The earth's crust is divided into seven major plates, that are about 50 miles thick, which move slowly and continuously over the earth's interior and several minor plates. Earthquakes are tectonic in origin; that is the moving plates are responsible for the occurrence of violent shakes. Deserts would cover North America, arid sand dunes would replace expanses of the Amazon rainforest in South America, and lush, green landscapes would flourish from central Africa to the Middle East.