Samples of the Earth's Mantle

This photo depicts a dark-colored, basaltic lava flow with an unusually high concentration of large inclusions composed of bright-green, coarse-grained crystalline rock. The inclusions were carried from great depth by magma ascending to the surface, and are termed xenoliths (literally, "foreign rocks"). These particular xenoliths are mostly composed of a rock called peridotite, which is predominantly composed of the green silicate mineral olivine, and is thought to make up most of the Earth's upper mantle. These rocks are thus actual samples of the mantle.

Our understanding of the seismic velocities of rocks at depth in the Earth, essential to our capacity to image deep structures using seismic waves, is rooted in laboratory measurements on rocks such as these. Xenolith studies also complement seismic studies of layering in the crust and mantle.

Not all volcanoes carry xenoliths from the mantle, and very few in such profusion. The locality is Peridot Mesa on the San Carlos Apache Nation in east-central Arizona. Peridot is gem-quality olivine, and this place was formerly mined for the gemstone. The rock hammer at lower right is approximately 25 centimeters (10 inches) long, indicating scale.

Image contributed by Steven Semken, Arizona State University.