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