The evidence for plate tectonics includes the past and present distribution of earthquakes, changes in the earth's magnetic field, meteorites, chains of islands and volcanoes. Increasingly, geologists study plate tectonics with the use of satellite laser ranging techniques and rock magnetism as well as echo-sounders and small explosions to detect shock waves in the earth's interior. The study of seismic waves also enables geologists to examine the theory of plate tectonics. By determining the speed and the path of these shock waves through the earth geologists are able to identify the density and thickness of rocks that lie thousands of kilometres within the earth's interior.
In the seventeenth century Frances Bacon noted that the coastline of the eastern side of the Americas was very similar to the coastline of the western side of Africa and could fit together like pieces in a jigsaw. In addition settlers in the New World found that huge coal deposits in the American continent were similar in location to their European counterparts. In 1857 the Irish civil engineer Robert Mallet produced the first scientific investigation of earthquakes. He produced a map which included land earthquakes as well as submarine earthquakes. Many of the seaquakes, as they were called, were found around the centre of the Atlantic Ocean. In the late nineteenth century and early twentienth century it was realised that many earthquakes were caused by rocks suddenly breaking along faults such as the 1906 San Francisco earthquake along the San Andreas fault.
In the early twentieth century an Amercan, Harry Hess, suggested that convection currents would force molten rock (magma) to well up in the interior and to crack the crust above and force it apart.
In the 1960s research on rock magnetism supported Hess. The rocks of the Mid-Atlantic Ridge were magnetised in alternate directions in a series of identical bands on both sides of the ridge. This suggested that fresh magma had come up through the centre and forced the rocks apart. In addition with increasing distance from the ridge the rocks were older. This supported the idea that new rocks were being created at the centre of the ridge and the older rocks were being pushed apart.
In 1965 a Canadian geologist J Wilson linked together the ideas of continental drift and sea floor spreading into a concept of mobile belts and rigid plates, which formed the basis of plate tectonics. He argued that the earth consists of an outer layer of six or more major plates which move over a hot partially molten asthenosphere. New material is formed at the mid-ocean ridges. Due to the sea floor spreading continents move relative to each other. In some areas there is subduction whereby one crustal block of dives underneath another and is melted and taken back into the asthenosphere.
In some areas geologists believe they have discovered rocks from part of the mantle. In North Italy, South East Turkey, the Persian Gulf and New Guinea dark heavy rocks known as peridotites are composed of olivine and pyroxene silicate minerals that are formed only at high pressure and these are very hich in iron and magnesium.
