Because of its flexibility, the earths surface, like the plastic, can be bent, but at a certain point, the stress will cause a break. The stress is eased when the sheet snaps back. For many years the theory was advanced that the cracking of the earths crust was caused by the shrinking of the earth as it cooled. Today, geologists have a completely different theory. Some of us during our childhood may have yearned to explore the mysteries of the earth by digging a tunnel all the way to China.
Scientist point out that the outer crust of the earth is extremely thin like the shell of an egg.
It varied from three to thirty miles in thickness. This outer crust is thinner under oceans and thicker under continents. Under huge mountain chains, there is undoubtedly an extremely thick crust. However, there is apparently no connection between the relative thickness or thinness of the crust and the location of earthquakes.