The eruption of diamonds on the ground was 25 million years ago

Diamonds on Earth's surface today likely came from ancient kimberlite eruptions after tectonic plates separated.

Diamonds are often found in kimberlite rock. Photo: Interesting Engineering

Powerful volcanic eruptions that blow diamonds into the sky and scatter this gemstone across the Earth's surface have long puzzled researchers. Currently, scientists have found a mechanism at a depth of more than 160 km below the water surface that pushes diamond crystals upward with such strong force. New research reveals the mysterious process that causes the rare eruption and where diamond-rich ore is most likely to be found.

"Diamonds form in a location within the continent. The question is why do they shoot up from deep underground after spending billions of years there," said Professor Tom Gernon, a geologist at the University of California. Southampton student, said.

To solve the mystery, in research published at the end of July in the journal Nature, Gernon and his colleagues analyzed historical data and continental plates, giant slow-moving plates of the Earth's crust, and kimberlite, a type of rock that contains Diamonds shot out by eruption. They found that over billions of years, the majority of kimberlite eruptions occurred about 25 million years after the continental plates separated.

On closer inspection, the researchers found that the first kimberlite eruption occurred after the continental plate broke close to the edge. Subsequent eruptions increasingly moved toward the middle of the plate. Continental plates separate and merge on extremely large time scales. About 300 million years ago, North America and South America were connected to Africa and Europe, belonging to the same supercontinent called Pangea, but this land mass began to separate about 175 million years ago.

Based on computer models of rock and magma, scientists piece together the sequence of events that prompted the diamond eruption. The process begins when the continental tectonic plate is stretched as it begins to separate. This stretching causes the rock layer to become thinner and disrupts the normal flow of material in the Earth's mantle directly underneath. Disturbances in the mantle are strong enough to cause rocks to separate from the base of the tectonic plate. They have been under enormous pressure for hundreds of millions of years. Carbon deposits can change structure to form diamonds.

As the rocks sink into the mantle, they spur more and more turbulent flows to spread outward, sliding layers of rock tens of kilometers thick from the base of the tectonic plate above. The spillover effect collects all the ingredients needed to create diamond-bearing kimberlite magma. When molten enough, it rises very quickly, bursting through the crust in the form of a green eruption.

The last kimberlite eruption occurred 11,000 years ago at the Igwisi Hills in Tanzania, but most took place during the Cretaceous period 146 to 66 million years ago. Unlike regular volcanic eruptions, kimberlite eruptions leave vertical tubular holes in the ground, providing the basis for many diamond mines.