• xenolith
    A xenolith is a rock trapped inside another rock.

    Photograph courtesy Woudloper (Wikipedia)

    Xenocrysts are a Girl's Best Friend
    Diamonds are formed in the Earth's upper mantle. A type of igneous rock called kimberlite is known for carrying xenocrysts of diamonds to the surface, where they can be mined. Kimberlite was named after the rich Kimberley mine in South Africa, where a diamond rush began in 1871.

    A xenolith is a piece of rock trapped in another type of rock.

    Most of the time, a xenolith is a rock embedded in magma while the magma was cooling. Magma is the molten rock beneath the Earths crust that emerges as lava during a volcanic eruption. The rock that forms from cooled magma is called igneous rock. Xenoliths are different types of rock embedded in igneous rock.

    Xenoliths are torn from deep cracks, or pipes, in the Earths surface. Magma rises to the Earths surface through these pipes between the Earths crust and mantle. As the molten material rises, it tears off bits and pieces of the magma pipe in which it is traveling. These bits and pieces, trapped in the magma but not melting into it, become xenoliths. Crystals that are torn from the sides of magma pipes are called xenocrysts.

    As magma erupts or flows from the Earths surface, it is cooled by exposure to air or water. Lava cools fairly quickly, and various types of igneous rocks are formed. Xenoliths are usually visible. They have a different color and density than the surrounding igneous rock. Xenoliths can be as small as a grain of sand or as large as a football, and as long as several meters.

    Xenoliths and xenocrysts are affected by temperature. A xenolith may lose its unique qualities if it melts into the surrounding magma. As it cools, the material may cease being a xenolith at all and become a metamorphic rock. Metamorphic rock is rock that has changed from one form (sedimentary or igneous) to another.

    Xenoliths and xenocrysts are often identified by the names of the two rock types involved. A peridotite xenolith in a basaltic lava flow, for instance, means a chunk of the rock peridotite is embedded in basalt rock. The peridotite is usually yellow and dense, while the basalt is usually grey and light.

    Xenoliths and xenocrysts provide valuable information about the geology of the Earths mantle. Scientists study the chemical properties of xenoliths to understandthe depth at which they were formed. Many xenocrysts were created hundreds of kilometers within the Earth, far below the deepest mines and wells. The information about the condition of the mantle at these depths would be impossible to understand without xenoliths and xenocrysts. Some of the features studied by geologists are temperature, pressure, construction, and movement within the Earth's surface.

    Xenoliths can be a piece of rock trapped in a piece of sedimentary rock, but this is rare. Xenoliths have also been found in meteorites, or rocks from outer space that have crashed into Earth. The xenoliths in meteorites were formed from collisions with other objects outside the Earths atmosphere.

  • Term Part of Speech Definition Encyclopedic Entry
    atmosphere Noun

    layers of gases surrounding a planet or other celestial body.

    Encyclopedic Entry: atmosphere
    basalt Noun

    type of dark volcanic rock.

    cease Verb

    to stop or end.

    chemical property Noun

    unique identity of a substance expressed by its type and arrangement of molecules.

    collision Noun

    crash.

    construction Noun

    arrangement of different parts.

    crust Noun

    rocky outermost layer of Earth or other planet.

    Encyclopedic Entry: crust
    crystal Noun

    type of mineral that is clear and, when viewed under a microscope, has a repeating pattern of atoms and molecules.

    density Noun

    number of things of one kind in a given area.

    Encyclopedic Entry: density
    diamond Noun

    type of crystal that is pure carbon and the hardest known natural substance.

    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
    embed Verb

    to attach firmly to a surrounding substance.

    emerge Verb

    to develop or come into view.

    eruption Noun

    release of material from an opening in the Earth's crust.

    geology Noun

    study of the physical history of the Earth, its composition, its structure, and the processes that form and change it.

    igneous rock Noun

    rock formed by the cooling of magma or lava.

    kimberlite Noun

    type of magma or igneous rock associated with diamonds.

    lava Noun

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

    magma Noun

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

    Encyclopedic Entry: magma
    magma pipe Noun

    channel that transports molten rock to the surface from very deep volcanoes.

    mantle Noun

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

    Encyclopedic Entry: mantle
    metamorphic rock Noun

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

    meteorite Noun

    type of rock that has crashed into Earth from outside the atmosphere.

    Encyclopedic Entry: meteorite
    mine Verb

    to extract minerals from the Earth.

    molten Adjective

    solid material turned to liquid by heat.

    peridotite Noun

    type of igneous rock

    pressure Noun

    force pressed on an object by another object or condition, such as gravity.

    sedimentary rock Noun

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

    xenocryst Noun

    crystal embedded in an igneous rock.

    xenolith Noun

    piece of rock embedded in another type of rock, usually igneous.

    Encyclopedic Entry: xenolith
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