earth is a dynamic, ever-changing planet!. if we could go back in time 2000 million years, there would be no Grand Canyon. or Rocky Mountains. Moreover,we would find landmasses with unfamiliar shapes and located in different positions from today's continents. earth scientists have shown that the landmasses are not fixed, but slowly migrate across the globe. large land messes have split apart resulting in the formation of oceans. while the floor of the ocean has been recycled back into earth's interior. further, land messes that once separated by vast oceans have since collided and formed larger continents. the movement of earth's other layer continues today. results of this movement include earthquakes. volcanoes and building of earth's mountains. the modern scientific theory which describes the movement of earth's other layer is called plate tectonics. according to the plate tectonics theory, the uppermost mantle, along with the overlying crust, behave as a strong, rigid layer known as the lithosphere. the lithosphere is thinnest in the oceans where its thickness may very from as little as a few kilometers as the oceanic ridges to 100 kilometers in the deep-ocean basins. by contrast, continental lithosphere is generally 100-150 kilometres thick but may be more than 250 kilo thick below older portions of the continents. this rigid outer shell overlies a hotter, weaker layer in the upper mantle know as the asthenosphere. scientists think that the weak rock within the asthenosphere allows earth's rigid outer shell to move. further, the lithosphere is broken into numerous segment, called lithospheric plates, or simply plates. seven