Earth’s crust | geology | Britannica (original) (raw)

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• magnetization
  • 
    In geomagnetic field: Crustal magnetization
    …of the materials constituting the crust is cool enough for them to exist in solid form. The solids may become magnetized by Earth’s main field and cause detectable anomalies.
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composition and structure

chemical elements

• 
  In chemical element: Solar system
  The chemical composition of Earth’s crust, oceans, and atmosphere can be studied, but this is only a minute fraction of the mass of Earth, and there are many composition differences even within this small sample. Some information about the chemical properties of Earth’s unobserved interior can be obtained by…
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• 
  In chemical element: Early history of the Earth
  …on the composition of the Earth’s crust is available in the form of thousands of analyses of individual rocks, the average of which provides a reasonably precise estimate of the bulk composition. For the mantle and the core the information is indirect and thus much less precise. The origin of…
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• alkali metals
  • 
    In alkali metal
    4 percent of Earth’s crust. The other alkali metals are considerably more rare, with rubidium, lithium, and cesium, respectively, forming 0.03, 0.007, and 0.0007 percent of Earth’s crust. Francium, a natural radioactive isotope, is very rare and was not discovered until 1939.
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• barium
  • 
    In barium: Occurrence, properties, and uses
    03 percent of Earth’s crust, chiefly as the minerals barite (also called barytes or heavy spar) and witherite. Between six and eight million tons of barite are mined every year, more than half of it in China. Lesser amounts are mined in India, the United States, and Morocco.…
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• iridium
  • 
    In iridium
    …nature; its abundance in the Earth’s crust is very low, about 0.001 parts per million. Though rare, iridium does occur in natural alloys with other noble metals: in iridosmine up to 77 percent iridium, in platiniridium up to 77 percent, in aurosmiridium 52 percent, and in native platinum up to…
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• oxygen group elements
  • 
    In oxygen group element: Natural occurrence and uses
    …46 percent of the solid crust of the Earth, and about 89 percent of the water.
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• phosphorus
  • 
    In phosphorus: Occurrence and distribution
    …distributed element—12th most abundant in Earth’s crust, to which it contributes about 0.10 weight percent. Its cosmic abundance is about one atom per 100 atoms of silicon, the standard. Its high chemical reactivity assures that it does not occur in the free state (except in a few meteorites). Phosphorus always…
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• tellurium
  • 
    In tellurium
    …one part per billion of Earth’s crust. Like selenium, it is less often found uncombined than as compounds of metals such as copper, lead, silver, or gold and is obtained chiefly as a by-product of the refining of copper or lead. No large use for tellurium has been found.
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petrology

• feldspar

  • 
    In feldspar
    …up more than half of Earth’s crust, and professional literature about them constitutes a large percentage of the literature of mineralogy.
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• metamorphic rocks

  • 
    In metamorphic rock: Distribution of metamorphic rocks
    More-stable regions of Earth’s crust tend to be covered with sediments, and only deep drilling will reveal the metamorphic rocks below.
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• sedimentary rocks

  • 
    In sedimentary rock
    …sedimentary rocks are confined to Earth’s crust, which is the thin, light outer solid skin of Earth ranging in thickness from 40–100 kilometres (25 to 62 miles) in the continental blocks to 4–10 kilometres in the ocean basins. Igneous and metamorphic rocks constitute the bulk of the crust. The total…
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• silica mineral

  • 
    In silica mineral: General considerations
    …up approximately 26 percent of Earth’s crust by weight and are second only to the feldspars in mineral abundance. Free silica occurs in many crystalline forms with a composition very close to that of silicon dioxide, 46.75 percent by weight being silicon and 53.25 percent oxygen. Quartz is by far…
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• Antarctica

  • 
    In Antarctica: Structural framework
    …average thickness of the terrestrial crust for both East and West Antarctica approximates that of other continents. Although it has been postulated that West Antarctica might be an oceanic island archipelago if the ice were to melt, its crustal thickness of about 20 miles indicates an absence of oceanic structure.…
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• crust–mantle model

  • In crust–mantle model
    …the phenomena observed about the crust, the mantle, and their interface. Many years ago, seismic evidence showed a discontinuity, called the Mohorovičić Discontinuity, anywhere from 3 to 60 kilometres (about 2 to 40 miles) beneath the Earth’s surface. The model used to explain this discontinuity and the nature of volcanic…
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• lithosphere

  • 
    In Earth: The outer shell
    …rocky layer is called the crust. It is composed of low-density, easily melted rocks; the continental crust is predominantly granitic rock (see granite), while composition of the oceanic crust corresponds mainly to that of basalt and gabbro. Analyses of seismic waves, generated by earthquakes
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• North America

  • 
    In North America: General considerations
    …the lithosphere is called the crust. It is composed of low-density material crystallized from molten rock (magma) produced by partial melting of the lithosphere or asthenosphere. The average thickness of the oceanic crust is about 4 miles (6.4 km). Oceanic plateaus and seamounts are localized areas of abnormally thick oceanic…
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geologic history

• Archean Eon
  • 
    In Archean Eon
    …with the formation of Earth’s crust and extended to the start of the Proterozoic Eon 2.5 billion years ago; the latter is the second formal division of Precambrian time. The Archean Eon was preceded by the Hadean Eon, an informal division of geologic time spanning from about 4.6 billion to…
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• atmosphere development
  • 
    In evolution of the atmosphere: The atmosphere as part of the crust
    To the Earth scientist, the crust includes not only the top layer of solid material (soil and rocks to a depth of 6 to 70 km [4 to 44 miles], separated from the underlying mantle by differences in density and by susceptibility to surficial geologic processes) but also the hydrosphere…
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• geochronology
  • 
    In dating: Multiple ages for a single rock: the thermal effect
    …evident that many parts of Earth’s crust have experienced reheating temperatures above 300 °C—i.e., reset mica ages are very common in rocks formed at deep crustal levels. Vast areas within the Canadian Shield, which have identical ages reflecting a common cooling history, have been identified. These are called geologic provinces.…
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  • 
    In dating: Rhenium–osmium method
    … and extremely depleted in the crust, so that crustal osmium must have exceedingly high radiogenic-to-stable ratios while the mantle values are low. In fact, crustal levels are so low that they are extremely difficult to measure with current technology. Most work to date has centred around rhenium- or osmium-enriched minerals.…
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  • 
    In geologic history of Earth: The pregeologic period
    The earliest thin crust was probably unstable and so foundered and collapsed to depth. This in turn generated more gravitational energy, which enabled a thicker, more stable, longer-lasting crust to form. Once Earth’s interior (or its mantle) was hot and liquid, it would have been subjected to large-scale…
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• oceans
  • 
    In seawater: Chemical evolution of seawater
    …an early stage in which Earth’s crust was cooling and reacting with volatile or highly reactive gases of an acidic reducing nature to produce the oceans and an initial sedimentary rock mass. This stage lasted until about 3.5 billion years ago. The second stage was a period of transition to…
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geomorphic processes

• basins and valleys
  • 
    In tectonic basins and rift valleys
    …by an extension of the Earth’s crust and the subsequent dropping of a block of crust into the space created by the divergence of large crustal blocks or lithospheric plates. The extension of the brittle crust causes it to fracture, and as the adjoining crustal blocks or plates move apart,…
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• isostasy
  • In isostasy
    …all large portions of Earth’s lithosphere as though they were floating on the denser underlying layer, the asthenosphere, a section of the upper mantle composed of weak, plastic rock that is about 110 km (70 miles) below the surface. Isostasy controls the regional elevations of continents and ocean floors in…
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• mountains
  • 
    In mountain: Mechanisms that support elevated terrains
    …composition, and therefore density, the Earth’s crust is lighter than the underlying mantle. Beneath the oceans, the typical thickness of the crust is only six to seven kilometres. Beneath the continental regions, the average thickness is about 35 kilometres, but it can reach 60 or 70 kilometres beneath high mountain…
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• oceans
  • 
    In marine ecosystem: Geography, oceanography, and topography
    …theory of plate tectonics, the crust of the Earth is made up of many dynamic plates. There are two types of plates—oceanic and continental—which float on the surface of the Earth’s mantle, diverging, converging, or sliding against one another. When two plates diverge, magma from the mantle wells up and…
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• plate tectonics
  • 
    In plate tectonics: Earth’s layers
    A relatively thin crust, which typically varies from a few kilometres to 40 km (about 25 miles) in thickness, sits on top of the mantle. (In some places, Earth’s crust may be up to 70 km [40 miles] thick.) The mantle is much thicker than the crust; it…
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research

• Bullard
  • In Sir Edward Bullard
    …research on the structure of Earth’s crust and Earth’s internal constitution, he made valuable studies of radioactive heat generation within Earth and of Earth’s thermal history. One of his most important contributions to the study of geomagnetism is his theory of the geomagnetic dynamo, based on convective motion within Earth’s…
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• mass spectrometry
  • 
    In mass spectrometry: Geochronology and geochemistry
    The Earth’s crust is generally richer in oxygen-18 (18O) than is the mantle, as a result of the reaction of these upper-layer rocks with the hydrosphere and atmosphere. This fact allows oxygen-18 to be used to assess the degree to which ascending magmas have incorporated crustal…
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• Mohorovičić
  • In Andrija Mohorovičić
    …discovered the boundary between Earth’s crust and mantle—a boundary subsequently named the Mohorovičić discontinuity.
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• scientific exploration
  • 
    In Earth exploration: Conclusions about the deep Earth
    …led to a spherically symmetrical crust–mantle–core picture of the Earth. The crust–mantle boundary is marked by a fairly large increase in velocity at the Mohorovičić discontinuity at depths on the order of 25–40 kilometres on the continents and five–eight kilometres on the seafloor. The mantle–core boundary is the Gutenberg discontinuity…
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  • 
    In undersea exploration: Exploration of the seafloor and the Earth’s crust
    The ocean floor has the same general character as the land areas of the world: mountains, plains, channels, canyons, exposed rocks, and sediment-covered areas. The lack of weathering and erosion in most areas, however, allows geological processes to be seen more clearly on…
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• seismography
  • 
    In seismograph: Applications of the seismograph
    …to determine the structure of Earth’s crust to a depth of 40–50 km (about 25–30 miles) by detonation of a small amount of explosive.
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  • 
    In earthquake: Structure of the Earth’s interior
    …surrounding the mantle is the crust, whose lower boundary is called the Mohorovičić discontinuity. In normal continental regions the crust is about 30 to 40 km thick; there is usually a superficial low-velocity sedimentary layer underlain by a zone in which seismic velocity increases with depth. Beneath this zone there…
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