The movement of tectonic plates is caused by convection currents in the Earth's mantle. The mantle is a layer of hot, semi-solid rock that lies between the Earth's crust and core. Convection currents are created by the heat from the Earth's core, which causes the mantle to flow in a circular motion. As the mantle flows, it drags the tectonic plates along with it.
There are three main forces that drive the movement of tectonic plates:
- Ridge push: This force is caused by the upward movement of hot mantle material at mid-ocean ridges. As the new mantle material rises to the surface, it pushes the older, cooler mantle material away. This creates a ridge push force that moves the tectonic plates apart.
- Slab pull: This force is caused by the sinking of cold, dense mantle material at subduction zones. As the old mantle material sinks back into the Earth, it pulls the tectonic plates along with it.
- Friction: Friction between the tectonic plates also plays a role in their movement. However, friction is a resisting force, so it slows down the movement of the plates.
Let's play with some numbers. Assume that average continental crust weighs in at about 10 cubic feet to the ton. If a ton equals 2000 pounds, this means that each cubic foot of continental crust weighs about 200 pounds. How big is North America?
Since we're just trying to get an "order of magnitude" number here, lets assume that on average it's approximately 2500 miles from east to west, 5000 miles from north to south, and 35 miles thick. Let's see... converting to feet that makes 1.3 X 107 feet times 2.6 X 107 feet times 35 feet which equals 1.2 X 1016 cubic feet, times 200 pounds equals 2.4 X 1018 pounds. In normal numbers this is the same as 2,400,000,000,000,000,000 pounds (2.4 quintillion). Okay, nudge it 2 centimeters to the west, please!
Probably the most logical place to look is inside the earth. We already know that it's pretty hot down there, and we also know that heat and density are related. The short version is that when something is heated up, there is a corresponding decrease in density, and the material rises. This is the reason hot air balloons work, and why the cold water is near the bottom of the lake and not on top.
This process also drives the major wind patterns. Rock is no different, and when heated it lowers in density and wants to go up. So far so good. The problem is that if it rises it's going to leave a void where it was, and we sure can't have that. Just like in the lake (or the atmosphere), water, air, or in this case rock, moves in from the side to fill the hole. But that just leaves another hole, so something has to move in to fill that one, and so on and so on until the stuff which moved in the first place does the filling and the circle is complete.
We call this circular pattern of density driven material a "convection cell," and as I said it's this convection which powers the major global wind patterns, affects the ocean currents... and may well drive the plates. Deep beneath the crust the rock is VERY hot, and while there is probably too much pressure for it to be a liquid, it's still somewhat mobile, and individual atoms and such can kinda/sorta ooze about, setting up convection cells which may carry the plates along where they rub up against the surface crust.
Most geologists accept convection currents as the most likely mechanism for plate motions. One of the biggest questions now relates to how deep within the mantle the convection cells extend. Pictured below is a diagram of "Deep Mantle Convection." Many geologists favor this concept. Many others favor shallow mantle convection. Others postulate multiple layers of convection. And I know 3 big boys in Grants Pass who are still looking for a rope.