Why is a crater called a dome?
Actually, the Vredefort Dome is only the core of a very much larger crater that stretches from Johannesburg to Welkom. The Dome, Ring or Structure around Vredefort and Parys is not itself a crater – it is merely the central uplift, rebound mount, or upheaval dome, which marks the centre of the impact. It is “ground zero” of the impact.
What is the “Dome”?
The Vredefort Dome is a plug of granite that welled up when – as seems most likely – an asteroid hit the Earth some two billion years ago. It is has been deeply eroded over time so that today what you see is only the remnants of the central uplift. The whole feature is called an astrobleme, which means the eroded remains on the Earth’s surface of an ancient impact structure produced by a large, cosmic body.
Where is the Dome? Is it at Vredefort?
A visitor centre is now being built atop the granite dome at Vredefort, so at least visitors will now get the whole story. Vredefort happens to be near the centre, but so is Parys. The Vredefort granite dome is what is called an isostatic dome. Such domes “bubble up” when the weight of overlying landscape is lifted off them, as occurred when the Karoo system eroded to expose the older granite basement. Also, granite tends to erode into rounded shapes because it exfoliates, or cracks off in leaf-like shards. There are many small domes in the area – another is at the Leeukop quarry, near Otters’ Haunt. From the top, most afford good views of the surrounding Dome landscape with its ring of hills and rolling central plain.
This needs further explanation…
Yes it does. It is sometimes difficult for first-time visitors to understand the structure of the landforms. The problem is partly that people come looking for a “dome” and often end up at Vredefort where you can see an impressive, but relatively small, 300-metre granite dome (where an information centre has now been built). This is not the Vredefort Dome! The Vredefort Dome is the central core of a very large crater that cannot be seen as a whole except from space. The structure is like a champagne cork that popped out of the bottle when the blast blew a hole in the Earth and rock boiled up from below. The false-colour illustration in the sidebar (for geological use) is derived from a photo taken from space.
What was the process of forming the crater?
In the greatest energy release that we have any record of on Earth, a hole around 50km deep was blown in the planet’s crust. In the centre, the uprushing rock made the upheaval dome. The crater spread outwards in a series of rings beyond the core. It’s rather like what happens when you drop a stone in a pool of water. The stone makes a hole in the water which immediately splashes together, sending up a column of water in the middle while waves radiate outwards in rings. Some of the water splashes upwards and outwards, falling down around the rings. In the case of a large meteorite strike, some of the rocks hurled up into the atmosphere may leave Earth and head for other parts of the solar system as new meteors. (You can see a droplet of water at the top of the splash, a meteor, in the picture alongside). The column of water collapses, leaving a depressed area in the middle of the expanding rings of water. The water soon flattens out again, but in the case of rock, the central uplift and the rings remain – cast in stone. In the Vredefort blast, the whole process of upheaval and collapse is thought to have taken less than four minutes.
What is the “collar”?
The Dome is surrounded by the upended strata of the Witwatersrand Supergroup, which were blown apart and virtually capsized by the impact blast. These strata, now standing virtually upright, form the semi-circular ring of mountains called the Bergland. The Bergland is the “collar” partly surrounding the core. For further explanation of the form of the Dome as a “central uplift” with a surrounding collar go here.
Why is the asteroid also called a meteorite?
The term meteorite is applied to any space rock or bolide that hits the Earth. So an asteroid, large or small, becomes a meteorite when it enters our atmosphere and smashes into the ground. Millions of smaller bits of space matter burn up in the atmosphere before they hit the ground and these are “meteors” or shooting stars, some of which come in waves at certain times of the year like the Leonid showers.
Why is the Dome sometimes called a Structure?
Most scientists don’t like the term “Dome” and feel that the entire geological phenomenon should be labelled a “Structure”. There are many domes in central South Africa – Johannesburg itself rests on one – but these granite formations have little in common with the structure causes by the Vredefort blast. It is also a bit of a misnomer to call the Dome a “Ring” because it is, at best, half a ring or rather a pear-shaped remainder of what might once have been a more completely rounded Ring. As explained below, there has been very little tectonic plate action or other geological disturbance in this area since the Dome was formed, which is why we can still see it. But it does seem to have been faulted and compressed over time, and it is deeply eroded.
Are there any bits of the meteorite left?
In the central core, a rock called granophyre does seem to contain chemicals from an asteroid blast. This impact melt-rock is black with tiny intrustions. The asteroid from space would probably have been completely vaporised – along with much earth rock – during the blast. 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 10-15km 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 trillions of H-bombs. Had any scraps of the foreign body existed, they would have been removed during the two billion years of subsequent erosion. However, certain minerals (stishovite and coesite) are metamorphosed forms of quartzite that tend to be found at the site of meteorite impacts. The same can be said of the granophyre.
Why do scientists differ about the crater?
For all sorts of reasons, scientists do not agree about major features of the Vredefort Structure – what caused them, what they reveal about earth processes, and how they are related to South Africa’s mineral wealth.
Although the vast majority of earth scientists today agree that the Dome is, indeed, the central uplift of a crater caused by an asteroid impact, a tiny minority still argue that it is evidence of 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 surrounding the Dome. It may have been 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, and it is possible, though unlikely, that it caused changes to the development of life – perhaps by altering the DNA of primitive life.
Why is the Dome hard to see or understand?
Only from space can the entire structure be seen, and it has been photographed numerous times from the Shuttles and from satellites (eg Google Earth). So until the dawn of the space age it was difficult for the ordinary person to see the ring structure. It is still hard, since the Dome Bergland is just a semi-circle of mountains in the northwest segment of the structure. However, starting with geological surveyor Louis Nel in the 1920s, earth scientists have comprehensively mapped the surface and underlying layers of the structure and have also recorded its gravity and magnetic imprints (which are anomalous and puzzling). All of this is fairly advanced science, and unless you have someone explain the origins, shape and development of the crater to you, you are likely to miss many of its features.
How do we get to see the Dome?
Drive up to the ridges of the Bergland, or collar, and hike to the top. From a lookout point such as Boplaas, above Venterskroon, you can just see the core as well as the ring of hills that partly surrounds it 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 some can be viewed on this website. 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 crater and its rings. It is very large, estimated to be 270-320 km across in its entirety, stretching from Johannesburg to Welkom.
How did the Bergland get its name?
The crescent of mountains in the northwest part of the Dome collar is called the Bergland. This term apparently was to be used by right-wing Afrikaners who aimed to found a white homeland in the area. Although there has been some resistance to its use, the term Bergland (like Dome) has stuck, and today it is uncontroversial. It well describes the jutting sections of Witwatersrand strata that stand up as a result of having being thrown on edge by the blast. As the Witwatersrand strata are about 7+ km thick, the Bergland configuration shows these strata as a band of mountains, with valleys in between, that are about 7km from Dome core to the outer Ventersdorp lavas where the landscape becomes rolling grassland.
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. There is possibly a larger crater under the Antarctic icecap in Wilkes Land, some 500km across, but it cannot be seen and has only been revealed with geological soundings. 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 2.023 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.
So there were larger impacts?
Yes, definitely. There is no doubt that the largest impact ever suffered by our planet was during Earth’s formation as a “planetisimal” another similar body collided with us about 4.4 billion years ago. After the crash the two bodies separated. The Earth retained many of the heavier elements, including its iron core, while the Moon spun away carrying lighter rocks which have much in common with our drifting continents. The Moon is still floating away from us and may eventually separate to go its own way, though by then the solar system itself could be in a state of decay.
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.