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.
Discovery of the Crust
Until just a century ago, we didn't know the Earth has a crust. From astronomical measurements, we knew in the late 1800s that Earth wobbles in relation to the sky as if it had a large, dense core. Beyond that we had no clue, until the advent of seismology. Even today, almost all we know for sure about the deep Earth comes from just one type of evidence: the speed of sound in rock as measured using seismic waves, usually called seismic velocity. The rest is known from intricate, subtle modeling studies.
In 1909 the seismologist Andrija Mohorovicic published a paper establishing that about 50 kilometers deep in the Earth there is a sudden change in seismic velocity—a discontinuity of some sort. The discontinuity makes the seismic waves bounce (reflect) and refract (bend), just as light behaves at the discontinuity between water and air. Ever since, that discontinuity, the Mohorovicic discontinuity or "Moho," has been accepted as the boundary between the crust and the mantle beneath.
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 combination is stiff and brittle and is called the lithosphere ("stony layer" in scientific Latin). The lithospheric plates lie on top of a layer of softer, more plastic mantle rock (the asthenosphere or "weak layer") that allows the plates to move slowly upon it like a raft in thick mud.
We know that the Earth's crust is made of two grand categories of rocks: basaltic and granitic. Basaltic rocks underlie the seafloors and granitic rocks make up the continents. The seismic velocities of these rock types in the lab match the velocities in the crust down to 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, the crust has two types, oceanic crust and continental crust.
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 never 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 pressure upon the underlying mantle is released and the peridotite there begins to melt in response. The part 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 the basaltic component from the mantle as they go. What that means has to do with rock chemistry. Basaltic rocks contain more silicon and aluminum than the peridotite left behind, which has more iron and magnesium. Basaltic rocks are 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 refine the rocks of the upper mantle into new rocks with a lighter blend of elements. 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.
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 end 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. They 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. When continents collide, the crust can thicken to almost 100 km, but that is temporary. The limestones and other sedimentary rocks that form on the continents are likewise lighter than basalt. 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.