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  • Hi, this is Emily from MinuteEarth.

  • From the tip of Mt. Everest to the bottom of the Mariana trench, elevations on Earth

  • span over 65000 feet.

  • But we all know that those extreme elevations are super rare, and that the vast majority

  • of our planet's surface falls somewhere in the middle range So it seems like, if you

  • added up the points at the very top, and then a little lower, and all the way down to the

  • bottom you'd end up with something like this – a simple, normal distribution, with

  • very little area at the top and bottom, and a big hump in the middle near sea level.

  • And if we were talking about our sister planet Venus, we'd be rightthis is what the

  • elevation-distribution curve looks like for Venus.

  • But Earth's elevation is distributed like this: it has TWO humpsso, very few points

  • very high and very low, but a lot of points around sea level and then, again, a lot of

  • points several thousands feet below.

  • And if you took all of those points and arranged them from lowest to highest, you could see

  • that Earth's surface has two sort of levels to it.

  • The main reason is that Earth's outer layer, the crust, is made up of two different materials:

  • it's all rock, but the rock that makes up the seafloor is denser than the rock that

  • makes up the continents.

  • And this density difference has major consequences, because ocean floors and continents don't

  • just sit theretogether with the rocky mantle below them, they're broken up into

  • big plates that ride around on convection currents flowing deep inside of Earth.

  • And when two plates collide, the outcome is pretty much determined by density: oceanic

  • plates are denser than continental onesin fact, they're so dense that when the two

  • plates collide, the oceanic one sinks back down into the planet.

  • On the other hand, when two continents collide, neither is dense enough to go down, so instead

  • they both go up, creating mountain ranges and thickening up the continental crust.

  • As a result, the continental crust is, on average, about four times thicker than the

  • ocean crust, so its average elevation is much higher than the average elevation of the ocean

  • floor, which explains this weird, double-humped elevation-distribution curve.

  • And actually, if our planet's surface was more like Venus's, with it's so-called

  • normal distribution, only about 5% of itan area a little smaller than Africawould

  • be above sea level, leaving not much space for landlubbers like us.

  • Just one more reason to be glad there's nothing normal about Earth.

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B1 中級 美國腔

为什么地球有两个层次(Why Earth Has Two Levels | Hypsometric Curve)

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    joey joey 發佈於 2021 年 04 月 30 日
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