Why Diamonds Can Be Found at Crater of Diamonds
Ever wonder why the earth produces diamond sites and why we have a field in Arkansas where diamonds are found? The story of the diamonds found at the Crater of Diamonds State Park begins over three billion years ago with the formation of diamonds as the stable form of carbon in the earth’s mantle. At the tremendous pressures and temperatures some 60 to 100 miles below the earth’s surface, diamond crystallized from carbon, and under those conditions it remained stable.
During the past three billion years, many geologic changes have taken place on the surface of the earth. Crust formed and was destroyed, continents formed and migrated, and mountain ranges were built and eroded away. About 300 to 250 million years ago, the continent we now call South America collided with the southern portion of present day North America. This collision formed the Ouachita Mountains from sediments that were deposited in a deep ocean environment. The Ouachitas began to erode and during the Cretaceous Period (144 to 66 million years ago), the southern area of this eroded mountain range was covered by seas and the area of the Park was near-shore, but under shallow seawater. About 100 million years ago, an instability in the Earth's mantle caused the movement of gas and rock to the surface. This volcanic vent, known as the “Prairie Creek” diatreme by geologists, rose rapidly through the upper mantle and crust, carrying with it fragments of mantle and crustal rocks and minerals, until it came near enough to the surface to explode due to the release of gases. When it exploded, it created an 83-acre funnel-shaped crater with sides sloping inward at about 45 degrees. Much of the airborne material formed by the initial explosion fell back into the vent. The speed of rise of the mass allowed the diamonds to be preserved in this material.
Geologists calculate that only about 160 feet of the original vent has been eroded away, concentrating the heavy minerals, including diamond, in the present day soil. At the Crater, diamonds are often found loose in the soil, having been released during the rapid weathering of this unstable mantle rock.
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Kimberlite versus Lamproite
The original host rocks, described from Africa and other sites around the world, including those of the Crater of Diamonds were first described as kimberlite and peridotite. But since the discovery of diamonds in Australian "lamproite rocks," many of these localities have been reevaluated. The rock types at the Crater have been found to more closely resemble lamproite than was previously known, therefore, we now refer to them as "lamproite rocks." Differences are subtle and only by detailed scientific studies can they be determined. You can research these rock names on the Internet to learn more.
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