How the Earth Makes Diamonds

A young African tribesman was walking near the Orange River in what is now South Africa in 1868 when he discovered a glassy stone that caught his fancy.

The youth, known today only as Swartboy, took the stone to a local man, who was known to like colorful rocks, in hopes of exchanging it for a night's lodging. Schalk van Niekerk, so the story goes, astonished Swartboy when he offered to trade all of his livestock, including 500 sheep, 10 cows and a horse, for the stone.

It wasn't the first diamond discovered in southern Africa, but at that point it was the most spectacular, an 83.5-carat gem that Van Niekerk sold for a small fortune, setting off a frantic and ruthless quest for riches.

All these years later, scientists still puzzle over forces buried in the Earth that shaped those diamonds, because those same forces also built a continent. As a result, teams of scientists are now wrapping up an ambitious, interdisciplinary study of the same region where Swartboy found that nifty rock.

Reading Earth's Surface

The research isn't focused on diamonds, but diamonds play a role, and it's possible that the high tech equipment used to probe the region more than 150 miles below the Earth's surface may also help modern fortune hunters find new sources of diamonds. But that's not what the scientists set out to do.

"The scientific goal was to try and understand how continents form on Earth," says Richard W. Carlson of the Carnegie Institution of Washington's Department of Terrestrial Magnetism, principal investigator on the multinational project. The research has been underway in southern Africa for four years now, and it probably wouldn't have happened there had it not been for the diamonds.

For more than a century now the Earth's crust has been probed, mined and gutted in that area in the relentless search for diamonds. Part of the legacy of that quest is a massive amount of geological data about the region, and the mysterious geological structures that brought the diamonds to the surface in the first place.

So in 1996 Carlson and colleagues at Carnegie and the Massachusetts Institute of Technology proposed a major study of southern Africa to see what they could learn about the forces that built continents, as well as diamonds. The research was funded by the National Science Foundation and the field work has been completed. Preliminary findings were published in the July 1 issue of Geophysical Research Letters, but it will take years for the scientists to analyze all their data.

This is a difficult area for scientists to study, because they can't put their instruments 150 miles below the ground, so they are left with indirect ways of studying it, such as rocks heaved to the surface during a violent eruption long, long ago. But there is one tool that lends itself well to determining what lies so far beneath the surface — earthquakes.

When a fault moves, it sends out seismic shock waves that travel clear through the Earth. By measuring the time it takes those waves to travel through certain areas, scientists are able to create images through a process known as "seismic tomography," which is actually quite similar to CAT scans used in medical imaging.

But since seismic stations are normally spread all over the planet, usually at great distances from each other, the images are very low in resolution, offering researchers the big picture, but few details.

The Planet, in 3-D

So the scientists took 82 seismometers with them and placed them about 60 miles apart across Zimbabwe, Botswana and the country of South Africa. The seismometers recorded shockwaves from more than 200 earthquakes during the four-year period, mostly from the far-off Himalayan and Andean mountain ranges.

That data is being used to create a three dimensional image of the region of the Earth where the diamonds formed, and where the life of the continent actually began, according to seismologist Matt Fouch, assistant professor of geology at Arizona State University, a member of the research team.

"This is one of the oldest parts of the Earth, in terms of the age of the rocks," Fouch says. Some of the rocks, he says, are about 3.6 billion years old. That makes them younger than the planet's oldest rocks, found in Australia and Canada, but still quite old.

What's really intriguing to scientists is not just that the rocks are so old, but the fact that they were thrust to the surface through a violent event that, coincidentally, also brought diamonds to the surface. The high resolution images produced by the seismic network tell part of that story.

Diamond Pipes

Diamonds are crystals of pure carbon and they form in the Earth's upper mantle, the region of rock that lies just below the outer crust. This is an area of immense pressure, at least 840,000 pounds per square inch, Carlson says. The diamonds, as well as other mantle rocks, jetted to the surface during an astonishing event. Magma known as kimberlite flowed up through weaknesses in the crust, forming "diamond pipes" which were later mined for the precious stones.

Normally, magma flows pretty slowly, but some scientists think that in this case, it was remarkably fast, possibly even supersonic in speed.

We know that, Carlson says, because in addition to diamonds the magma carried other stones to the surface. Early geologists recognized the greenish stones as unusual, although they had no way of knowing what they were, and preserved them. That's fortunate, because the stones are pure mantle rocks from beneath the Earth's crust, and they offer Carlson and his team a chance to study their chemical composition and geological structure in an effort to reconstruct events of so long ago.

The rocks, Carlson says, would not have survived the trip to the surface had the magma moved slowly. The mantle rocks are so heavy they would have simply sank down as the magma moved up the pipes. So the magma was moving so rapidly the heavy rocks were swept up like grains of sand in a raging river, moving to the surface in a matter of hours, or possibly days.

With the rocks came the diamonds. For centuries millions of diamonds lay undisturbed on the ground. Some were picked up by river waters and carried miles away, even into the Atlantic Ocean.

And then one day Swartboy found his stone, and all hell broke lose. Thousands of fortune seekers flooded southern Africa, combing through the sands for tiny rocks that could make them wealthy beyond their dreams.

What they didn't know was where the diamonds came from, and how they got there. That's a story scientists are still piecing together.

Lee Dye’s column appears weekly on A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.