Diamonds form under great pressures in the depths of the Earth and are hundreds, or even billions of years old. They are usually found in types of volcanic rocks called kimberlites. These are located in the oldest, strongest, and thickest parts of the continents, such as in South Africa, where the well-known diamond rush of the 19th century occurred. However, how diamonds reach the Earth’s surface has largely remained a mystery until recently.
Now, a team of researchers led by the University of Southampton and the University of Birmingham has found that the breakup of tectonic plates is the main driving force behind the generation and ultimate eruption of diamond-rich magmas from deep inside the Earth.
“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,” explained co-author Stephen Jones, an associate professor of Earth Systems at Birmingham.
Studying how diamonds reach Earth’s surface
The experts used statistical analysis and machine learning algorithms to forensically examine the connection between continental breakup and kimberlite volcanism.
The investigations revealed that the eruptions of most kimberlite volcanoes occurred 20 to 30 million years after the initial tectonic breakup of our planet’s continents.
“Using geospatial analysis, we found that kimberlite eruptions tend to gradually migrate from the continental edges to the interiors over time at rates that are consistent across the continents,” said co-author Thea Hincks, a senior research fellow in Geology at Southampton.
What the researchers discovered
These findings prompted the scientists to examine what geological processes may drive this pattern. They discovered that the Earth’s mantle – the convecting layer between the planet’s crust and core – is often disrupted by rifting, or stretching, of the crust.
This phenomenon occurs even thousands of kilometers away. The result leads to a domino effect which can help explain how continental breakup causes the formation of kimberlite magma.
“The pattern of diamond eruptions is cyclical, mimicking the rhythm of the supercontinents, which assemble and break up in a repeated pattern over time. But previously we didn’t know what process causes diamonds to suddenly erupt, having spent millions – or billions – of years stashed away 150 kilometers beneath the Earth’s surface,” said lead author Tom Gernon, an associate professor of Earth Science at Southampton.
These findings could be used to identify the locations and timings of past volcanic eruptions linked to these processes, providing important insights that could enable the future discovery of additional diamond deposits.
The study is published in the journal Nature.
More about diamond deposits
Diamond deposits are where diamonds can be found in the Earth’s crust. The vast majority of natural diamonds are found in kimberlite pipes, which are volcanic in origin. These pipes originate deep in the Earth’s mantle, over 100 miles below the surface, where the pressure and temperature conditions are right for diamond formation.
The pipes then burst through to the surface in violent eruptions, bringing diamonds with them. These deposits are mined in many parts of the world, with the most significant sources being in Russia, Botswana, Canada, Australia, and South Africa.
Another form of diamond deposit is called an alluvial deposit. These diamonds are found in riverbeds and along the coast where rivers meet the sea. They originate from kimberlite pipes, but over millions of years, they’ve been eroded out of the pipes and transported by water to their current location.
Alluvial diamonds are often of very high quality because the process of erosion tends to destroy lower-quality stones. Alluvial diamond deposits are found in several places, including Brazil, Venezuela, and Namibia.
Finally, there are also diamond deposits in certain types of meteorites known as ureilites. These diamonds were likely formed by high-velocity impacts in space, though their exact origin is still a subject of scientific study.
The process of mining diamonds has significant environmental and social impacts, and many diamonds are sold on the global market under strict protocols intended to prevent the sale of “blood diamonds” that finance conflict.
These protocols include the Kimberley Process, which is an international certification scheme to prevent the trade in conflict diamonds. However, these protocols have been criticized for their effectiveness and for not addressing broader social and environmental issues associated with diamond mining.