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Secrets of Earth’s Deepest Volcanoes

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I’ll just say it up front: National Geographic Committee for Research and Exploration grantee Rich Brown did not discover diamonds on his recent research expedition to the Igwisi Hills Volcanoes in the Tanzanian highlands. Nor did he expect to find any. But the story behind his geological trek reveals the violent ways in which diamonds are flung into the air and oozed out in lava flows during unusual volcanic eruptions.

Most people carry a mental image of volcanoes that involves mountains with smoke and fire spewing from their summits. But the kind Brown studies—kimberlite volcanoes—present a different picture. Kimberlite volcanoes can be imagined as deep, steep-sided holes in the ground full of crystalized, fragmented magma. Kimberlite pipes, as these structures are called, tap gas-rich magma that originated in the Earth’s mantle, up to 400 miles below the surface.

When a kimberlite blows, a vertical column of rock explodes from the ground, then falls in a ring around a bowl-shaped crater on the surface. Roughly one out of every 200 kimberlite volcanoes belches forth diamonds, and occasionally semi-precious garnets and peridots to boot. In 1871, the discovery of an 83.5 carat diamond near the town of Kimberley, South Africa, spawned a diamond rush and gave the bling-gifting volcanoes their name.

The trouble with kimberlites is that almost all of them erupted more than 45 million years ago and have since been heavily eroded, which means that we don’t really understand how they work. Typically, scientists can only examine their “guts”—the subterranean parts deep within a kimberlite “pipe” revealed after tens of thousands of millennia of erosion. Brown chose to study the Igwisi Hills Volcanoes because they are the only ones known with well-preserved surface remains, thanks to their young age—probably much less than two million years.

Reaching them is a real journey, requiring a two day drive on badly maintained dirt roads to the middle of the remote Miombo forest plain. Once there, Brown’s team was introduced to the locals and blessed by a village elder who claimed to own the volcanoes. According to Brown, “he was extremely frail, almost completely blind and bed-bound in a small mud hut. He fought with the Germans against the British in Tanzania during WWI, which makes him at least 110 years old.”

The scientists proceeded to create the first geological map of a kimberlite volcano to include the typically absent surface deposits. They collected data in the field…

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… that will allow them to determine the size and date of eruptions, understand what causes eruptions to occur, and figure out why there have been so few of them lately.

The scientists’ activities did not go unnoticed by Igwisi villagers. Despite their reassurances, some villagers suspected the scholars of secretly prospecting for diamonds. “We explained daily, via our Tanzanian colleagues, that we were simply interested in the rocks, but as there is not a widespread culture of academic geological research in Tanzania, and we were far-removed from the cities, they didn’t believe us,” said Brown. By the end of their stay, locals were carting off rock samples on their bicycles, mimicking the geologists’ sampling procedure. “Our strong suspicion is that on return, there will be many new trenches around the volcanoes and many disappointed locals.”

Learn more about the explosive power of volcanoes.

Photographs of a kimberlite volcano in Tanzania by Rich Brown