Two of the most important geologic features of our planet, that shape our oceans and continents, are Earth’s two types of crusts – oceanic and continental.

Both types float on Earth’s asthenosphere – its semi-molten mantle.

Oceanic crust is thin, averaging just 5 miles thick. It rises out of the mantle at mid-oceanic ridges and sinks below continents at subduction zones.

It’s constantly being recycled like this, and is therefore relatively young. Our most ancient oceanic crust is just 180 million years old.

But the defining quality of oceanic crust is that it’s dense. The minerals that form it are heavier than those of continental crust, meaning it floats lower on Earth’s mantle. The average elevation of oceanic crust is 16,000 feet below sea level.

Continental crust is less dense, so it floats higher. When it contacts oceanic crust, it tends to ride up over it, pushing oceanic crust under and back into the mantle.

As a result, continental crust is recycled much less frequently. It’s much older, piles up much higher, and is 10 times thicker than oceanic crust in places.

Continental crust forms Earth’s land masses, from sea shores to mountain ranges, where land-dwelling creatures live. Without it, Earth might have only sea life.

Earth’s two mobile crusts are unique in our solar system, geologic features that shape life as we know it.