The Earth’s crust is the outermost, thin shell of our planet. This layer is entirely in a solid state of matter, composed of various rocks and minerals. The crust varies significantly in thickness, ranging from 5 kilometers beneath the oceans to over 70 kilometers in mountainous continental regions.
The Rigid State of the Earth’s Crust
The crust maintains its solid form because the rocks that compose it are rigid and brittle. The materials are primarily silicate rocks, which have relatively high melting points. While temperature increases with depth, the pressure exerted by the overlying rock is substantial and keeps the material tightly bound.
This rigidity means the crust fractures and breaks when subjected to stress, which is the mechanism behind earthquakes. The base of the crust can reach temperatures between 700 and 1,600 degrees Celsius, but the materials remain solid because the pressure prevents the crystal structure from breaking down into a liquid state.
Oceanic and Continental Composition
The solid crust is structurally divided into two distinct types: continental and oceanic. Continental crust, which underlies the landmasses, is thicker, with an average depth of about 40 kilometers, but can extend up to 90 kilometers beneath mountain ranges. This crust is primarily composed of granitic rock, a felsic material rich in silica and aluminum, and has a relatively low density, allowing it to float higher on the mantle. Oceanic crust, found beneath the ocean basins, is significantly thinner, measuring only 5 to 10 kilometers thick, and is made up of denser mafic rocks like basalt and gabbro, which contain higher concentrations of iron and magnesium.
Separating the Crust from the Mantle
The crust ends abruptly at a boundary known as the Mohorovičić discontinuity, or Moho. This boundary marks a shift in rock composition and density between the crust and the underlying mantle. The Moho is found at depths ranging from 5 kilometers beneath the oceans to around 35 kilometers beneath the continents.
The layer immediately beneath the Moho is the upper mantle, which is also solid but behaves differently from the crust. The crust and the uppermost, brittle part of the mantle together form the lithosphere. Below this rigid section lies the asthenosphere, a region of the upper mantle that is highly viscous and described as plastic. This plastic region is solid rock under immense pressure, but the high temperatures allow it to deform and flow very slowly over geologic timescales, which allows the tectonic plates to move across the Earth’s surface.