Earth’s crust

geology
Also known as: crust

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Assorted References

  • magnetization

composition and structure

    chemical elements

    • geochemical cycle
      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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    • geochemical cycle
      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
      • periodic table
        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
      • 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
      • 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
    • phosphorus
      • 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
      • 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
        • Figure 1: Schematic diagram showing ordered (left) and disordered (right) arrays within a structure having two kinds of sites (type 1 and type 2) and two types of occupants (x atoms and y atoms). In the ordered structure all x atoms are distributed uniformly in the spaces between the y atoms, whereas in the disordered structure no regular arrangement obtains.
          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
      • sedimentary rocks
        • chemistry of 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
        • Smoky quartz from St. Gotthard, Switz.
          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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      geologic history

        • Archean Eon
          • 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
        • geochronology
          • Morrison Formation
            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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          • Morrison Formation
            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
          • The Bahamas
            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

          research

            oceanic crust, the outermost layer of Earth’s lithosphere that is found under the oceans and formed at spreading centres on oceanic ridges, which occur at divergent plate boundaries.

            Oceanic crust is about 6 km (4 miles) thick. It is composed of several layers, not including the overlying sediment. The topmost layer, about 500 metres (1,650 feet) thick, includes lavas made of basalt (that is, rock material consisting largely of plagioclase [feldspar] and pyroxene). Oceanic crust differs from continental crust in several ways: it is thinner, denser, younger, and of different chemical composition. Like continental crust, however, oceanic crust is destroyed in subduction zones.

            The lavas are generally of two types: pillow lavas and sheet flows. Pillow lavas appear to be shaped exactly as the name implies—like large overstuffed pillows about 1 metre (3 feet) in cross section and 1 to several metres long. They commonly form small hills tens of metres high at the spreading centres. Sheet flows have the appearance of wrinkled bed sheets. They commonly are thin (only about 10 cm [4 inches] thick) and cover a broader area than pillow lavas. There is evidence that sheet flows are erupted at higher temperatures than those of the pillow variety. On the East Pacific Rise at 8° S latitude, a series of sheet flow eruptions (possibly since the mid-1960s) have covered more than 220 square km (85 square miles) of seafloor to an average depth of 70 metres (230 feet).

            Below the lava is a layer composed of feeder, or sheeted, dikes that measures more than 1 km (0.6 mile) thick. Dikes are fractures that serve as the plumbing system for transporting magmas (molten rock material) to the seafloor to produce lavas. They are about 1 metre (3 feet) wide, subvertical, and elongate along the trend of the spreading centre where they formed, and they abut one another’s sides—hence the term sheeted. These dikes also are of basaltic composition. There are two layers below the dikes totaling about 4.5 km (3 miles) in thickness. Both of these include gabbros, which are essentially basalts with coarser mineral grains. These gabbro layers are thought to represent the magma chambers, or pockets of lava, that ultimately erupt on the seafloor. The upper gabbro layer is isotropic (uniform) in structure. In some places this layer includes pods of plagiogranite, a differentiated rock richer in silica than gabbro. The lower gabbro layer has a stratified structure and evidently represents the floor or sides of the magma chamber. This layered structure is called cumulate, meaning that the layers (which measure up to several metres thick) result from the sedimentation of minerals out of the liquid magma. The layers in the cumulate gabbro have less silica but are richer in iron and magnesium than the upper portions of the crust. Olivine, an iron-magnesium silicate, is a common mineral in the lower gabbro layer.

            The oceanic crust lies atop Earth’s mantle, as does the continental crust. Mantle rock is composed mostly of peridotite, which consists primarily of the mineral olivine with small amounts of pyroxene and amphibole.

            Investigations of the oceanic crust

            Knowledge of the structure and composition of the oceanic crust comes from several sources. Bottom sampling during early exploration brought up all varieties of the above-mentioned rocks, but the structure of the crust and the abundance of the constituent rocks were unclear. Simultaneously, seismic refraction experiments enabled researchers to determine the layered nature of the oceanic crust. These experiments involved measuring the travel times of seismic waves generated by explosions (such as dynamite blasts) set off over distances of several tens of kilometres. The results of early refraction experiments revealed the existence of two layers beneath the sediment cover. More sophisticated experiments and analyses led to dividing these layers into two parts, each with a different seismic wave velocity, which increases with depth. The seismic velocity is a kind of fingerprint that can be attributed to a limited number of rock types. Sampled rock data and seismic results were combined to yield a model for the structure and composition of the crust.

            Study of ophiolites

            Great strides in understanding the oceanic crust were made by the study of ophiolites. These are slices of the ocean floor that have been thrust above sea level by the action of plate tectonics. In various places in the world, the entire sequence of oceanic crust and upper mantle is exposed. These areas include, among others, Newfoundland and the Pacific Coast Ranges of California, the island of Cyprus in the Mediterranean Sea, and the mountains in Oman on the southeastern tip of the Arabian Peninsula. Ophiolites reveal the structure and composition of the oceanic crust in astonishing detail. Also, the process of crustal formation and hydrothermal circulation, as well as the origin of marine magnetic anomalies, can be studied with comparative clarity. Although it is clear that ophiolites are of marine origin, there is some controversy as to whether they represent typical oceanic crust or crust formed in settings other than an oceanic spreading centre—behind island arcs, for example.

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            The age of the oceanic crust does not go back farther than about 200 million years. Such crust is being formed today at oceanic spreading centres. Many ophiolites are much older than the oldest oceanic crust, demonstrating continuity of the formation processes over hundreds of millions of years. Methods that may be used to determine the age of the crustal material include direct dating of rock samples by radiometric dating (measuring the relative abundances of a particular radioactive isotope and its daughter isotopes in the samples) or by the analyses of fossil evidence, marine magnetic anomalies, or ocean depth. Of these, magnetic anomalies deserve special attention.

            A marine magnetic anomaly is a variation in strength of Earth’s magnetic field caused by magnetism in rocks of the ocean floor. Marine magnetic anomalies typically represent 1 percent of the total geomagnetic field strength. They can be stronger (“positive”) or weaker (“negative”) than the average total field. Also, the magnetic anomalies occur in long bands that run parallel to spreading centres for hundreds of kilometres and may reach up to a few tens of kilometres in width.