Plate Tectonics

a map of the most important plates of the world.
The seven large plates are named in the picture. The smaller ones are: 1 Nazca Plate, 2 Carribbean Plate, 3 Cocos Plate, 4 Juan de Fuca Plate, 5 Arabian Plate, 6 Indian Plate, 7 Philippine Plate, 8 Scotia Plate

Plate Tectonics is a geologic theory explaining the movements and forces in the crust of Earth. It was developed during the mid 20th century, but the basic ideas were first published by the German geographer Alfred Wegener during the 1920s. He published the first version of his theory in 1915, and he called it Theorie der Kontinentalverschiebung (continental drift). His theories were based on the form of the continents, similarities in fossils and contemporary life on different continents, and much more. However, the theory lacked two basic parts: an explanation for the motor of the drift, and a single impressive circumstantial evidence. The bunch of little pieces he had, were to inconclusive to convince the authorities of geology. But over decades more and more facts were discovered, most important probably the so called sea floor spreading, and so in the fifties a new theory was developed which met all those new discoveries. This is the theory of plate tectonics as we know it today, and although it was adapted to new discoveries in details, the basic idea of the continental drift - the movement of whole continents - was supported by any new find.

Plate Tectonics is the "World Theory" of geology, as it explains most of the important geologic processes, which were enigmatic before. Some say, it has the same importance for geology as has Darwins' evolution theory for biology. However, in science, especially if it is a descriptive science which tries to explore nature, it is a good idea to be careful and keep in mind the difference between facts and theories. Still, plate tectonics is the most important theory in geology and - after half a century of scientific research in this field - a very well supported one.

The basic point in plate tectonic is, that earth's surface is broken into seven large and many small moving plates, hence the name. These plates move relative to one another an average of a few centimeters per year. Three types of movement are recognised at the boundaries between plates: convergent, divergent and transform-fault. This is a rather simple result of the the fact that earth is a sphere with more or less constant diameter. So if two plates move in different direction, they either collide or move away from each other. A special case is when they both move parallel to their boundary, but with different speed.

There are only two types of plates, continental plate, which is higher than sea level and oceanic plate, which is below sea level. The reason is simple: they are composed of different materials. Continetal plate is thick and composed of sediments and volcanic rocks, oceanic crust is thinner and consist only of a certain kind of volcanic rocks. You can throw wood and polystyrol foam into water and you will see: the foam is lighter, so it swims higher. The same is with the plates, they swim in the (more or less) "liquid" upper mantle. Oceanic crust is thinner and heavier, so it swims deeper, continental crust is lighter because of the sedimentary rocks, and that is why it is higher.

  1. Convergent boundaries:
    Plates move toward each other and collide. As we have two kinds of plates, oceanic and continental, there are three theoretically possible combinations:
    1. oceanic plate collides with a continental plate: the oceanic plate tips down and slides beneath the continental plate forming a deep ocean trench. This is called subduction. This happens all along the western coast of South America, at the boundary between the oceanic Nazca Plate and the continental South American Plate.
    2. continental plates collide: the collision presses the rocks from two sides, the easiest way to escape is up and down. This forms major mountain systems which go up several kilometers, but also go down rather deep. The plate becomes twice as thick as normal. Many mountain ranges on earth were formed by this, fo example the Himalaya and the Alps.
    3. oceanic plate collides with a oceanic plate: one of the plates will be subducted below the other, similar to the ocean-continent collision. But on top of the rim of the upper plate the accumulation of sediments from the lower plate and the volcanism caused by the subduction forms a chain of islands. There are only small examples on earth but they are very impressive though, for example the chain of islands of the Philippines is located on such a rim.
  2. Divergent boundaries:
    Plates move away from each other, the gap is filled from below by volcanic material. It often starts inside a continental plate, for example in the Great Rift Valley in Africa, the Dead Sea or the Rheingraben (Rhine Rift System) in Germany. The material on top of the boundary goes down, because the parts of the plate move away, and so it forms a ExplainGraben. Later the graben gets deeper and wider and sea water enters. This stage may be seen at the Red Sea, which is such a young ocean. On the boundary volcanic lava from below forms oceanic plate, a process known as sea-floor spreading. And as oceanic plate is always below the sea level, this rims are always below the sea. A typical example for this stage is the Mid-Atlantic Ridge.
  3. Transform-fault boundaries:
    Plates move horizontally past each other. There is neither production nor consumption of plate material, but numerous earth quakes and probably some volcanism. A typical example is the San Andreas Fault zone where the Pacific Plate is moving slowly northwestward along the North American Plate.

After all those results of plate tectonics, the cause, or better the motor of the movement, is also very interesting. The upper mantle of earth is formed of molten rocks, called magma, which is not really liquid, but it flows extremely slow. Because of the temperature difference between inside and outside, convection currencies formed. Hot material is expanding and thus lighter and so it ascends, here it cools down, shrinks, becomes heavier and goes down again. While flowing horizontally below the crust, these flows introduce a force and thus a movement into the plate. This is the force behind all tectonics in the crust.