Xenolith

Gabbroic xenolith in a granite; eastern Sierra Nevada, Rock Creek Canyon, California.

Peridotite (green) mantle xenolith in a (dark) volcanic bomb from the Vulkan-Eifel, Germany. One euro coin for scale

A xenolith (foreign rock) is a rock fragment which another rock surrounds.

It happens most in igneous rock when magma is fluid enough to flow round more solid rock. Xenoliths may be covered in the margins of a magma chamber, torn loose from the walls of a flow of lava or picked up along the ground by flowing lava on Earth's surface. A xenocryst is an individual foreign crystal included within an igneous body. Examples of xenocrysts are quartz crystals in a low-silica lava and diamonds in kimberlite diatremes.

Although the term xenolith is most commonly associated with igneous inclusions, a broad definition could include rock fragments which have become encased in sedimentary rock. Xenoliths are sometimes found in meteorites.

Xenoliths and xenocrysts provide important information about the composition of the otherwise inaccessible mantle. Basalts, kimberlites, lamproites and lamprophyres, which have their source in the upper mantle, often contain fragments and crystals assumed to be a part of the lower mantle mineralogy.

Xenolith Media

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  Gabbroic xenolith in andesite, Tertiary of Wyoming, United States

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  Olivine weathering to iddingsite within a mantle xenolith

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  Xenoliths in granodiorite of the Alta Stock, Little Cottonwood Canyon, Utah

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  Large xenolith of sandstone (probably from the Albee Formation) within the Fairlee Pluton in Vermont

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  Rounded, yellow, weathered peridotite xenolith in a nephelinite lava flow at Kaiserstuhl, SW Germany

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  Large xenolith of Baltimore Gneiss in the Guilford Quartz Monzonite in a wall of the old Waltersville Quarry, Granite, Maryland (about 1895)

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  Lamprophyre with xenolith at Ontario, Canada.

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  Xenolith in granite near Donner Pass, California (foot for scale).

References

Pink granite in basalt, Georgeville, Nova Scotia