You don’t need to go on a Jules Verne journey down to the center of the Earth, sometimes the center of the Earth is brought up to us!
These rocks are examples of peridotite xenoliths. A xenolith is a fragment of rock that becomes enveloped by magma without being melted or incorporated into it. In these photos, there are abundant fragments of xenolith peridotite surrounded by basalt. Peridotite is a rock composed of the minerals olivine and pyroxene, and it is the dominant rock composing the upper-part of Earth’s mantle.
As this basaltic magma was quickly rising through the mantle and the crust, it tore away solid chunks of mantle and brought them up in-tact to the surface. So when you look at these xenoliths, you’re looking at the composition of the Earth at tens to hundreds of kilometers depth!
These samples come from the San Carlos volcanic field near the town of Peridot, AZ. (Gem-quality olivine is called ‘peridot,’ so you can see where the town gets its name!) The San Carlos volcanic field covers a 50 km2
area of cinder cones and lava flows and is one of the world’s leading sources in gem-quality peridot.
All images by author
Emily Zawacki, Ph.D. 