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• 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