Why Earth Has Two Levels | Hypsometric Curve


This video is sponsored by NordVPN. 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 right – this is what the elevation-distribution curve looks like for
Venus. But Earth’s elevation is distributed like
this: it has TWO humps – so, 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 there – together 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 ones – in
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 it – an
area a little smaller than Africa – would 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. Have you ever had that experience where some
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