The Vredefort Dome-Scar of an Ancient Asteroid

The Vredefort Dome in South Africa offers a unique record of the earth’s geological history, which is crucial to our understanding of the planet’s evolution. Erosion and geological activity that have destroyed other asteroid-impact sites also threaten the integrity of the Vredefort Dome.

As if there weren’t enough earthly forces to worry about, don’t forget the threats that come from outer space. Take this gentle ring of hills rising from the South Africa plains—who would have guessed that it was caused by a giant asteroid, walloping the earth some 2 billion years ago? We can only guess how huge the asteroid was; geologists estimate it was some 10km (6< miles) in diameter. The mark of its forcible landing is what geologists call an “impact structure”—and the one at Vredefort is not only the largest, it’s the oldest one on the earth.

The village of Vredefort, which nestles within the dome, lies 100km (62 miles) southwest of Johannesburg, just off the N1 highway to Cape Town. Actually, this ring of hills, some 180km (112 miles) across, isn’t the entire crater; it’s a secondary spot at the crater’s center where the earth’s surface sprang back violently. (The original crater, long eroded away, may have been 250-300km/155-186 miles in diameter.) The asteroid itself was simply vaporized by the ruthless blast. Such enormous energy was released in this cataclysm, it must have triggered major global changes, perhaps even altering the course of evolution. No, this wasn’t the one that killed the dinosaurs—that meteor hit the earth many years later, in Mexico. The Vredefort collision was even more important—it kicked the earth’s oxygen levels up to a threshold that would finally sustain multicellular life.
Designated a World Heritage Site in 2005, the Vredefort Dome is still relatively undeveloped as a tourist destination, though plans are afoot to lay out interpretive hiking trails and organize sightseeing flights over the crater. Given all that primeval furor, the Vredefort Dome today presents a surprisingly placid, pastoral face. Rich in birds and butterflies, the land shelters rare rooikats, aardwolves, leopards, and the endangered rock dassie, not to mention the world’s largest olive-tree forest. The rounded, eroded hills are mantled in veldt grass, with erratic outcrops of granite. But where the rock has broken away, you can see a distinctive pattern of turbulence: blobs of light gray granite trapped within an ancient flow of molten dark-gray granite. The magnetic field hereabouts remains so severely affected, it sets compasses spinning. Earth hasn’t forgotten its ancient injury—and neither should we.