Scientists think they’ve discovered what brings diamonds to the surface of the Earth, and where to find more
We’ve long known how diamonds are formed: Intense heat and pressure deep under the Earth’s surface cause carbon atoms to crystallize into diamonds. What we haven’t known, until now, is how those shiny rocks we love so much make it all the way to the surface after millions or billions of years below. The answer, it appears, lies in the breakup of tectonic plates, and it could provide clues as to where to find more.
“We found that a domino effect can explain how continental breakup leads to formation of kimberlite magma. During rifting, a small patch of the continental root is disrupted and sinks into the mantle below, triggering a chain of similar flow patterns beneath the nearby continent,” Stephen Jones, part of a team of researchers led by the universities of Southampton and Birmingham who published their findings in Nature, told Earth.com.
Geologists have been in general agreement that this constant cycle of continents coming together and breaking apart pushed diamonds to the surface, but they didn’t know how. The prevailing theories had some holes in them. The lithosphere is too hard for kimberlites to shoot up through suddenly, discrediting one theory, and kimberlites don’t contain enough of the right chemicals to support the mantle plume theory. The research team believes it has found a Goldilocks theory that explains how the eruptions that push diamonds to the surface occur.
Using machine learning and statistical models, the team determined that it takes about 20 to 30 million years after tectonic plate breakups for most kimberlite eruptions to occur. Focusing on Africa, North America, and South America, the continents with the most kimberlites, the researchers made a crucial discovery. They found that the eruptions start at the edges of continents then make their way inland at the same rate everywhere.
When that continental root Jones mentioned sinks under the mantle, hot mantle wells up, causing edge-driven convection.
“Our models show that while sweeping along the continental root, these disruptive flows remove a substantial amount of rock, tens of kilometers thick, from the base of the continental plate,” lead author of the paper Thomas Gernon wrote. “Various other results from our computer models then advance to show that this process can bring together the necessary ingredients in the right amounts to trigger just enough melting to generate gas-rich kimberlites. Once formed, and with great buoyancy provided by carbon dioxide and water, the magma can rise rapidly to the surface carrying its precious cargo.”
Finding More Diamonds
We’re still a couple hundred million years away from the next supercontinent forming, let alone breaking up. So there won’t be any more kimberlite explosions for quite a while, and we’re more likely to mine asteroids than the Earth’s crust. But the researchers’ discoveries might be able to help find diamonds that have already made their way to the surface.
The “highly systematic” processes of kimberlite eruption have a consistent pattern of moving from the edges of continents to the interior. “This information could be used to identify the possible locations and timings of past volcanic eruptions tied to this process, offering insights that could enable the discovery of diamond deposits and other rare elements needed for the green energy transition,” Gernon wrote.
Knowing when, where, and why kimberlites form is useful for finding diamonds, Gernon told the Guardian. Searching areas inland from diamond-rich places where kimberlite eruptions are known to have taken place could prove fruitful for speculators. Most kimberlite eruptions happened during the Cretaceous period, but the most recent came 11,000 years ago in Tanzania.
“These eruptions are quite rare in Earth’s history because they require this perfect storm of conditions and events to generate them,” he said. “The dinosaurs would have been walking around in some of these areas, watching these events, and they would have been quite perplexed. They are extremely rapid events, and they probably wouldn’t be expecting them.”
That perfect storm is a big part of what makes diamonds so valuable to us, and solving this mystery could lead us to unearth more.