Step by Step Formation of the Vredefort Dome

© David Fleminger

An Enormous Amount of Energy

As the Vredefort bolide hurtled towards the Earth, its speed could have been anywhere between 40 000 and 270 000 kilometers per hour. When it smashed into the ground, an enormous amount of energy was instantly released in an explosion that was roughly 5000 times more powerful than Little Boy, the incongruously named nuclear bomb which destroyed Hiroshima.

It is estimated that the impact would have measured 14 on the Richter Scale, or 100 000 times more powerful than the worst earthquake ever recorded.

The bolide penetrated about 10 kilometers into the Earth’s surface before it exploded under the pressure. It sheared through the rocks of the Transvaal, Ventersdorp, Witwatersrand and Dominion Supergroups to expose the granite basement of the Kaapvaal Continent.

Dimension Stone

This steep-sided transient crater was unstable, however, and quickly collapsed in the moments after impact. The crater rim fractured into a series of parallel terraces, which proceeded to slip down into the deep basin.

Within minutes, the crater had widened out into a shallow-bowl shape that had a diameter of roughly 300 kilometers. It extended from Pretoria to Welkom, and is still identifiable by a series of concentric ridges (including the Witwatersrand) that were tilted up by the impact.

At the same time that the rim was collapsing, an uplift was forming in the middle of the crater. This is because, when the bolide hit, the rocks at the epicentre of the impact were heavily compressed. After the bolide vaporised, the cessation of pressure from above allowed these rocks to bounce back up, to regain their equilibrium.

This release caused a section of Archaean granite to punch through the younger rock layers that lay above it. The previously horizontal rock layers of the Witwatersrand and Transvaal Supergroups were thus flung onto their side and overturned, forming a collar around the rising mass of older basement rocks.

The uplift then settled down again, but still protruded above the rim of the crater. Much later, the ancient Vredefort granite that had been uplifted and exposed was quarried, and slabs of this ‘dimension stone’ were used to decorate the pillars of the new local terminal at Johannesburg airport.

Dome Mountain Land

The resulting landscape, as characterised by a central uplift surrounded by a broad, flattened basin and a broken rim, is called a complex crater and should be compared with smaller ‘simple’ craters, such as the one at Tswaing. Technically, only the central uplift (originally measuring about 100 kilometers across) qualifies as the Vredefort Dome. The remainder of the crater structure is not protected by any heritage legislation.

At the time of the impact, the ground level was much higher than it is today and, over the past 2 billion years, the Vredefort dome structure was heavily eroded. In the south west part of the crater, new rocks of the Karoo Supergroup covered the dome structure, while other sediments filled in the broad basin around the dome.

Today, the only visible remainder of the dome is a beautiful semi-circle of parallel ridges that curves between the towns of Potchefstroom and Parys. This remarkably geometric feature is clearly visible on aerial and satellite images, and it is now known as the Dome Bergland or Dome Mountain Land.

By David Fleminger