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VREDEFORT DOME

Frequently Asked Questions (FAQs)

A simplified guide to puzzling issues

Where is the Dome? Is it at Vredefort?

The “Vredefort Dome” is not actually at Vredefort but includes the whole central area of the crater core. Vredefort happens to be near the centre, as is Parys. There is an isostatic dome (bubble of granite) at Vredefort that is several hundred metres across and from the top affords a good view of the surrounding landscape with its ring of hills and rolling central plain. But this is not the Dome, only a tiny part of it. There are many small granite domes in the area – another is at the Lion’s Head quarry, near Otters’ Haunt, where very good three-dimensional views of the granites and blast melts or pseudotachylites can be seen. It is worth making the 2.5km hike from Otters to the Lion's Head quarry to walk up and view the Dome Bergland and nearby plains.

So what then is the Dome? Why is it called that and how did it form?

The diagram explains why it's called the Dome. In schematic form, this shows a cross-section of the earth at the time of the impact. When an asteroid or comet blew a hole in the earth's surface, molten rock from deep down popped up like a champagne cork. First the impactor made an enormous hole, anything up to 50 km deep, liquifying the existing rocks. Into the hole rushed molten matter from below with tremendous force. The rebound from below initially uplifted an “upheaval dome”, which then collapsed into itself. It has subsequently been eroded down to its roots (see next diagram). The strata of the Witwatersrand system were violently smashed apart and bent upwards, partly capsizing near the centre. Concentric circles or rings marking the crater formed on the surface within minutes.

How big is the crater?

The visible crater is some 190km wide but can be assumed to be about 270-300km across judging by other evidence. It would be better to call it the Vredefort Structure or Ring. The widest of the concentric rings forms today's "Ring of Gold" - the external ring of the crater stretching from Johannesburg around to Welkom where South Africa's gold wealth has been mined.

Are there any bits of the meteorite left?

No. The rock from space would have been completely vaporised – along with much earth rock – during the blast. The diagram above shows how as the impactor body struck, ejecta were hurled upwards and outwards (some probably becoming meteorites headed for other planets, and some falling back to earth). Physicists have described the Vredefort explosion as the greatest single energy release ever to occur on earth (at least that we can identify). The impactor that formed the crater was either a large body such as an asteroid with a diameter of about 12 km travelling at a relative velocity of 20 km/sec, or a smaller one, such as the head of a comet, approaching at a much higher speed. The kinetic energy released was equivalent to millions of H-bombs. Had any scraps of the foreign body still existed afterwards, they would have been removed during the two billion years of subsequent erosion.

How do we get to see the Dome? Can we drive or walk to viewsites?

Drive up to the ridges of the Bergland, or crater’s inner rim, and hike to the top. From a lookout point such as Boplaas, above Venterskroon, you can look towards the heart of the Dome in the rolling Free State plains, and also observe the ring of hills that partly surrounds the Dome on the northwest side. To the south and southeast the rim is buried under the plains of the Karoo system and cannot be seen. The best images of the Dome have been taken from Nasa space shuttles and can be viewed on our website, www.otters.co.za . It is possible to see the lay of the land from airliners, especially how the Vaal River cuts its way through the landscape, but even from 10,000 metres it is not possible to see the entire remains of the crater.

Is this the biggest-ever crater on the earth’s surface?

It’s big, but we can speculate there were bigger ones. It is the biggest that can still be clearly seen. The earth went through a period of heavy bombardment by meteorites (asteroids, comets) during much of the first two billion years of its existence. At 2.02 billion years old, Vredefort occurred less than half the age of the earth ago, and hence was a relatively late phenomenon. Of course, large meteorites can still hit the earth, and we know that our planet has had a couple of close shaves even in the recent past.

What caused this crater to be preserved?

The reason we can still discern the crater is that this region of Africa has been geologically almost inert for billions of years. Other very large craters on the earth have been buried by tectonic plate action (landmass plates moving and subsiding under the crust) or have eroded away. Three major factors have preserved what remains of the crater for us to see.

• First, the blast itself capsized and buried much of the rock strata that later ages have exposed to our view, seen standing on end.

• Secondly, the blast occurred in the middle of a great shield of rock called the Kaapvaal Craton (no relation to crater). The Craton has lain unmoving on what is now Southern Africa since continental landmasses first formed and it is still there today.

• Third, the Vredefort Structure is heavily eroded but we are fortunate that it was buried for aeons under the sedimentary layers of the Karoo system. Subsequent erosion of this system by the Vaal and Orange Rivers has once again exposed what is left of the original core of the crater, deep down. The rivers have, in fact, played a major role in the remoulding of the landscape.

Why is Vredefort scientifically interesting – and also controversial?

For all sorts of reasons, scientists do not agree about major features of the Vredefort Structure – what caused them, what they reveal about impact catastrophes and rock mechanics, and how they are related to South Africa’s mineral wealth.

• Although the vast majority of earth scientists today agree that this is, indeed, a crater caused by an asteroid impact, a tiny minority still argue that it was a blast from within the earth.

• Some believe that the crater core is discontinuous with the surrounding collar and rings, giving us a window into the deeper earth’s crust – others say there is no discontinuity.

• There is a lot of debate about how gold came to be concentrated at great depths in certain strata. Was it by the original placement of gold dust in river deltas, by the blast itself, or by later hydrothermal action (hot water under the surface), or by a complex of such factors?

• And finally there is the question whether the Vredefort event influenced biological evolution on our planet. The evidence is sparse and debatable, since only single-celled creatures existed at the time. The blast caused devastating global change, including, as seems likely, major changes to the development of life.

Apart from the geology, what else is there of interest?

This region occupies a central place in Southern African history – right from the origins of humankind and the appearance of the “first nation” (the Khoisan), through the battles between the Matabele and the Voortrekkers, the Anglo-Boer wars, the discovery of gold and diamonds, the development of Johannesburg as the industrial powerhouse of Africa, and the mass protests against apartheid which erupted at Sharpeville in 1960. The Vaal River with its recreational appeal has drawn artists to Parys, and today the area is a mecca for tourists and outdoor lovers from mountainbikers to adventure racers.

Graeme Addison, Otters’ Haunt, March 2006. 

The "Dome" covers the entire central core of the crater, not just the town of Vredefort.

There is plenty to do in the area apart from crater-hunting. Arts & crafts, battlefields, coffee bars and restaurants attract visitors. So does the great outdoors, with hiking, mountain biking, whitewater rafting and adventure sports.