These plates carry the continents, but scientists still do not understand the forces that drive the movement. There are two theories about what pushes the tectonic plates. The first says that heating effects, or 'convection', in the mantle layer directly beneath the crust moves the plates, dragging one plate beneath its neighbour. The second theory says that gravitational forces drive the plates. As rock moves upwards at the mid-ocean ridges between plates, neighbouring plates are squeezed together.
Gravity then drags the heavier plate beneath the other.
A seismogram recorded by a seismograph in Russia shows earth tremors during the massive Turkish earthquake in August, 1999. Recordings of thousands of earthquakes, known as seismograms, from many places around the world have helped scientists to pinpoint the origin and times of each. From these they have built up a kind of X-ray image of the inside of the earth. This has then helped them see how tectonic plates move relative to the mantle below and so identify the direction of the forces involved. North America, it turns out, is moving because the deeper mantle pulls it along.