Rather, these topographic highs are supported in large part by buoyancy, because they are made of rocks which are of lower density than those comprising the underlying mantle. Hence on Earth, the continents float in the mantle, like icebergs floating in the ocean. The rock in the mantle is solid but is nevertheless able to flow over geologic time - albeit very slowly and under great duress. Circulation in the mantle is driven by heat escaping from the interior of the Earth, and it drags the continents around the world at the majestic rate of centimeters per year, causing the so-called continental drift. To be a bit more precise, the continents are carried on rafts which we call plates and which make up the uppermost, relatively cool and rigid, layer of the mantle.
Circulation occurs in the hotter, lower layers of the mantle.
The entire crust of the Earth is carried on plates in relative motion, with thinner crust on plates under the oceans and thicker crust on plates holding the continents. Some plates collide, and these monstrous collisions build mountain ranges, produce earthquakes, and induce volcanism, thereby driving much of the geologic activity on Earth. Plates are continually being recycled, with new plate material forming at ocean ridges and old plate material being pulled down into the mantle at ocean trenches. This picture of moving plates is called plate tectonics, and in this form, it is found only at Earth. However, various types of tectonism, meaning crustal deformations, are found at Mercury, Venus, and Mars as well.