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A blast from the past

New scientific findings undermine the claim that the oldest and largest meteorite crater on earth is right here in South Africa, centered on the sleepy dorp of Vredefort. The Vredefort Dome is being proposed as a unique World Heritage Site. GRAEME ADDISON suggests it’s time to revise the motivation and recognise the real value of the Dome.

Scientific discoveries have a way of upsetting the applecart. Two centuries ago the dilettantes and aristocrats in charge of the Royal Geographical Society scoffed at early reports that Mount Kilimanjaro could have a permanent snowcap, but eventually explorers proved them wrong. The mandarins of established knowledge also refused to believe that the Himalayas could be higher than the Andes, until British surveyors showed that at least one peak—Nanda Devi—was 25 479 feet high (7766 metres).

Nanda Devi, oddly known in those days as “A2”, was soon celebrated as “the world’s highest mountain”, a position it occupied for 25 years until dislodged by Kangchenjunga at 27 176 feet (8283m). This peak was demoted within a decade by the pre-eminence of Mount Everest, named in 1865 after the Surveyor-General of India, Sir George Everest. This eccentric martinet insisted he could never be wrong, but he was. In 1999 the peak was found to be 6 ft (2.52m) higher than thought, at 29,035 (8850m).

The name Everest was an accidental but happy choice for the 60million year-old hunk of rock that today represents everything grand and tragic in mountain mythology. It is poetically known as Chomulungma (mother goddess of the universe) in Tibet and Sagarmatha (goddess of the sky) in Nepal. Less evocative is the name of what has been claimed to be the world’s oldest and largest meteorite crater, the Vredefort Dome, situated around the Vaal River and stretching from Johannesburg to Welkom.

Those who hear about it for the first time are usually surprised to learn that we have a grand geological phenomenon lurking in our midst. Gautengers are not entirely taken aback because many have felt for a long time that they teeter on the edge of ghastly devastation; but the name Vredefort somehow lacks an aura of romance. It could be time to delve into African place names or legends of the fall. For a mighty rock did fall from the sky here about half the age of the earth ago—2.3 billion years ago to be more accurate, give or take few million. 

If anything has a claim to being goddess of the sky, it is the roving asteroid that paid us a visit that day. Some 10-15km in diameter, travelling at a speed of 10-20km per second, it slammed into the earth with a force experts estimate to have been about 87 million megatons. As one megaton equals a million tons of TNT, and the Hiroshima nuclear bomb was only one-fifth of a megaton (20 kilotons), the cosmic bomb was infinitely worse than anything mankind is able to detonate.

The Vredefort crater, estimated at 300-320km in diameter, is one of three known massive impact sites amongst many hundreds or even thousands of meteorite blemishes on the earth. The other two really large sites are at Sudbury in Ontario, Canada, and Chicxulub on Mexico’s Yucatan Peninsula. The Sudbury area has the dubious record of being the target area of two major hits: the first, almost as ancient as Vredefort, causing a 200km-wide crater, and the second a smaller one just 37 million years ago.

Before Vredefort, the largest and (still the most famous) crater was Chicxulub (pronounced CHEEK-shoe-lube). This impact was almost certainly the cause of the extinction of the dinosaurs about 65 million years ago, the clouds of dust following it blanketed the earth and killed most animal life.

What happened in the Vredefort blast? Like a drop of water falling into a pond, the asteroid punched a hole in the earth’s crust and caused a recoil of molten matter into the atmosphere. The meteorite itself vaporised in the explosion, and the earth was liquidised down to at least 10km as the crater walls slumped inwards, effectively capsizing part of the crust. What we can see today are merely the remnants, like broken teeth in a skeleton’s jaw, of the mountainous upwelling that has eroded over the past 2 billion years.

 The effect of it all was to bury, deep down, the gold-bearing strata of the older Witwatersand basin. As the “astrobleme” (eroded crater) was worn away, so the tilted strata of the gold reefs were exposed—and this also explains why the gold industy is perched in a ring around Vredefort (see map). South Africa has the meteorite to thank for first preserving, then exposing, its mineral riches in the upturned edges of the Witwatersrand basin, which have now broken the surface.

Asteroids are actively sought and plotted by astronomers around the world because of their potential for causing devastation on Earth, and there are known to be a few big ones on their way towards us in the next few decades. Unlike many of these “near-earth objects” which harmlessly pass us by, the Vredefort meteorite struck with the force of a cosmic ultrabomb. This occurred towards the end of a long period of cratering by space objects, and we have no way of knowing how many blasts there were or how big they were.

This is precisely the problem for those in South Africa who are now promoting the Vredefort Dome as a future World Heritage Site (WHS) under Unesco’s wing. Signboards as you enter the Vaal River town of Parys, near the centre of the crater, say this is the oldest and biggest impact site on earth. South Africa already has four WHS areas—Sterkfontein, Robben Island, St Lucia and the Drakensberg. Vredefort would be the fifth. According to Coen Erasmus, researcher for Free State’s Nature Conservation, a draft motivation has gone to Unesco in Paris.

The process of investigation by leading scientific and cultural experts should begin soon and is bound to take several years as they question the validity of the claim, its tourism viability, and compliance with Unesco rules. At this early stage it is not disloyal to the country to suggest that work had better be done to prove the relative merits of Vredefort—not necessarily as the biggest and oldest meteorite crater on earth, but currently the biggest and oldest clearly identifiable impact structure" (just beating the Sudbury double crater on both counts). Hence it continues to have a valid claim to the uniqueness that is demanded of any candidate for WHS status.

A forum of local authorities and private sector stakeholders has been formed to realise the benefits of heritage through tourism, attracting investment and creating jobs in the Potchefstroom-Parys region. In February this year, planning and promotion of the site began in earnest with the launch of a report by Contour Project Managers and Grant Thornton Kessel Feinstein, on behalf of the Vredefort Dome Conservancy. The report estimated that the Dome would attract 113 000 visitors 2002, growing at 2.5 percent a year. But competition from the likes of Pilanesberg, Madikwe, Dinokeng and Hartbeespoort Dam will make it “difficult for the Vredefort Dome to attract large numbers (over 300 000 visitors per annum)” says the report.

Obviously, gaining WHS status will give the Vredefort area a competitive advantage. But no sooner had the conservancy report appeared than researchers based in the United States published an article in the August 23 issue of the journal, Science, knocking Vredefort’s claim down a peg or two. They presented evidence that an even larger meteorite blast—or several of them—occurred in the area of the earth’s crust that were once joined but are now separated by the Indian Ocean: Mpumalanga and Western Australia.

“The Barberton greenstone belt of South Africa and the eastern Pilbara block of Western Australia provide information about Earth’s surface environments between 3.2 and 3.5 billion years ago, including evidence of four large…impacts that likely created large craters, deformed the target rocks, and altered the environment,” said the authors, Gary Byerly, Donald Lowe, Joseph Wooden and Xiaogang Xie. Byerly, one of the principal authors, was reported as saying the the discovery had come by chance but was a “delightful surprise”.

“We were working in these areas because we know these are very old rocks and we were hoping they would shed some light on ancient volcanism and early evolution of life. The last thing we expected to find were these impact layers,” he told ABC television.

The scientists proposed that the related impact was many times larger than the one associated with the dinosaurs’ extinction. Particle evidence in the rocks show that a powerful impact took place 3.47 billion years ago. Apparently the crater has been eroded away and the evidence takes the form of “buckyballs” (trapped in fullerene carbon molecules) found by the researchers in sedimentary rocks.

It is estimated that a 1 km-size asteroid impacts the Earth every million years. Smaller asteroid impacts, with the force of a several kiloton nuclear weapon, occur several times a year in the upper atmosphere, and cause extensive damage on the ground about every 100 years—as they did at Tunguska Northern Siberia in 1908 when a 30 meter asteroid exploded, flattening and burning the forests.

 Business commitments and PR spin are already far advanced in selling the Vredefort Dome as the Big One. It may not be. Local scientists, who have shown only fairly recently that this is indeed an impact crater, may now have to revise the record in the light of new findings. Asteroids are by no means unique, so the task of the Vredefort planners and promoters is to convince the world that our local astrobleme really does have a substantial claim to fame—and what better than it preserved the gold wealth of South Africa? Call it the Egoli Dome.

•  Graeme Addison is a writer now living in Parys and working on a book about the Vaal River and the Dome.