• crust
    The crust is the hard, rocky outer shell of the Earth.

    Photograph by Noelia Magnusson, My Shot

    Tectonic Plates
    There are actually more than 30 tectonic plates. One of the smallest is the Galapagos microplate, smaller than the U.S. state of Connecticut.

    Thin Crust
    Earth's crust is only 40 kilometers (25 miles) thick on average, making it the thinnest of Earth's three layers of crust, mantle, and core. This thin band contains all known life in the universe.

    The crust is the outermost layer of the Earth.

    Earth has three layers. Beneath the crust is the mantle, which is made of semi-solid magma and solid rocks and minerals. At the center of the Earth is the extremely hot, metal core. The lack of air, water, and moderate temperature prevents organisms from living in the mantle or core.

    Earth's crust is divided into 15 major tectonic plates: the North American, Caribbean, South American, Scotia, Antarctic, Eurasian, Arabian, African, Indian, Philippine, Australian, Pacific, Juan de Fuca, Cocos, and Nazca plates. Tectonic plates actually slide around on the mantle, causing earthquakes, mountain formation, continental drift, volcanoes, and other geologic activity on the crust.

    Billions of years ago, the Earth started out as a hot, gooey ball of rock. The heaviest material, mostly iron and nickel, sank to the center of the Earth and became the core. The surface of the Earth slowly cooled off and hardened. These surface rocks became the crust.

    The crust is divided into two types: oceanic crust and continental crust. Oceanic crust, found under the ocean floor, is made of dense rocks such as basalt. It is about 7 kilometers (4 miles) thick. Continental crust, found under land masses, is made of less dense rocks such as granite. Its thickness varies between 10 and 75 kilometers (6 to 47 miles).

    Continental crust is almost always much older than oceanic crust. Some of the oldest rocks in the world can be found in the Nuvvuagittuq greenstone belt in Quebec, Canada. This continental crust formation has rocks that are about 4 billion years old. Unlike continental crust, oceanic crust is still being formed in places called mid-ocean ridges. Here, magma from the mantle erupts through cracks in the ocean floor, creating crust as it cools.

    Oceanic crust is heavier than continental crust. The heavy oceanic crust is constantly sinking, very slowly, underneath the lighter continental crust. This important process is called subduction. A chain of volcanoes formed at a subduction zone is called a volcanic arc. One such volcanic arc exists where the oceanic crust of the Australian plate subducts under the continental crust of the Eurasian plate. The Indonesian Island Arc, which includes the islands of Sumatra and Java in Indonesia, has some of the most powerful volcanoes in the world.

    Eventually, oceanic crust sinks low enough to enter the mantle. Once this happens, the crust melts, then rises up again as magma in the mid-ocean ridges. In this way, the Earth enjoys a brand-new oceanic crust once every 200 million years or so.

  • Term Part of Speech Definition Encyclopedic Entry
    air Noun

    the layer of gases surrounding Earth.

    Encyclopedic Entry: air
    basalt Noun

    type of dark volcanic rock.

    continental crust Noun

    thick layer of Earth that sits beneath continents.

    continental drift Noun

    the movement of continents resulting from the motion of tectonic plates.

    Encyclopedic Entry: continental drift
    core Noun

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

    Encyclopedic Entry: core
    crust Noun

    rocky outermost layer of Earth or other planet.

    Encyclopedic Entry: crust
    dense Adjective

    having parts or molecules that are packed closely together.

    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
    earthquake Noun

    the sudden shaking of Earth's crust caused by the release of energy along fault lines or from volcanic activity.

    eventually Adverb

    at some point in the future.

    granite Noun

    type of hard, igneous rock.

    iron Noun

    chemical element with the symbol Fe.

    magma Noun

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

    Encyclopedic Entry: magma
    mantle Noun

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

    Encyclopedic Entry: mantle
    metal Noun

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

    mid-ocean ridge Noun

    underwater mountain range.

    mineral Noun

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

    mountain Noun

    landmass that forms as tectonic plates interact with each other.

    nickel Noun

    chemical element with the symbol Ni.

    oceanic crust Noun

    thin layer of the Earth that sits beneath ocean basins.

    organism Noun

    living or once-living thing.

    rock Noun

    natural substance composed of solid mineral matter.

    subduction Noun

    process of one tectonic plate melting or going beneath another.

    subduction zone Noun

    area where one tectonic plate slides under another.

    tectonic activity Noun

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

    tectonic plate Noun

    large, moveable segment of the Earth's crust.

    universe Noun

    all known matter, energy, and space.

    volcanic arc Noun

    chain of volcanoes formed at a subduction zone.

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