or conservative boundary the plates slide past each other, crust is neither made nor destroyed, and earthquakes are frequent; the San Andreas Fault in California is the example always quoted. Rates of movement differ widely: the Arctic Ridge opens at less than two and a half centimetres a year while the East Pacific Rise near Easter Island moves at more than fifteen.

The drift of the Indian plate

India was part of Gondwanaland, joined to Africa, Australia, Antarctica and South America, and lay far to the south of the equator. When Gondwanaland broke up, the Indian plate drifted northwards about five thousand kilometres, the fastest long journey known among the plates. As it moved, enormous fissure eruptions laid down the Deccan Trap basalts of peninsular India about sixty million years ago. The Tethys Sea between India and Asia closed, its sediments were squeezed and folded upward, and the Himalaya rose; the plate has not stopped, so the mountains are still rising and the belt from Kashmir to Arunachal Pradesh remains the most earthquake-prone part of the country. The trans-Himalayan Zanskar and Karakoram ranges, the Indus and Brahmaputra suture zones, and the frequent earthquakes of the north-east all belong to the same unfinished collision.