What is the origin of plate tectonics? The continents drift slowly (the timescale for substantial change is 10-100 million years), but that they drift at all is remarkable. The following figure illustrates the structure of the first 100-200 kilometers of the Earth's interior, and provides an answer to this question.
The crust is thin, varying from a few tens
of kilometers thick beneath the continents to to less than 10 km thick beneath
the many of the oceans. The crust and upper mantle together constitute the
lithosphere, which is typically 50-100 km thick and is broken into large plates
(not illustrated). These plates sit on the aesthenosphere.