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The Earth's crust is an extremely thin layer of rock, like the skin of an apple in relative terms. It amounts to less than half of 1 percent of the planet. But the crust is exceptionally important, and not just because we live on it. The crust can be thicker than 80 kilometers in some spots, less than one kilometer in others. Underneath it is the mantle, a layer of rock some 2700 kilometers thick that accounts for the bulk of the Earth. The crust is primarily made of granite and basalt while the mantle beneath is made of peridotite. More about all that below. How We Know the Earth Has a Crust
Just a century ago, we didn't know the Earth has a crust. Until the 1900s all we knew was that our planet wobbles in relation to the sky as if it had a large, dense core. Astronomical observations told us so. Then along came seismology, which brought us a new type of evidence from below: seismic velocity, or the speed of sound in rock as measured using seismic waves from earthquakes. In 1909 a paper by the seismologist Andrija Mohorovicic established that about 50 kilometers deep in the Earth there is a sudden change in seismic velocity—a discontinuity of some sort. Seismic waves bounce off it (reflect) and bend (refract) as they go through it, the same way that light behaves at the discontinuity between water and air. That discontinuity, named the Mohorovicic discontinuity or "Moho," is the accepted boundary between the crust and mantle. Crusts and Plates
The crust is not the same thing as the plates of plate tectonics. Plates are thicker than the crust and consist of the crust and the shallow mantle just beneath it; the two-layered combination is stiff and brittle and is called the lithosphere ("stony layer" in scientific Latin). The lithospheric plates lie on a layer of softer, more plastic mantle rock (the asthenosphere or "weak layer") that allows the plates to move slowly over it like a raft in thick mud. We know...
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