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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

  • both of these tasks?

It seems a bit odd, but the tectonic plates that cover huge areas of Earth's surface have

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B2 US plate nazca oceanic slab pacific ridge

Rates of Plate Motions

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    羅紹桀 posted on 2015/11/12
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