Xenolith

A xenolith is a piece of rock that gets trapped inside another rock as it forms. Imagine a piece of cookie getting caught inside a cake while it’s being baked—that’s what a xenolith is like in rocks. In the study of Earth’s surface, scientists use the word xenolith mainly to talk about pieces of rock found inside molten rock, called igneous rock. These pieces can get caught when hot, moving rock, known as magma, pushes up through the Earth’s crust. They might get pulled in from the sides of a magma chamber, torn loose from the walls of a volcanic pipe, or picked up from the ground as lava flows.

A xenocryst is just a single crystal caught inside the rock, like a tiny gem trapped in glass. For example, you might find quartz crystals in lava that doesn’t usually contain quartz, or even diamonds inside special rocks called kimberlite.

Xenoliths are important because they give scientists clues about parts of the Earth we can’t usually see, like the deep mantle below the surface. Rocks such as basalts and kimberlites often carry pieces of the mantle inside them. By studying these trapped pieces, scientists can learn about the composition and conditions deep inside the Earth. For instance, the type of minerals in a xenolith can tell us how deep it came from—whether it was near the surface or far deeper down.

Examples

Xenoliths can be found in many places around the world. For example, they appear in granodiorite in the Alta Stock in Little Cottonwood Canyon, Utah. There is also a large xenolith of sandstone in the Fairlee Pluton in Vermont. In Germany, peridotite xenoliths have been found within volcanic bombs from Vulkaneifel.