How does a xenocryst reach Earth's surface?

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ncchemist | eNotes Employee

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Basically, a xenocryst is a subset of a broader term called xenoliths.  In geology, a xenolith is piece or fragment of a rock that is embedded in a host rock of different chemical composition.  A xenocryst is similar to a xenolith but it refers only to mineral crystals like quartz embedded in host rocks.

Xenocrysts (and xenoliths) are generally found in igneous rocks, or rocks that are formed from cooling magma.  Since the rocks have their origin in magma, they reach the surface of the Earth in any way that magma itself does.  They reach the surface either through lava flows from volcanoes or any other openings in the Earth's crust like tectonic plate boundaries or hotspots.  Additionally, xenocrysts can also be found in some meteorites that originate from outer space and impact the Earth's surface.

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sid-sarfraz | Student, Graduate | (Level 2) Salutatorian

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Xenolith

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

Magma

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.

Igneous Rock

The rock that forms from cooled magma is called igneous rock.

Xenoliths are different types of rock embedded in igneous rock.

How does Xenoliths reach earth surface:-

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.

Characteristics of Xenoliths:-

  • 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.

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