The theory of plate tectonics says that the lithosphere is broken into pieces called tectonic plates. The plates have a thin layer of crust above a thin layer of cooler, rigid mantle rock. Most of them have both continental and oceanic crust. These tectonic plates fit together like a jigsaw puzzle. There are about twelve large plates and a few smaller ones that make up the earth's surface. The plates float like rafts on the asthenosphere which is a flowing layer below the stiffer top layer of the earth's mantle called the lithosphere. When one plate moves, it affects all the others. The movement of some of these plates causes sea-floor spreading, collisions, and faults like San Andreas Fault.
There are several types of plate boundaries. Most are found on the ocean floor.
The first type is a divergent boundary. This is where two plates move away from each other. A mid-ocean ridge is an example of this type of boundary. The mid-Atlantic ridge is where new ocean floor is made, causing North America and Europe to be moving apart. When the plates move apart, magma flows up between them. It cools and forms new crust. This is why divergent boundaries are also called constructive boundaries. New ocean floor is being constructed at mid-ocean ridges.
Another type of boundary is called a convergent boundary. This is where plates come together and one plate is pushed under another. These boundaries are also called destructive boundaries since plate material is destroyed by subduction here; in particular, it is stirred back into the mantle. A deep-sea trench is an example of a convergent boundary. Along the edge of the Pacific plate, the largest plate is an area called the Ring of Fire. This is a line of volcanoes that are found along major trenches in that area. As plates collide along convergent boundaries, there is much friction and pressure causing earthquakes to occur. Such earthquakes can be severe. As the plate material is subducted, ocean crust and sediments are dragged down and at several hundred kilometers depth, they break down and release water that was dissolved in their minerals; the water weakens the overlying mantle rock and makes it easy to melt them rises to form volcanoes.
Other boundaries are called transform faults. A fault is a deep crack in the earth's surface. At these fault boundaries, the plates slide past each other. For example, one plate may be moving toward the north while the other moves south. Crust is neither produced nor destroyed at these boundaries. Most of these boundaries are found on the ocean floor.