How the Earth Makes Diamonds

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.

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