The surface of the Earth is broken up into "tectonic plates" that move in different directions and speeds (Figure 1). The plates are made of the

To study present-day plate motions, we can anchor GPS instruments firmly in bedrock and measure how they move along with the tectonic plate. Each arrow in Figure 3 shows the

Tectonic plates are on a globe, not on a flat surface. Each plate rotates about a "pole" and each plate also has a different speed. The farther you are from the pole of rotation, the bigger the speed of your GPS station. By observing tectonic motion with GPS instruments, we can learn what is pushing and pulling at the Earth's surface—and what happens when it does.

It is a little complicated to calculate the motion of a tectonic plate. But you can understand the motion of a tectonic plate if you think about a simple two-dimensional case. In Figure 5 we have a Ferris wheel. It is rotating counterclockwise. How fast it rotates is called its

You can think of a tectonic plate the same way (Figure 6). The speed of any point on the plate is the product of the angular speed

A final thing to point out - all the motion takes place in this two-dimensional space, which means there is no vertical motion.

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