Subtitles section Play video Print subtitles It seems a bit odd, but the tectonic plates that cover huge areas of Earth's surface have something in common with your fingernails. Our nails grow at an average rate of about 3 centimeters a year. The tectonic plates move at about the same rate, essentially traveling a few centimeters across Earth's surface every year. In this lesson we will describe the relative motions of plates along different types of plate boundaries and we will explain why we have fast and slow plates. Here's a world map with the major plate boundaries indicated. Could you draw arrows on this map to illustrate the relative motion directions associated with these plate boundaries? We will start you off with a few examples. Can you add some more arrows? We can use our knowledge of plate boundaries to constrain the plate motions. Can you identify where different types of plate boundaries are present? For example, convergent, divergent, or transform plate boundaries. You might have come up with a map that looks something like this. The divergent boundaries are shown in red, blue lines indicate convergent boundaries, and green indicates a transform boundary. Rule #1 is that tectonic plates move away from the oceanic ridges that mark the locations of divergent plate boundaries. And rule #2 is that plates generally move toward oceanic trenches that characterize many convergent boundaries, such as those around the rim of the Pacific Ocean. Now let's look at a map with a little more detail about both the direction of plate motions and also the rates of motion. While the direction of motion relative to the oceanic ridges is consistent in each location, the rates of motion are not. For example, the North Atlantic ocean is opening at a rate of a couple of centimeters or about one inch per year. In contrast, the oceanic ridge that makes up the East Pacific Rise spreads 15 centimeters or 6 inches each year. That is, the Pacific and Nazca plates each move about 7.5 centimeters away from each other each year. Notice too that the Nazca plate is moving toward the trench that marks the western limit of the South American plate. The Nazca plate will actually get a little smaller each year as South America will slowly over-ride its eastern margin, forcing the trench to migrate to the west, steadily reducing the size of the Nazca plate. A similar process is taking place in other locations. We see the same thing in the Indian Ocean where the Indian-Australian plate migrates toward the Java trench. And in the western Pacific Ocean, where the Pacific plate is consumed beneath the Kurile and Mariana trenches. Elsewhere, relative plate motions are essentially parallel to portions of plate boundaries defined by transform faults. We see examples of this along the margins of the smaller Caribbean plate, along transform segments in ocean ridges, and along the San Andreas fault separating the North American and Pacific plates. The Nazca and Pacific plates move faster than other plates. While the plates around the Atlantic Ocean move relatively slowly. This helps explain why the Pacific Ocean basin is wider than the Atlantic. But what's the explanation for this contrast in rates? Before we figure out why some plates are fast and some are slow, think about why plates move at all. Recall that plates are in motion as a result of convection currents in the mantle. Convection releases internal heat left over from the planet's formation and heat generated by the decay of radioactive elements in the core and mantle. OK, so we have convection, but why do some plates move faster than others? The answer to that question brings us back to our consideration of the differences between the South American and Nazca plates. In particular, we are interested in the structure of the plates and the processes operating along their margins. Every major plate is bounded on at least one side by an oceanic ridge. The opposite side of several plates is often defined by an oceanic trench. Some plates descend into the mantle below the trench at a subduction zone, while others over-ride the trench. For example, the Nazca plate is consumed in a subduction zone that is over-ridden by the western margin of the South American plate. The Nazca plate descends into the subduction zone while the South American plate remains at the surface. All major plates form at oceanic ridges and are pushed outward and downward from the high elevations by a process we call ridge push. After traveling along the seafloor for millions of years, the slab of cold dense lithosphere is then pulled down into the subduction zone. Think of the slab of lithosphere acting almost like a giant weight, pulling the plate behind it down into the mantle by a process we label slab pull. Plates that are driven by ridge push alone tend to move slowly, while plates with a combination of ridge push and slab pull have faster rates of motion. So a final examination of the plate map allows us to identify fast moving plates like Nazca and the Pacific plate. These relatively rapid motions are due to the combination of the push forces from the ridge and pull forces along the opposing convergent boundaries. In contrast, slow moving plates, such as the North, and South American plates, only experience the push force from the oceanic ridge in the Atlantic Ocean. Both plates have active margins associated with trenches but they over-ride these margins and are not attached to a down-going slab, so they don't have the benefit of slab pull. So we had two learning objectives for today. How confident are you that you could complete both of these tasks?