Vredefort Dome
Vredefort Dome, the largest known meteorite impact structure on Earth. The Vredefort Dome is an area of raised land near the town of Vredefort in the Free State province of South Africa. The dome is at the centre of a crater formed by the impact of a very large meteorite about 2.023 billion years ago, making it the second oldest known impact structure on Earth. (Only the Yarrabubba impact structure in Western Australia state is older, with an age of 2.229 billion years.)
The meteorite that formed the Vredefort Dome was about 20–25 km (12–16 miles) in diameter. It may have been moving as fast as 25 km (16 miles) per second when it struck. The impact created a crater that originally may have been about 250–280 km (155–170 miles) across. A layer of solidified droplets of molten rock (called spherules) 10 cm (4 inches) thick from the Vredefort impact has been found in Russia’s Karelia region, which at the time of the impact was about 2,000–2,500 km (1,200–1,600 miles) away. The dome formed when underlying granite shifted upward at the centre of the crater after the meteorite blasted away many layers of soil and rock.
Billions of years of weathering have made the Vredefort Dome and the surrounding crater difficult to recognize from the ground. Because the surrounding crater has weathered away, the Vredefort Dome is considered an impact structure. The Vredefort Dome was originally considered to be possibly volcanic in origin, but in 1937 American geologists John Boon and Claude Albritton noted the absence of volcanic material, similarities between Vredefort and Arizona’s Meteor Crater, and evidence that Vredefort rocks had been exposed to enormous pressures to suggest that a gigantic meteorite impact had occurred there. For decades the meteorite hypothesis was not considered a likely explanation for the dome, but in 1961 American geologist R.B. Hargraves showed the existence there of shatter cones, conical structures in rocks formed by strong shocks, such as those from nuclear explosions or meteorite impacts. The cones pointed to the centre of the Vredefort Dome.
In 2005 UNESCO declared the Vredefort Dome a World Heritage site. It is the seventh World Heritage site in South Africa.