32. Vredefort, South Africa
The Vredefort structure is located near the center of the Witwatersrand Basin, about 100 kilometers from Johannesburg. It is expressed as a central core, about 40 kilometers in diameter, composed of old crystalline rocks. A deformed collar of uplifted and overturned younger sediments and lavas surrounds the core. Younger, flat-lying sediments resulting in the arcuate shape observed in this image bury much of the structure. The diameter of the collar rocks is approximately 80 kilometers, but reconstructions based on the distribution of shock metamorphic effects suggest an original crater diameter of up to 300 kilometers, encompassing the whole extent of the Witwatersrand Basin. It is very likely that the preservation of the gold deposits of the Witwatersrand Basin, the largest on Earth, is due to the structural constraints provided by the Vredefort impact structure. Formed about 2 billion years ago, it is the oldest and largest recognized impact structure on Earth.
Space shuttle image STS51I-33-56AA.