crust (geology)

crust (geology)
Name	Crust
Caption	Diagram showing the Earth's crust as part of the lithosphere.
Thickness	5–70 km
Composition	Primarily silicate minerals

crust (geology). In geology, the crust is the outermost solid shell of a terrestrial planet, dwarf planet, or natural satellite. On Earth, it is the thinnest of the planet's major layers, forming the upper part of the lithosphere and resting upon the asthenosphere. The crust is distinct from the underlying mantle by its chemical composition and, on Earth, by the presence of the Mohorovičić discontinuity.

Overview

The crust is the primary interface between the solid Earth and external systems like the hydrosphere, atmosphere, and biosphere. Its formation and modification are driven by planetary-scale processes, most notably plate tectonics on Earth, which facilitates the creation of oceanic crust at mid-ocean ridges and the destruction of crust at subduction zones. The study of the crust's structure and history is fundamental to fields like geology, geophysics, and planetary science, providing insights into planetary differentiation and the evolution of habitable environments.

Composition and types

Earth's crust is chemically distinct from the mantle, being enriched in elements like oxygen, silicon, aluminum, and potassium. It is broadly divided into two principal types: continental and oceanic. Continental crust is primarily composed of granitic and other felsic rocks, is less dense, and is much thicker, averaging about 30–50 km but reaching up to 70 km under major orogenic belts like the Himalayas. Oceanic crust is predominantly composed of basaltic and gabbroic mafic rocks, is denser, and is thinner, typically only 5–10 km thick. The oldest continental crust, found in regions like the Canadian Shield, contains cratonic rocks over 4 billion years old, while the oldest oceanic crust is less than 200 million years old due to continuous recycling.

Formation and evolution

Crustal formation is a consequence of planetary differentiation and partial melting of the mantle. On Earth, new oceanic crust is continuously generated at divergent plate boundaries such as the Mid-Atlantic Ridge through the process of seafloor spreading. Continental crust forms and evolves through more complex processes including the partial melting of oceanic crust at subduction zones, producing magma that rises to form volcanic arcs like the Andes, and through the accretion and collision of terranes. Major events like the Archean and Proterozoic eons saw the assembly of the first continents, culminating in supercontinents like Rodinia and Pangaea.

Physical properties

The crust exhibits significant variation in its physical characteristics. Its density ranges from about 2.7 g/cm³ for continental crust to 3.0 g/cm³ for oceanic crust. The boundary between the crust and the mantle, marked by a sharp increase in seismic wave velocities, is known as the Mohorovičić discontinuity, discovered by Andrija Mohorovičić. The crust's strength and brittle behavior define the lithosphere, which moves atop the ductile asthenosphere. Heat flow from the Earth's interior is higher through oceanic crust due to its youth and proximity to mantle upwelling, influencing phenomena like hydrothermal vents at sites like the East Pacific Rise.

Interaction with other Earth layers

The crust is dynamically linked to the mantle and core through geochemical and geophysical processes. The convection currents in the mantle drive plate tectonics, which in turn shapes the crust. Material is exchanged at subduction zones, where crustal slabs descend into the mantle, and at hotspots like Hawaii, where mantle plumes generate volcanism directly through the crust. The crust also interacts with the outer core and inner core indirectly through the planet's magnetic field, which is generated by the core's dynamics and can influence crustal rocks through paleomagnetism.

Planetary crusts

Crusts are a common feature of other rocky bodies in the Solar System, though their compositions and evolutionary histories differ. The Moon has a crust composed largely of anorthosite, a relic of its early magma ocean. Mars possesses a primary basaltic crust, with major features like the Tharsis bulge and Olympus Mons indicating past volcanic activity, and a stark dichotomy between its northern lowlands and southern highlands. Mercury has a crust heavily cratered and contracted due to core cooling. The study of these extraterrestrial crusts by missions like NASA's Mars Reconnaissance Orbiter and the Lunar Reconnaissance Orbiter provides comparative planetology crucial for understanding the early history of the Earth.

Category:Structure of the Earth Category:Geology Category:Planetary geology