The Vredefort Dome
Centre of the World’s Largest Meteorite Impact Structure!
In the heart of South Africa, straddling the border between the North West and Free State provinces and on the doorstep of populous Gauteng, lies a truly unique, indeed mind-boggling site, the Vredefort Dome which has just been declared South Africa’s 7th World Heritage Site. What makes the Vredefort Dome so special that the entire world considers it worth of conservation and special status? The answer to this is easy: The Vredefort Mountain Land, involving the Dome Bergland Conservancy [give proper, new name], is the most spectacular landscape between the Magaliesberg Range in the northeast and the Drakensberg Mountains in the southern Free State Province. Anybody who has been on top of one of the prominent mountains overlooking the dome bergland has marveled on the beautiful ridge and valley country through which the idyllic Vaal River valley snakes.
That these mountains represent the eroded relic of an originally 300 km wide meteorite impact structure (actually produced by the impact of an asteroid or comet some 5 to 10 kilometres wide and perhaps travelling as fast as 200 000 kilometres per hour) is difficult to perceive. From the air, for example in a jumbo on the way to Cape Town at a height of 10 000 metres, one sees the semi-circular ring of mountains surrounding the less rugged interior part of the Vredefort Dome, and even on satellite images [the green satellite image here (small print underneath: Courtesy of M. Phillips, Greenwich] one can not discern much more than that; where is the giant crater one should be seeing? The problem is that the impact took place more than 2 000 million years ago. The original crater structure was some 100 km wide and 40 km deep. It , however, rapidly collapsed to a final crater size of 300 km width and about 10 km depth, and this has been gradually eroded, leaving us without a real crater to look at, but with a window into the deep interior of the original central uplift of this giant impact structure. Looking at the full moon and marvelling at its dark basin features that are many hundreds of kilometres wide [add a photo of it], we can try to imagine how the original Vredefort impact crater would have looked like [here a picture of a moon crater such as that on the mid-left page – small print beneath: Courtesy of NASA –Note the central mountain feature of uplifted rock and the terrace collapse along the rim]. At the time of impact the land surface was 7 to 10 kilometres higher, and all this has been weathered away by now.
The Witwatersrand basin around the Vredefort Dome is the remnant of the outer parts of the impact structure. This means, yebo, that the Vredefort impact structure extends as far as Johannesburg in the northeast and to Welkom in the southwest. What we admire today is a cross section through the lowest parts of the impact crater. In the central part of the Vredefort Dome, rocks that originally were located maybe as much as 20 kilometres down in the Earth’s crust have been uplifted by the impact – and today give us a ready view of the Earth’s inner secrets. And surrounding this core of deep-crystal rocks we have the steeply upturned and even overturned, and thoroughly shattered rocks of the Vredefort Mountain Land, which originally were closer to the impacted surface.
How strong was the impact event?
Take the entire nuclear weapons arsenal of the world and blast it to smithereens in one go, and this explosion would only be a semblance of an impact event like that at Vredefort, so long ago. To gouge a whole of 100x40 km out of the Earth, an unimaginably gigantic explosion must have taken place. The rock and dust ejected from the crater was deposited around the world. The projectile and much of the immediate target rock was vaporised and more entirely melted. Remnants of this melt still occur in the form of large dikes of a special rock, known as Vredefort Granophyre, in the region. Further out from the point of the explosion, the rock was shattered – and anybody looking at the quartzite ridges of the berg land can see what that means: the rocks are thoroughly fractured and in many places fragmented. Earthquakes of magnitudes far off the normal scale would have shaken the planet. The force of this event even surpasses that from another enormous hit, the impact that killed off 70% of all life some 65 million years ago, including all the dinosaurs. But, as things go, this catastrophe was also a very fortunate event for us humans, as it meant that from this catastrophe the mammals could evolve and eventually conquer the planet – ultimately resulting in the highest developed life form that Earth has ever seen – mankind. Whether Vredefort had a decisive influence on life evolution on this planet is unknown. There was only algal life at the time, and primitive life forms (such as the cockroach) are notoriously resilient.
How was Vredefort recognized as the result of catastrophic impact?
For the last 50 years, scientists have debated and battled – absolutely the right word – about the origin of the Vredefort Structure. Some believed that the dome was the result of an internal explosion of sorts, whereas others favoured meteorite impact. Definitive evidence in the form of rock and mineral deformations that require huge pressures and temperatures as they can only be produced in the outermost parts of the Earth, the crust, by the impact of a large, ultra-fast extraterrestrial bolide was mostly only discovered in the last 15 years. The Vredefort melt rock, for example, contains a tiny trace of the extraterrestrial projectile – only discovered in 1996. The Vredefort impact structure is the largest and oldest of its kind known on Earth and has been a unique laboratory for those scientists specialising in chaos and catastrophe.
What are the special features of the Vredefort Dome?
The Vredefort Dome is beautiful. Its mountains and valleys, the Vaal River bed, and many different climatic and soil conditions giving rise to a vast variety of different vegetations, exceptionally varied bird life, all this makes the Dome a very special place to live in and to visit. But considering that all this beauty and specialty ultimately is the result of the world’s darkest moment, provides another dimension. Humans have inhabited the Vredefort Dome for at least 150 000 years – as ample evidence of Stone Age habitation proves. In Iron Age times, from about the 1300s to 1850, maybe as many as tens of thousands of Tswana people lived in the mountains and hills of the dome, likely because the varied landscape provided shelter and excellent grazing alike. In even more recent times, the area around Venterskroon has seen a little gold rush, when it was discovered that the rocks already mined since a few years at Johannesburg cropped out at surface and just below in the hills surrounding this hamlet (soon to become a bustling - and perhaps raucous??? – mining camp. This was shortly followed by the intriguing combat of the Anglo-Boer War, when General Christiaan de Wet made use of the impact-produced geology, and the landscapes based on the underlying geology, to play cat-and-mouse with the British troops. Geology, archaeology, biodiversity, history and culture – all the ingredients of a fascinating and intriguing pie. Add to these already existing tourism facilities of highest standard, and you have the basis for a top World Heritage Site. Need a few pictures here of Iron Age ruins, some Anglo-Boer War feature, some plants , such as proteas from Thwane, …a nice landscape of the bergland….
What does the future hold for the Vredefort area?
The Dome will continue to attract geologists and archaeologists – but its multi-faceted attractions should be enough to make every South African wish to come and pay tribute to one of the world’s outstanding natural and cultural heritage sites. Recognized as national and world heritage means that there is an obligation to conserve and protect, but also to make use of the resource for education and research, and economic development. This resource does belong to all South Africans, and sharing it will bring economic benefits to the Dome and its people. Sharing it will also show to all visitors how precious this site is and hopefully contribute to all of us noticing how valuable and irreplaceable our natural environment is.
Prof. Uwe Reimold (Wits)
The Book "METEORITE IMPACT - The danger from space and South Africa's Mega - impact " can be orderd from
Parys Info (056) 811 4000 email@example.com
Dome Bergland Conservancy
World Heritage listing
South Africa Info
Hartebeesthoek Radio Astronomy Observatory http://www.hartrao.ac.za/other/vredefort/vredefort.html
South African World Heritage sites http://whc.unesco.org/en/statesparties/za/