Plate Tectonics
Plate Tectonics-a theory explaining the structure of the earth's crust and many associated phenomena as resulting from the interaction of rigid lithospheric plates that move slowly over the underlying mantle.
Thats the actual definition, but basically what its trying to say is over the mantel layer their is another small layer the lithosphere. The lithosphere is like a lot of broken uneven eggshells, theses eggshells types are called Plate tectonics.
Then the solid plastic layer of the mantel beneath the listhospere. This layer is what allows it to just be free and have a float type motion, sometimes these Plate tectonics might collide and thats how earthquakes form, when they try to go back into place is when they have an earthquake. They move cause the of its so hot down their that hot lava rises and when its at the very top it starts to cool, when it cools it drops and continues the cycle. Because of this the plates are allowed to move freely and might collide from time to time.