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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 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.
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