Convection Currents:The mantle is made of much denser, thicker material, because of this the plates "float" on it like oil floats on water.
Many geologists believe that the mantle "flows" because of convection currents. Convection currents are caused by the very hot material at the deepest part of the mantle rising, then cooling, sinking again and then heating, rising and repeating the cycle over and over. The next time you heat anything like soup or pudding in a pan you can watch the convection currents move in the liquid.
oceanic crust:is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium. It is thinner than continental crust, or sial, generally less than 10 kilometers thick, however it is denser, having a mean density of about 2.9 grams per cubic centimeter[1] as opposed to Continental which has a density of about 2.7 grams per cubic centimeter
continental cruThe continental crust is the layer of igneous, sedimentary, and metamorphic rocks which forms the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial because there is more felsic, or granitic, bulk composition, which lies in contrast to the oceanic crust, called sima because of the mafic or basaltic rock. (Based on the change in velocity of seismic waves, it is believed that at a certain depth sial becomes close in its physical properties to sima. This line is called the Conrad discontinuity.st: