INTRODUCTION

The world’s oldest and largest visible blast crater - probably caused by an asteroid impact - is centred on the Vredefort Dome, Ring, or Structure, situated in central South Africa. The Dome itself represents the central uplift, or upheaval dome, which rushed up to fill the hole made by the impactor. The blast occurred some 2.023 billion years ago (give or take a few million years), nearly half the age of the Earth, and today the crater is deeply eroded. In 2005, Unesco declared the Vredefort Dome World Heritage Site (WHS), although of  the entire vast crater, some 260-300km across, only part of the central portion forms the WHS. The land is mostly in private hands and will be legally protected under the Protected Areas Act.

The town of Vredefort is closest to the centre and has thus given its name to the feature, while Parys on the Vaal is the main tourism drawcard. The "Dome" is not the crater, only the core of it. This is where granite surged up from below like a champagne cork popping out of a bottle, forming a plug. For a more detailed explanation of the Dome's structure, with illustrations, read the Frequently Asked Questions. The article below is a summary of the main topics covered by this website.

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Major Dome themes

The Dome attracts scientists, students and the general public from all over the world for a great variety of reasons.

• PLANETARY SCIENCE: Geology has made the Dome famous, not only because the crater is the biggest we can see on Earth, but because erosion has exposed the deep structure of a what is a complex and fascinating planetary science phenomenon.

• EVOLUTION: The crater is also notable for what it tells us about the evolution of life from simple bacteria which lie fossilised in rocks adjacent to the Dome. The birth of life, the creation of our atmosphere, and speculation about extinctions in the course of evolution, can all be discussed with reference to evidence in and around the Dome.

• ECOTOURISM: It is extremely scenic, with its peculiar ring of mountains, the Bergland, and the picturesque Vaal River running through it. Ecotourism in all its forms is flourishing, as the Dome attracts visitors who come to enjoy its adventure offerings, from whitewater rafting to horse-riding and hiking.

• VAAL RIVER: Another notable feature of the Dome is the Vaal River, which is much younger than the Dome and yet still a very ancient river whose watercourse has been determined by faults in the Dome rocks. The many islands in the river as it crosses the granite plain of the Dome core play host to a great variety of riverine wildlife.

• HUMANKIND: This landscape was certainly where primitive humans roamed in the course of our own evolution, with another WHS, the Cradle of Humankind, lying just to the north of the Dome. The reason why no palaeontological remains of early man have been found here is that the Dome contains no limestone caves in which these remains could have been preserved.

• HISTORY: The entire crater from Johannesburg to Welkom is rich in archaeology and more recent history, with a great number of battlefields covering several epochs of conflict between tribes, nations and races. The region can truly be described as the central cauldron of South African history because this is where people from Africa and the world have struggled for possession and dominance over  land and mineral wealth.

• GOLD: Most importantly from an economic aspect, the edge of the Dome - the so-called Arc of Gold marking the goldfields of the Witwatersrand and Free State - is directly a product of the Dome's geology. The gold would no longer have been here (it would have eroded away) without the explosion which upended the gold-bearing strata and preserved the precious metal deep down for later mining.

• CONTROVERSY: Despite its status as a WHS and the fact that it has been extensively studied, much remains puzzling about the Dome. A few scientists believe that the Vredefort structure was not caused by an asteroid strike but by a “cryptoexplosion” or "Verneshot" from within the crust of the earth. This seems unlikely but debates continue. There are several other issues in dispute - for example, how South Africa's gold came to be so concentrated in the outer ring of the crater; so the study of the Dome remains hugely interesting with new findings being produced every year.

• EDUCATION: many school and university groups come to the Dome to study geology, geography, ecology and history. For youngsters, it is fascinating to hear about the forces of the blast and to speculate on when it is going to happen again. An exciting school-level experiment is described here - why not try it yourself?

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Viewing the Dome

It is impossible to see the whole of the crater from the level of the Earth - it is simply too big. But it can be clearly seen from space, as the NASA Shuttle picture above shows. It is also very difficult for visitors to understand what they are looking at on the landscape unless you have someone explain it to you. The whole epic story of the Dome is like a scientific Whodunnit - a detective tale in which we have the body (the crater itself with its central uplift) but not the killer beyond any further doubt. Whatever its origins, the site marks the greatest energy release on the Earth’s surface of which we have any record.

Talks and Trails

This site contains extensive background information and links to articles, books and videos. If you’d like to know more, contact us at Otters’ Haunt for the full briefing and location visits. We are the most up-to-date information centre in the area, specialising in briefings, excursions, research and publishing about the Dome. Go here for more about why we launched BLAST! to inform visitors. Go here for articles.

Writer

The author of this website is Graeme Addison, a popular science writer living near Parys. A former Professor of Communication, his hobbies are mountain biking and kayaking. He has written seven books including four on innovations in science and technology, and two on rivers, and has published extensively on the Dome in articles and books. He is currently working on a new book about the Dome, the Vaal River, and the evolution of life down to the present. Graeme was the founder of South Africa’s whitewater rafting industry in the 1980s and continues to run rivers, including Dome rafting and canoeing trails. He  is married to Karen, a leading mountain runner. They own and manage Otters’ Haunt, a guest house and bush camp on the Vaal River. Follow the links on this site to find out more.

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IN MORE DETAIL...

Crater River

The Vaal River, which flows across the crater and crosses into the central Dome, is one of the world's oldest water courses occupying what is arguably the oldest of river basins. The remarkable story of river systems and the Dome has been neglected in most literature about the crater. But Graeme has written books about the world's rivers and is a keen paddler, so he naturally set about researching the origins of the Vaal and its relationship with the Dome.

The Vaal is superimposed on the crater, which means it has come down from above and kept the meanders that it developed as an old, mature watercourse flowing across the flat Karoo. The original surface of the crater, which was possibly about 360km across, has long since been eroded away, and much of it has been reburied by newer formations. For a long period of hundreds of millions of years after the blast, the crater was steadily eroded by river systems and finally it was drowned under the Karoo sea. Heavy sedimentation completely covered the crater and its central Dome under about 10km of Karoo strata. But then the ancient predecessors of the Vaal and Orange Rivers began to remove the Karoo and what we can see of the crater today is the remaining deep structure on the northwest side, still partly buried under Karoo layers in the southeast.

Goldfields

The Bergland comprises rock strata of the Witwatersrand, some 100km away. There is little gold here, but between here and the outer perimeter of the crater are huge concentrations of gold. Much of the gold is deeply buried and now has to be mined at depths of 3-4km. The external ring is called the Arc of Gold and comprises the East Rand goldfields, the central Rand, the West Rand and the Free State goldfields. Along the ridge are cities whose distinctive yellow and grey mine dumps signal the presence of deep mining. From Germiston in the east through Johannesburg and Krugersdorp in the centre, to Klerksdorp in the west and Welkom to the southwest, the Arc of Gold follows the now indistinct (much eroded) external rim of the crater. 

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Evolution of life

Because the Vredefort blast happened so long ago it is highly unlikely to have caused a mass extinction. But it may well have changed the course of life’s development. Paradoxically, meteorite helped to give birth to complex new life forms by raising the world’s oxygen levels. Our species is first thought to have evolved as a walking ape some 3 million years ago on the highveld. And modern humans have inhabited this region for perhaps 150 000 years. It's hard to match this story at any other spot in the country, or the planet! There is more on the evolutionary story here.

Extinctions

It is considered possible that meteors and comets may have created conditions for life on Earth to develop in the first place. The heaviest meteor bombardment of Earth happened about 3,8 billion years ago, around the same time that life on the planet is believed to have started. Geologists researching the crater left when the Haughton meteor slammed into what is now Canada’s Arctic 23 million years ago found the impact created hydrothermal springs in the cracked rock and other conditions that would have made it easier for microbes to survive and evolve.

Most people think of meteorites in terms of life’s extinctions. A major meteorite that hit at Chixulub, Yucatan, some 65 million years ago is thought to have wiped out the dinosaurs. But at the at the time of the Vredefort impact just on 2 billion years ago there were no multicelled species to destroy. There were primitive one-celled forms of life, and it is possible that the cataclysmic effect of the blast mutated the DNA of these creatures, causing a change in the direction of evolution. So-called “stromatolites”, which are beds of ancient algae, can be seen in the rocks exposed in the Vredefort Dome, which pre-date the dome. Ancient signs of life have also been uncovered in the deep gold mines around Klerksdorp (part of the Dome). As these signs of ancient life are buried very deep in rock distorted by the blast, it is clear that life did exist beforehand.

Some evolutionary biologists speculate that the blast would have affected the direction of evolution, not only near the blast itself but in many parts of the planet because the cataclysm would have spread devastation across the planet. At the same time, it raised world oxygen levels.

Human history

The “First People” - the Bushmen - came to inhabit the plains and mountains, perhaps from tens of thousands of years ago. They hunted and gathered across the game-filled landscape , leaving etchings on the rocks which can be seen in the Dome and near Klerksdorp. But in time the Bushmen were driven out by successive waves of migrants, first the yellow-skinned Khoi people then the black Bantu-speaking tribes, all flowing in from the north, and finally by white trekboers and colonialists coming from the south. The tragic fate of the First People - hunted down, expelled into the deserts, and virtually exterminated - is one of the darkest chapters in Southern African history.

The first flood of migrants from the south included runaway slaves and fugitives from the Cape colonial administration. They formed a wild frontier society that depended on the hospitality of the black people living in the interior. From these origins came the mixed-blood Griquas. Later came the Afrikaner trekboers (itinerant herders), followed by the Voortrekkers bent on establishing new settlements.  And finally there was a flood of British colonists and international fortune seekers drawn by the diamond and gold wealth that had so surprisingly been discovered in a country that had been regarded, until then, as a backwater. The interior became cauldron of racial mixing, a true borderland of conflict, pioneering, oppression and, sometimes, co-operation between peoples. The discovery of diamonds in the Vaal River (which crosses the Dome), and then gold on the surface of the Witwatersrand, accentuated conflict over land and resources. Wars, banditry and finally apartheid caused endless friction among the multi-lingual and multi-cultural groupings. Yet today after the democratic settlement of 1994 the country is relatively peaceful and its people have learnt to call themselves the nation of South Africans.

Debates

Debate has never ended over the causes and consequences of the blast - whether it was an impact by a foreign body such as an asteroid or comet, or a "cryptoexplosion" out of the Earth itself. There are no fragments of space rock although there is chemical evidence in melt rock and high compression in surrounding quartizites. The best that can be said is that the asteroid thesis is 99% certain, judging by how the available evidence ties this crater to other proven impact sites. There is more on the crater-cryptoexplosion debate below.

Without this feature, South Africa might never have had its ring of goldfields, the original source of the country’s wealth. How the gold got there, and became so concentrated, remains in dispute. Around the crater core, a semi-circular ring of mountains known as the Bergland rears up from the rolling veld or platteland of the surrounding Free State and North West Province.

Belch or blast?

Visitors often look for “bits of meteorite” - but there aren’t any. However, many remaining signs of the blast are to be found in the “rock signatures”: melted seams of matter caused by great heat and friction; microscopic fractures indicating that the rocks were shocked; and semi-triangular rock fragments called shatter cones. There are also the mountains around Parys and Vredefort, representing what is left that is visible of the rings caused by the blast.

The Dome is the scene of the largest ever energy release to occur on the planet (that we know of), equivalent to 100 million megatons or 100,000,000,000,000 tons of TNT. The blast immediately melted a hole some 30-50km deep in the crust. What we can see today is the eroded remnant of this astrophysical event, which took place some 2.023 billion years ago.

 The Vredefort Dome “Blast Zone” is the subject of long-standing controversies about whether it was caused by an asteroid impact or by a blast from within the Earth’s crust. The idea that mass extinctions are caused by impacts from outer space has been one of the best marketed pieces of popular science but it is not the only explanation.

Asteroid theory

Today almost all experts agree that the Vredefort “anomaly” – a structure of puzzling mountains around the Vaal – was probably caused by an asteroid strike some 2.02 billion years ago. The asteroid theory was only recently broadly accepted by most, but not all, scholars. The evidence is indeed persuasive and tours of the region will allow you to touch and feel the rocks that felt the ancient blast. The presence of the element iridium (one of the two heaviest elements, iridum and osmium) in the rocks would probably prove that the impact was caused by an extra-terrestrial body. But according to a recent NASA/Los Alamos study no such component has been identified. There is no obvious terrestrial (or Earth) source of the widepsread iridium found in the Earth’s crust as a result of other impacts such as Chixulub in Mexico.

Verneshot theory

An older explanation is that the crater was caused by a volcanic eruption. The signs in the rock would be the same as for an impact:  the crater rings in the landscape, “melted glass”, shatter cones, and microscopic lines in the quartzite.

New scientific reasoning (not directly related to Vredefort but certainly relevant to it) has thrown the impact explanation into doubt yet again. The name given to a revived crytopexplosion explanation is “Verneshot” after the science fiction author Jules Verne, who proposed that spaceships could be shot to the moon using a mighty cannon. The cannon principle was put forward by scientists at the Geomar earth science institute at Kiel University to explain mass extinctions. They speculate that a huge eruption of gas took place from beneath the crust. This was not a volcanic event but a release of enormous gas pressures in a planetary belch which may well have hurled a large Earth meteor into space. If the Earth itself was responsible for the blast, the hole in the crust that occurred at Vredefort may not have come from outside but from within and would represent the mouth of the cannon.

Kaapvaal Craton

Could such a blast from within the earth happen on earth today? Don’t hold your breath… These things take a bit of time to develop, say a few million or billion years. But the huge lump of solid rock on which South Africa largely sits, the Kaapvaal Craton, is certainly rising at a relatively fast pace. Some students of the Vredefort phenomena believe that the Blast Zone is a window into the craton and can tell us a lot about what happened in the evolution of the Earth as well as what may happen in the future. The so-called “African Superswell”, which is a strange and so far unaccountable swelling up of the Earth’s surface (today) in our region of the planet, apparently as a result of massive forces underneath the crust.