kimberlite

Imagine the Earth's deep interior: not a sea of liquid lava, but solid rock moving very slowly over vast stretches of time. Very rarely, unusual volcanic eruptions rise from deep in the mantle, more than 150 kilometers below the surface, and bring up a rock called kimberlite. These are not calm eruptions. They are fast and explosive, and they often leave behind carrot-shaped pipes in the Earth's crust. This rock matters because those pipes are one of the main places where diamonds are found. So when geologists talk about kimberlite, they are talking about both a clue to the deep Earth and an important source of diamonds.

Kimberlite is a rare igneous rock, meaning it formed from magma, but unlike most volcanic rocks, it begins much deeper than the crust or uppermost mantle. It rises from depths greater than about 150 kilometers (93 miles) within the Earth's mantle. That deep origin is what makes it so important to geologists.

Kimberlite does not usually appear as a slow lava flow. Instead, it is linked to extremely violent eruptions that form vertical, pipe-like structures called kimberlite pipes. You can picture one as a giant carrot narrowing as it goes down through the Earth's crust. These eruptions are driven by gas-rich magma rising quickly from deep in the mantle, carrying pieces of rock and minerals upward at great speed.

The best-known reason kimberlite is studied is its connection to diamonds. Not every kimberlite pipe contains diamonds, but kimberlite is the main source of most diamonds mined from the Earth. Diamonds form under extreme pressure and heat deep in the mantle, and these eruptions can carry them to the surface before they break down.

Geologists study kimberlite not only because of diamonds, but also because it gives clues about the makeup and behavior of the deep mantle. In a sense, it brings up samples from parts of the Earth that humans could never reach directly. The name comes from Kimberley, South Africa, where this rock became famous as a source of diamonds in the late 19th century.