Vredefort
Key facts
- Country
- South Africa
- Diameter
- 300 km
- Age
- about 2.02 billion years
- Location
- 27.00° S, 27.50° E
An arc of low, wooded hills around the town of Parys, crossed by the Vaal River, marks the eroded heart of the structure. Rocks here show shatter cones, veins of dark melted rock and other signs of shock.
Vredefort is the largest confirmed impact structure on Earth, and one of the oldest. About 2.02 billion years ago an asteroid or comet, probably more than 10 km across, struck what is now the Free State and North West provinces of South Africa. The crater it made is thought to have been about 250 to 300 km wide, roughly the distance from London to Manchester.
You cannot see a crater bowl today. Over two billion years, erosion has stripped away perhaps 7 to 10 km of rock, leaving only the deep roots of the structure. What remains is the Vredefort Dome, the central uplift. In a large impact the ground beneath the crater rebounds upwards, like the splash at the centre of a raindrop hitting water. At Vredefort, rocks that once lay many kilometres underground were thrust up and tilted on end. Today they form a ring of hills about 70 km across, surrounding a core of ancient granite and gneiss, some of the oldest rocks in South Africa.
Proving it was an impact. For much of the twentieth century geologists argued over Vredefort. Some thought it was volcanic or caused by movements deep in the crust. The clinching evidence came from the rocks themselves. Shatter cones, cone-shaped fracture patterns that form only under the sudden pressure of a shock wave, are found throughout the dome. Microscopic features called planar deformation features in quartz, and rare high-pressure forms of silica such as coesite and stishovite, confirmed that the rocks had experienced pressures far beyond anything a volcano can produce.
Vredefort is also where geologists first described pseudotachylite, dark glassy veins of rock that melted by friction as huge blocks slid past each other during the impact. The name was coined in 1916 from rocks in this area.
Gold and geology. Vredefort sits at the edge of the Witwatersrand Basin, the source of a large share of all the gold ever mined. The impact helped shape the basin, and many geologists think it lowered and preserved gold-bearing layers that might otherwise have been eroded away. Without Vredefort, the history of Johannesburg might have been very different.
A World Heritage Site. In 2005 the Vredefort Dome was added to the UNESCO World Heritage List as the oldest and largest clear example of an impact structure on Earth. The landscape is a mixture of farmland, villages and hills, and it is easy to drive through without realising what lies beneath. Guided geology tours visit outcrops of shatter cones and pseudotachylite.
Why it matters. Vredefort shows that the Earth was struck by very large objects long after the heavy bombardment of the early Solar System. Studying such deeply eroded structures also lets geologists see the deep interior of a giant crater, something impossible at younger, better-preserved sites. As always when visiting geological sites, stay on public land or ask the landowner's permission, and never hammer protected outcrops.
Did you know?
The Vredefort impact released more energy than any other known event in Earth's history that has left a surviving crater.
Rocks in the centre of the dome were pushed up from depths of perhaps 20 km or more.
The towns of Parys and Vredefort both sit inside the structure.