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 that the Earth's outer layer is made of two grand categories of rocks: basaltic and granitic. Basaltic rocks underlie the seafloors and granitic rocks make up the continents. We know that the seismic velocities of these rock types, as measured in the lab, match those seen in the crust down as far as the Moho, so we're pretty sure that the Moho marks a real change in rock chemistry. The Moho isn't a perfect boundary, because some crustal rocks and mantle rocks can masquerade as the other, but even so everyone who talks about the crust, whether in seismological or petrological terms, fortunately means the same thing.
In general, then, there are two kinds of crust, oceanic crust (basaltic) and continental crust (granitic).
Oceanic crust covers about 60 percent of the Earth's surface. Oceanic crust is thin and young—no more than about 20 km thick and no older than about 180 million years. Everything older has been pulled underneath the continents by subduction. Oceanic crust is born at the midocean ridges, where plates are pulled apart. As that happens, the pressure upon the underlying mantle is released and the peridotite there responds by starting to melt. The fraction that melts becomes basaltic lava, which rises and erupts while the remaining peridotite becomes depleted.
The midocean ridges migrate over the Earth like Roombas, extracting this basaltic component from the peridotite of the mantle as they go. This works like a chemical refining process. Basaltic rocks contain more silicon and aluminum than the peridotite left behind, which has more iron and magnesium. Basaltic rocks are also less dense. In terms of minerals, basalt has more feldspar and amphibole, less olivine and pyroxene, than peridotite. In geologist's shorthand, oceanic crust is mafic while oceanic mantle is ultramafic.
Oceanic crust, being so thin, is a very small fraction of the Earth—about 0.1 percent—but its life cycle serves to separate the stuff of the upper mantle into a heavy residue and a lighter set of basaltic rocks. It also extracts the so-called incompatible elements, which don't fit into mantle minerals and move into the liquid melt. These in turn move into the continental crust as plate tectonics proceeds. Meanwhile, the oceanic crust reacts with seawater and carries some of it down into the mantle.
Continental crust is thick and old—on average about 50 km thick and about 2 billion years old—and it covers about 40 percent of the planet. Whereas almost all of the oceanic crust is underwater, most of the continental crust is exposed to the air.
The continents slowly grow over geologic time as oceanic crust and seafloor sediments are pulled beneath them by subduction. The descending basalts have the water and incompatible elements squeezed out of them, and this material rises to trigger more melting in the so-called subduction factory.
The continental crust is made of granitic rocks, which have even more silicon and aluminum than the basaltic oceanic crust; they also have more oxygen thanks to the atmosphere. Granitic rocks are even less dense than basalt. In terms of minerals, granite has even more feldspar, less amphibole than basalt and almost no pyroxene or olivine, plus it has abundant quartz. In geologist's shorthand, continental crust is felsic.
Continental crust makes up less than 0.4 percent of the Earth, but it represents the product of a double refining process, first at midocean ridges and second at subduction zones. The total amount of continental crust is slowly growing.
The incompatible elements that end up in the continents are important because they include the major radioactive elements uranium, thorium and potassium. These create heat, which makes the continents act like electric blankets on top of the mantle. The heat also softens thick places in the crust, like the Tibetan Plateau, and makes them spread sideways.
Continental crust is too buoyant to return to the mantle. That's why it is, on average, so old. When continents collide, the crust can thicken to almost 100 km, but that is temporary because it soon spreads out again. The relatively thin skin of limestones and other sedimentary rocks tend to stay on the continents, or in the ocean, rather than return to the mantle. Even the sand and clay that is washed off into the sea returns to the continents on the conveyor belt of the oceanic crust. Continents are truly permanent, self-sustaining features of the Earth's surface.
What the Crust Means
The crust is a thin but important zone where dry, hot rock from the deep Earth reacts with the water and oxygen of the surface, making new kinds of minerals and rocks. It's also where plate-tectonic activity mixes and scrambles these new rocks and injects them with chemically active fluids. Finally, the crust is the home of life, which exerts strong effects on rock chemistry and has its own systems of mineral recycling. All the interesting and valuable variety in geology, from metal ores to thick beds of clay and stone, finds its home in the crust and nowhere else.