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  • The idea that our planet’s continents drift around the globe, periodically glomming together

  • and breaking apart, is at least 200 years old. But most geologists didn’t believe

  • it until the 1960’s, when mounting evidence made it clear that the Earth’s crust is

  • broken up into fragments, and that those fragments, called tectonic plates, are moving. And these

  • days we directly track that motionwith millimeter precisionfrom space.

  • The common, simplified explanation for why tectonic plates are moving is that theyre

  • carried along on currents in the upper mantle, the slowly flowing layer of rock just below

  • Earth’s crust. Converging currents drive plates into each other; diverging currents

  • pull them apart.

  • This is mostly true; hot mantle rock rises from the core and moves along under the crust

  • until it grows cool and heavy and sinks back down again. But the plates aren't just passively

  • riding these conveyer-belt-like currents around like a bunch of suitcases at the baggage claim.

  • They can’t be, because some of the plates are moving faster than the currents underneath

  • them. For example, the Nazca plate – a chunk of ocean crust off the west coast of South

  • Americais cruising eastward at about 10cm per year, while the mantle underneath

  • it oozes along at just five. Neither tectonic plates nor luggage can move faster than the

  • belt theyre riding on unless something else is helping to push or pull them along.

  • And some of Earth’s plates, it turns out, are pulling themselves.  When an ocean plate

  • collides with another ocean plate or a plate bearing the thick crust of continental landmasses,

  • the thinner of the two plates bends and slides under the other. As the edge of the seafloor

  • sinks into the mantle, it pulls on the plate behind it, the same way a chain dangling further

  • and further off a table will eventually start to slide. The bigger the sunken portion of

  • the plate becomes, the harder it pulls and the faster the remaining plate behind it moves.

  • You can find where this is happening by looking at google earththe incredibly deep, narrow

  • ocean trenches visible off the coasts of some continents and island chains mark the creases

  • formed as ocean crust plunges downward, bending the edge of its neighbor in the process.

  • What’s more, those chunks of seafloor are actually helping to drive convection in the

  • mantle beneath them. Sunken slabs of ocean crust block flowing rock from moving further

  • sideways, forcing it to turn downward and sink. Eventually those slabs get too heavy

  • and break off, plunging slowly toward the core and creating a suction force that pulls

  • mantle material along behind it. So, in some ways, seafloor crust really is more like part

  • of the conveyor belt than something riding on top of it. The continents, on the other

  • hand, are baggage.

The idea that our planet’s continents drift around the globe, periodically glomming together

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プレートテクトニクスの説明 (Plate Tectonics Explained)

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    稲葉白兎 に公開 2021 年 01 月 14 日
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