The crust is the outermost layer making up the solid Earth. It is distinguished from the underlying mantle layer by its more silicon- and aluminium-rich composition, lower density, and the lower velocity at which it conducts seismic energy. Most of the crust can be classified either as ‘continental’ or ‘oceanic’ (see below). The crust beneath Antarctica is continental, but it is surrounded by the oceanic crust of the Southern Ocean.
Oceanic crust underlies most of the two-thirds of the Earth’s surface which is covered by the oceans. It has a remarkably uniform composition (mostly 49% ± 2% SiO2) and thickness (mostly 7 ± 1 km). The ocean floor is the most dynamic part of the Earth’s surface. As a result, no part of the oceanic crust existing today is more than 200 million years old, which is less than 5% of the age of the Earth itself. New oceanic crust is constantly being generated from the upper mantle by sea-floor spreading at mid-ocean ridges, while other parts of the oceanic crust are being recycled back into the mantle at subduction zones. Maps of oceanic crustal ages have been produced as a result of studies of the record of reversals of the Earth’s magnetic field, which are preserved in the crust as it forms.
Continental crust is more silica-rich and thicker than oceanic crust, and by virtue of generally surviving longer, is on average older. However, it is highly variable in all of these respects. The average thickness of the continental crust is about 40km, but beneath parts of the Andes and the Himalaya mountain ranges the crust is more than 70km thick. The oldest parts of the continental crust, known as ‘shields’ or ‘cratons’, include some rocks that are nearly 4 billion years old. Most of the rest of the continental crust consists of the roots of mountain belts, known as ‘orogens’, formed at different stages in Earth history. Over large areas, however, these orogens are covered by younger sedimentary rocks.
Ironically, new continental crust is produced by the destruction of oceanic crust at subduction zones, a process that continues today. Continental crust is not immortal, however: it is continuously being eroded and turned into sediment. Some of this sediment ends up on the ocean floor where it can be returned to the mantle at subduction zones.
Beneath the Crust
Beneath the crust lies the mantle, a thick dense layer that extends 2700km down to the Earth’s outer core. The boundary between the crust and the mantle is known as the ‘Mohoroviçic discontinuity’, or ‘moho’, after the seismologist who first discovered it. Contrary to popular myth, the mantle material beneath the moho is not generally molten or even partially molten. The mantle only becomes partially molten in special circumstances such as occur mid-ocean ridges, subduction zones or ‘hotspots’.
The crust is firmly attached to the uppermost part of the mantle and together they make up a rigid layer known as the ‘lithosphere’. The rigid surface of the Earth is made up of ‘plates’ of lithosphere which move relative to one another and relative to the underlying part of the mantle, known as the ‘asthenosphere’. The asthenosphere is also solid, but over millions of years it deforms in a manner similar to Plasticine (although it is actually many times more viscous).
Hotspots are areas of the Earth’s surface, away from tectonic plate boundaries, that have a high rate of basaltic volcanism. They are thought to be underlain by hot plumes of mantle material rising from deep within the Earth. Hotspots are responsible for the volcanic activity we see in Hawaii and Iceland today and have been implicated in the break-up of continents in the past.