Plate Tectonic Theory

For several centuries it was believed that the continents were fixed permanently in their place. In 1965, an English scientist, Sir Edward Bullard using computer tested the fit of Africa and America and found that at a depth of 2000 meters the match was close indeed. It was then proved that continents are continually drifting away from each other on the surface of the earth like logs floating on water. This is called continental drift. Seabed is also recycled every 200 million years. Magma (molten rock) rises from inner layers of the earth at certain sites on ocean floor called ocean ridges.

The mid _Atlantic Ridge represents the boundary between two huge blocks of crystal material. The magma solidifies and moves outwards before being swallowed up at sites called ocean trenches. Thus new crust is being created by volcanic material welling up from below and spreading outwards. This process is called sea floor spreading and the boundary is called a constructive margin. These margins lie along ocean ridges. Since, the earth is not getting bigger this plates and are carried by their apart the continents. The edge of a plate where it is being destroyed is called destructive plate margin. These lie along ocean trenches.

Here, one block or plate or plate of crystal material is forced beneath another. This movement results in the earth quakes and volcanic activities. The idea of sea floor spreading combined with the idea of continental drift forms a theory what is called plate tectonics.