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 Earth's 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 Earth's surface. Magma rises to Earth's surface through these pipes between Earth's 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 Earth's 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 Earth's mantle. Scientists study the chemical properties of xenoliths to understand the depth at which they were formed. Many xenocrysts were created hundreds of kilometers within 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 Earth's atmosphere.