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  • Crust” describes  the outermost shell of a terrestrial planet. Our planet’s thin, 40-kilometer (25-mile) deep crust—just 1% of Earth’s mass—contains all known life in the universe.
     
    Earth has three layers: the crust, the mantle, and the core. The crust is made of solid rocks and minerals. Beneath the crust is the mantle, which is also mostly solid rocks and minerals, but punctuated by malleable areas of semi-solid magma. At the center of the Earth is a hot, dense metal core
     
    Earth’s layers constantly interact with each other, and the crust and upper portion of the mantle are part of a single geologic unit called the lithosphere. The lithosphere’s depth varies, and the Mohorovicic discontinuity (the Moho)—the boundary between the mantle and crust—does not exist at a uniform depth. Isostasy describes the physical, chemical, and mechanical differences between the mantle and crust that allow the crust to “float” on the more malleable mantle. Not all regions of Earth are balanced in isostatic equilibrium. Isostatic equilibrium depends on the density and thickness of the crust, and the dynamic forces at work in the mantle. 
     
    Just as the depth of the crust varies, so does its temperature. The upper crust withstands the ambient temperature of the atmosphere or ocean—hot in arid deserts and freezing in ocean trenches. Near the Moho, the temperature of the crust ranges from 200° Celsius (392° Fahrenheit) to 400° Celsius (752° Fahrenheit).
     
    Crafting the Crust
     
    Billions of years ago, the planetary blob that would become the Earth started out as a hot, viscous ball of rock. The heaviest material, mostly iron and nickel, sank to the center of the new planet and became its core. The molten material that surrounded the core was the early mantle.
     
    Over millions of years, the mantle cooled. Water trapped inside minerals erupted with lava, a process called “outgassing.” As more water was outgassed, the mantle solidified. Materials that initially stayed in their liquid phase during this process, called “incompatible elements,” ultimately became Earth’s brittle crust.
     
    From mud and clay to diamonds and coal, Earth’s crust is composed of igneous, metamorphic, and sedimentary rocks. The most abundant rocks in the crust are igneous, which are formed by the cooling of magma. Earth’s crust is rich in igneous rocks such as granite and basalt. Metamorphic rocks have undergone drastic changes due to heat and pressure. Slate and marble are familiar metamorphic rocks. Sedimentary rocks are formed by the accumulation of material at Earth’s surface. Sandstone and shale are sedimentary rocks.
     
    Dynamic geologic forces created Earth’s crust, and the crust continues to be shaped by the planet’s movement and energy. Today, tectonic activity is responsible for the formation (and destruction) of crustal materials.
     
    Earth’s crust is divided into two types: oceanic crust and continental crust. The transition zone between these two types of crust is sometimes called the Conrad discontinuity. Silicates (mostly compounds made of silicon and oxygen) are the most abundant rocks and minerals in both oceanic and continental crust. 
     
    Oceanic Crust
     
    Oceanic crust, extending 5-10 kilometers (3-6 kilometers) beneath the ocean floor, is mostly composed of different types of basalts. Geologists often refer to the rocks of the oceanic crust as “sima.” Sima stands for silicate and magnesium, the most abundant minerals in oceanic crust. (Basalts are a sima rocks.) Oceanic crust is dense, almost 3 grams per cubic centimeter (1.7 ounces per cubic inch).
     
    Oceanic crust is constantly formed at mid-ocean ridges, where tectonic plates are tearing apart from each other. As magma that wells up from these rifts in Earth’s surface cools, it becomes young oceanic crust. The age and density of oceanic crust increases with distance from mid-ocean ridges.
     
    Just as oceanic crust is formed at mid-ocean ridges, it is destroyed in subduction zones. Subduction is the important geologic process in which a tectonic plate made of dense lithospheric material melts or falls below a plate made of less-dense lithosphere at a convergent plate boundary
     
    At convergent plate boundaries between continental and oceanic lithosphere, the dense oceanic lithosphere (including the crust) always subducts beneath the continental. In the northwestern United States, for example, the oceanic Juan de Fuca plate subducts beneath the continental North American plate. At convergent boundaries between two plates carrying oceanic lithosphere, the denser (usually the larger and deeper ocean basin) subducts. In the Japan Trench, the dense Pacific plate subducts beneath the less-dense Okhotsk plate.
     
    As the lithosphere subducts, it sinks into the mantle, becoming more plastic and ductile. Through mantle convection, the rich minerals of the mantle may be ultimately “recycled” as they surface as crust-making lava at mid-ocean ridges and volcanoes.
     
    Largely due to subduction, oceanic crust is much, much younger than continental crust. The oldest existing oceanic crust is in the Ionian Sea, part of the eastern Mediterranean basin. The seafloor of the Ionian Sea is about 270 million years old. (The oldest parts of continental crust, on the other hand, are more than 4 billion years old.)
     
    Geologists collect samples of oceanic crust through drilling at the ocean floor, using submersibles, and studying ophiolites. Ophiolites are sections of oceanic crust that have been forced above sea level through tectonic activity, sometimes emerging as dikes in continental crust. Ophiolites are often more accessible to scientists than oceanic crust at the bottom of the ocean.
     
    Continental Crust
     
    Continental crust is mostly composed of different types of granites. Geologists often refer to the rocks of the continental crust as “sial.” Sial stands for silicate and aluminum, the most abundant minerals in continental crust. Sial can be much thicker than sima (as thick as 70 kilometers kilometers (44 miles)), but also slightly less dense (about 2.7 grams per cubic centimeter (1.6 ounces per cubic inch)). 
     
    As with oceanic crust, continental crust is created by plate tectonics. At convergent plate boundaries, where tectonic plates crash into each other, continental crust is thrust up in the process of orogeny, or mountain-building. For this reason, the thickest parts of continental crust are at the world’s tallest mountain ranges. Like icebergs, the tall peaks of the Himalayas and the Andes are only part of the region’s continental crust—the crust extends unevenly below the Earth as well as soaring into the atmosphere.
     
    Cratons are the oldest and most stable part of the continental lithosphere. These parts of the continental crust are usually found deep in the interior of most continents. Cratons are divided into two categories. Shields are cratons in which the ancient basement rock crops out into the atmosphere. Platforms are cratons in which the basement rock is buried beneath overlying sediment. Both shields and platforms provide crucial information to geologists about Earth’s early history and formation.
     
    Continental crust is almost always much older than oceanic crust. Because continental crust is rarely destroyed and recycled in the process of subduction, some sections of continental crust are nearly as old as the Earth itself. 
     
    Extraterrestrial Crust
     
    Our solar system’s other terrestrial planets (Mercury, Venus, and Mars) and even our own Moon have crusts. Like Earth, these extraterrestrial crusts are formed mostly by silicate minerals. Unlike Earth, however, the crusts of these celestial bodies are not shaped by the interaction tectonic plates. 
     
    Despite the Moon’s smaller size, lunar crust is thicker than crust on Earth. Lunar crust is not a uniform thickness and in general tends to be thicker on the “far side,” which always faces away from Earth.
     
    Although Mercury, Venus, and Mars are not thought to have tectonic plates, they do have dynamic geology. Venus, for instance, has at partly-molten mantle, but the Venusian crust lacks enough trapped water to make it as dynamic as Earth’s crust.
     
    The crust of Mars, meanwhile, features the tallest mountains in the solar system. These mountains are actually extinct volcanoes formed as molten rock erupted in the same spot on the Martian surface over millions of years. Eruptions built up enormous mountains of iron-rich igneous rocks that give the Martian crust its characteristic red hue.
     
    One of the most volcanic crusts in the solar system is that of Jupiter’s moon Io. The rich sulfide rocks in the Ionian crust paint the moon a dappled collection of yellows, greens, reds, blacks, and whites.
    crust
    Earth's crust is made of young oceanic material and older, thicker continental material.
    Silicates, Silicates Everywhere
    Silicate minerals, mostly feldspars and quartz, are the most abundant minerals in Earth’s rocky crust.
    Old Crust
    The oldest rocks yet identified on Earth were discovered in the Jack Hills of Western Australia, part of the Yilgarn Craton, a shield formation. The Jack Hills zircons are about 4.4 billion years old. (The Earth itself is about 4.6 billion years old!)
    Mining Temperature
    The TauTona and Mponeng gold mines of South Africa are the deepest in the world, descending about 4 kilometers (2.5 miles) below the surface of the Earth. Although those are deep mines, they are shallow crust. Still, temperatures at the bottom of the mines can climb to 55° Celsius (131° Fahrenheit). A sophisticated air conditioning system lowers the temperature to allow miners to work.
  • Term Part of Speech Definition Encyclopedic Entry
    abundant Adjective

    in large amounts.

    accessible Adjective

    relatively easy to approach, use, or obtain.

    accumulation Noun

    a buildup of something.

    ambient Adjective

    having to do with the surrounding area or environment.

    arid Adjective

    dry.

    atmosphere Noun

    layers of gases surrounding a planet or other celestial body.

    Encyclopedic Entry: atmosphere
    basalt Noun

    type of dark volcanic rock.

    basement rock Noun

    oldest underlying rock formation in any region.

    boundary Noun

    line separating geographical areas.

    Encyclopedic Entry: boundary
    characteristic Noun

    physical, cultural, or psychological feature of an organism, place, or object.

    Conrad discontinuity Noun

    seismic boundary between the continental crust and oceanic crust.

    continental crust Noun

    thick layer of Earth that sits beneath continents.

    convergent plate boundary Noun

    area where two or more tectonic plates bump into each other. Also called a collision zone.

    core Noun

    the extremely hot center of Earth, another planet, or a star.

    Encyclopedic Entry: core
    craton Noun

    old, stable part of continental crust, made up of shields and platforms.

    crucial Adjective

    very important.

    crust Noun

    rocky outermost layer of Earth or other planet.

    Encyclopedic Entry: crust
    dappled Adjective

    spotted, or having areas of differently colored shades or tones.

    dense Adjective

    having parts or molecules that are packed closely together.

    desert Noun

    area of land that receives no more than 25 centimeters (10 inches) of precipitation a year.

    Encyclopedic Entry: desert
    dike Noun

    a barrier, usually a natural or artificial wall used to regulate water levels.

    Encyclopedic Entry: dike
    ductile Adjective capable of withstanding a certain amount of force by changing form before fracturing or breaking.
    dynamic Adjective

    always changing or in motion.

    Earth Noun

    our planet, the third from the Sun. The Earth is the only place in the known universe that supports life.

    Encyclopedic Entry: Earth
    erupt Verb

    to explode or suddenly eject material.

    eventually Adverb

    at some point in the future.

    extinct volcano Noun

    volcano that will no longer erupt.

    extraterrestrial Adjective

    located or formed outside Earth's atmosphere.

    geologic Adjective

    having to do with the physical formations of the Earth.

    granite Noun

    type of hard, igneous rock.

    granite Noun

    type of hard, igneous rock.

    hue Noun

    tint or general variety of color.

    iceberg Noun

    large chunks of ice that break off from glaciers and float in the ocean.

    Encyclopedic Entry: iceberg
    igneous rock Noun

    rock formed by the cooling of magma or lava.

    incompatible element Noun

    in geochemistry, an element that stays in a liquid phase during the melt or crystallization process.

    initially Adverb

    at first.

    iron Noun

    chemical element with the symbol Fe.

    isostasy Noun

    equilibrium of Earth's crust, where the forces tending to elevate landmasses balance those tending to depress them. Also called isostatic equilibrium.

    lava Noun

    molten rock, or magma, that erupts from volcanoes or fissures in the Earth's surface.

    lithosphere Noun

    outer, solid portion of the Earth. Also called the geosphere.

    Encyclopedic Entry: lithosphere
    lunar Adjective

    having to do with Earth's moon or the moons of other planets.

    magma Noun

    molten, or partially melted, rock beneath the Earth's surface.

    Encyclopedic Entry: magma
    magnesium Noun

    chemical element with the symbol Mg.

    malleable Adjective

    flexible and capable of reforming itself without breaking when under stress.

    mantle Noun

    middle layer of the Earth, made of mostly solid rock.

    Encyclopedic Entry: mantle
    mantle convection Noun slow movement of Earth's solid mantle caused by convection currents transferring heat from the interior of the Earth to the surface.
    metal Noun

    category of elements that are usually solid and shiny at room temperature.

    metamorphic rock Noun

    rock that has transformed its chemical qualities from igneous or sedimentary.

    mid-ocean ridge Noun

    underwater mountain range.

    mineral Noun

    inorganic material that has a characteristic chemical composition and specific crystal structure.

    Mohorovicic discontinuity Noun

    point between Earth's crust and the mantle below. Also called the Moho.

    molten Adjective

    solid material turned to liquid by heat.

    ocean basin Noun

    depression in the Earth's surface located entirely beneath the ocean.

    oceanic crust Noun

    thin layer of the Earth that sits beneath ocean basins.

    ocean trench Noun

    a long, deep depression in the ocean floor.

    Encyclopedic Entry: ocean trench
    ophiolite Noun

    remnant of oceanic crust (certain igneous rocks) embedded in continental crust.

    orogeny Noun

    the way mountains are formed.

    outgas Verb

    to release a gas that was dissolved, trapped, frozen or absorbed in another material.

    planet Noun

    large, spherical celestial body that regularly rotates around a star.

    Encyclopedic Entry: planet
    platform Noun

    ancient rocks that formed as part of continental crust, now overlain with sediment and sedimentary rock, located in the interior of continents.

    rock Noun

    natural substance composed of solid mineral matter.

    sediment Noun

    solid material transported and deposited by water, ice, and wind.

    Encyclopedic Entry: sediment
    sedimentary rock Noun

    rock formed from fragments of other rocks or the remains of plants or animals.

    shield Noun

    ancient rocks that formed as part of continental crust and are located in the interior of continents.

    sial Noun

    rocks, mostly silicates and aluminum, making up most of Earth's continental crust.

    silica Noun

    chemical compound (SiO2) that makes up most of the Earth's rocks.

    silicate Noun

    most common group of minerals, all of which include the elements silicon (Si) and oxygen (O).

    sima Noun rocks, mostly silicates and magnesium, making up most of Earth's oceanic crust.
    subduction Noun

    process of one tectonic plate melting, sliding, or falling beneath another.

    subduction zone Noun

    area where one tectonic plate slides under another.

    submersible Noun

    small submarine used for research and exploration.

    sulfide Noun

    (S2-) negatively charged ion of sulfur, or a chemical compound containing such an ion.

    tectonic activity Noun

    movement of tectonic plates resulting in geologic activity such as volcanic eruptions and earthquakes.

    tectonic plate Noun

    massive slab of solid rock made up of Earth's lithosphere (crust and upper mantle). Also called lithospheric plate.

    temperature Noun

    degree of hotness or coldness measured by a thermometer with a numerical scale.

    Encyclopedic Entry: temperature
    terrestrial planet Noun

    one of the four planets closest to the sun: Mercury, Venus, Earth, or Mars.

    transition zone Noun

    areas in the Earth's interior between the upper mantle, near the Earth's crust, and the lower mantle, near the Earth's core.

    uniform Adjective

    exactly the same in some way.

    universe Noun

    all known matter, energy, and space.

    viscous Adjective

    liquid that is thick and sticky.

    volcano Noun

    an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.

    Encyclopedic Entry: volcano