A glance at any atlas shows that the Atlantic coastlines of Africa and (South America have very similar shapes. The coastline does not truly represent the shape of a continent, as it depends on the level of the oceans. A much more significant feature is the edge of the continental shelf where the sea bottom falls comparatively rapidly to a depth of around four kilometers. The fit between the two continents at this edge is quite striking. Other coastlines fit together nearly as well and it is possible to assemble all the continents into just two pieces which themselves fit together, although not was well. This led to the suggestion that at some time in the past there was a single continent called Pangaea, which split into two parts, Laurasia and Gondwanaland. These drifted apart and subsequently broke themselves up to give the present-day northern and southern continents respectively. This whole process was called CONTINENTAL DRIFT. More direct evidence for such movements of the continents across the face of the Earth comes from the direction of the permanent magnetism of the rocks, which shows that in many places there has been a substantial change in latitude since the rocks were laid down. There is also the existence of coal deposits, which form in tropical areas, in the now frozen wastes of Antarctica.

This simple theory of continental drift has now been superseded by PLATE TECTONICS since it is now realized that the crust under the oceans is involved just as much as the continents. The present theory says that the entire surface of the Earth is divided into a series of internally rigid, but relatively thin (about 100km) PLATES which together form the lithosphere. There are at least fifteen plates of various sizes but most of the Earth’s surface is accounted for by seven large plates. A plate may include both oceanic and continental areas. The plates are in continuous motion and this causes virtually all the earthquake, seismic, volcanic and mountain-building activity. This activity marks the active zones of the Earth’s crust and closely follows the boundaries of the plates. There are four principal types of boundary between plates. At spreading ridges the plates are separating and material rises from the mantle to fill the gap and form a ridge of new plate material. These ridges mainly occur in the centers of oceans and in places reach the ocean surface to form volcanic islands. At subduction zones two plates are moving towards one another and one is consumed. They often occur at the edge of a continent and the plate carrying oceanic crust is forced under the continental plate and down into the mantle where it is destroyed. The crustal material is less dense than the mantle and so it rises back towards the surface. Some breaks through to the surface to form a string of volcanoes, such as those in the Andes towards the of volcanoes, such as those in the Andes towards the western coast of South America. In other places the subduction zone is under the ocean and a string of volcanic islands is formed such as the Aleutian, Kurile, Japanese and Marianas islands in the northwest Pacific. Ultimately an ocean can be completely destroyed by a subduction zone so that two continents come into collision and a collision zone is formed. Some material is still forced into the mantle but some is pushed upwards to form a chain of mountains, such as the Himalayas. .Finally there are transform faults where two plates are simply gliding past one another with no formation or destruction of plate material. These occur at intervals along spreading ridges to form a series of offsets along the plate boundary.

The molten material ejected by volcanoes is called MAGMA. Although the mantle below the lithosphere is semi-molten so that the slow convection currents responsible for driving the plate motions are possible, it is not magma. The heat-flow in the mantle forms magma preferentially at the disturbed areas along plate boundaries. Only some of this is released to the surface by vol¬canoes ; the rest cools and solidifies underground to form rocks such as granite.

Venus has almost the same size and density as the Earth so it is quite possible that tectonic processes similar to those on the Earth are taking place. Radar mapping is the only technique used so far. To study large-scale surface features and the best maps have a resolution of about ten kilometers. This makes it difficult to distinguish impact craters from the results of tectonic activity but it is most probable that both are present. The best evidence of tectonic activity is a linear trough, 1500km long, that runs northeast-southwest across the equator. This can be compared to the East African rift valley on the Earth which is known to have- been caused by crustal movement. Other features found on Venus are a large volcano and mountain areas. The volcano is about 300km across and one kilometer high with a central crater 80 km across.

Mars,too, shows similar evidence of tectonic activity. It has the largest volcano in the Solar System. Olympus Mons, which is about 600km across and whose summit rises 25km above the surrounding terrain! Near the summit are a number of calderas which were once the vents for the molten lava that built up the volcano. The volcano forms a shield whose edge is marked by an escarpment two kilometers high; presumably this cliff face was formed by erosion of the softer surrounding rocks to leave the harder volcanic rock. Close to Olympus-Mons are four more large shield volcanoes (one of them greatly eroded) located along the Tharsis ridge. It seems that tectonic activity lifted the edge of a plate by about ten kilometers to form this ridge and that the shield volcanoes formed at places of particularly high stress. On the Earth, plate movements are mainly horizontal but on Mars there is no sign of such movements and the plates can only move vertically. This difference may account for the gigantic size of the shield volcanoes on Mars. There is also a giant canvon. the Valles Marineris (also known as Coprates Canyon) which is 4000km long, 150km or more wide and 2-3km deep. This looks like a rift valley and is therefore further evidence of tectonic activity.

The driving force for tectonic activity is the convection in the asthenosphere, the semi-molten layer of the mantle below the rigid lithosphere. If a planet has a sufficiently thin lithosphere then the convection currents can break it into plates and move them around. The Moon has a vast lithosphere and tectonic activity there is quite impossible. Although Mercury has an Earth-like interior, its surface is very like the Moon and it too does not appear to have present-day tectonic activity.

Comments are closed.