But as the rock cooled and solidified, the particles aligned themselves with the earth's magnetic field, like tiny compass needles. Thus, the particles continue to point in the direction of the magnetic field that was present during the time that the rock cooled. So when the plate containing the rock either drifts to a different latitude or rotates, the particles no longer align with the earth's magnetic field. A comparison of the direction in which the particles now point in the rock with the direction of the earth's present magnetic field provides information about where the plate was when the rock solidified. Causes of plate movement.
Tectonic plates slide mostly because of temperature changes and gravity.
As an edge that has formed on the ocean floor cools, it shrinks, becoming denser. After about 25 million years of cooling and shrinking, the edge becomes so dense that gravity can pull it down into the asthenosphere. There, the intense heat and increased pressure due to the great depth change the crust of the sunken plate edge into even denser rock. Because of the additional density, gravity pulls the plate edge into the asthenosphere even more strongly. This sinking action is known as slab-pull because the sinking edge pulls the remainder of the slablike plate behind it.