Encyclopedic Entry

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.

Vocabulary

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 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

Credits

Media Credits

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Writers

Kim Rutledge
Melissa McDaniel
Diane Boudreau
Tara Ramroop
Santani Teng
Erin Sprout
Hilary Costa
Hilary Hall
Jeff Hunt

Illustrators

Tim Gunther
Mary Crooks, National Geographic Society

Editors

Kara West
Jeannie Evers

Educator Reviewer

Nancy Wynne

Producer

Caryl-Sue, National Geographic Society

Sources

Dunn, Margery G. (Editor). (1989, 1993). "Exploring Your World: The Adventure of Geography." Washington, D.C.: National Geographic Society.

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