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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