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USGS Geology in the Parks

Visual Glossary

Limestone xenolith preserved in basalt flow


A xenolith is a piece of foreign rock enclosed within an igneous rock. The foreign rock is usually picked up from the walls surrounding the igneous rock and is frozen in place before it has a chance to melt. Xenoliths can be sedimentary, metamorphic, or igneous rocks.

This xenolith is limestone encased in basalt of the Bonito Lava Flow at Sunset Crater Volcano National Monument. Here at Sunset Crater, molten basalt traveled through a dike, an almost vertical conduit, up to Sunset Crater Volcano's central vent. As the fiery liquid made its way through thick layers of sedimentary rock, pieces of limestone were plucked from the sides and carried upward. Most were probably melted by the liquid basalt, but some limestone chunks from the layers nearest the surface are preserved in the lava flows, providing evidence that layered sedimentary rocks lie beneath the 1000 feet-thick volcanic mantle.

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