The centre of South Africa is an amazing place. The long drive from Cape Town in-land is exciting for the first two hours, while the road climbs up through the mountains, past vineyards and valleys full of fynbos. Once on the high interior plateau of South Africa, the landscape becomes dry and monotonous – and it stays like that for hour after long hour. This is the Karoo’s dry shrubland. It was covered by the Karoo flood basalt, 180 million years old, associated with the break-up of Gondwana. Half of the flood basalt is now in Antarctica. Not much seems to have changed since: in the Karoo, Gondwana counts as recent history. Slowly, the land becomes less dry; after many hours, grassy fields and even trees appear. It also grows ever more ancient with the passing hours. Cape Town is the new addition to South Africa – Table Mountain is a mere 500 million years old. In the deep interior lies one of Earth’s original cratons which has been here for more than 2 billion years. Our goal is the region of Johannesburg, near the heart of craton. Go further, several hours beyond Johannesburg, and the age of the land reaches well over 3 billion years and the lava becomes komatiite. That is too much time. We don’t want to go there.

As we approach Johannesburg, the landscape becomes one of rolling grass lands and rock outcrops. There is not a single volcano on view anywhere. But underneath the surface lies history. 50 km northwest of Johannesburg is a place known as the Cradle of Mankind (now renamed into Cradle of Humankind). It an area of limestone caves which is our largest source of hominin fossils. Something like a third of our ancestral fossils known in the world come from here. 1000 fossils from a variety of hominid species have been found. Here is also the oldest known use of fire, dated to 1 million years ago. The Australopithecus africanus  (the Southern Ape of Africa) known as ‘Mrs. Ples’ lived here. If you have time, the Maropeng visitor centre comes highly recommended. It will take you on a journey through time, starting with a thundery boat ride. Do be aware of snakes in the surrounding fields.

But there is much more to see. If volcanoes are high on your list, another 150 km to the northwest is the Pilanesberg, the oldest visible volcano in the world, dating to 1.4 billion years ago. If you want entertainment, of a type that also feels slightly outdated, the make-believe world of Sun City is there as well. If you need elephants, again the Pilanesberg is a good place to go. Many more wild elephants as well as the other ‘Big Five’ (and the countless smaller ones) can be found a few hours east of Johannesburg with some of the best (and malaria-free) wildlife viewing in Africa. The Kruger National Park may take you a week to explore. If you like money, the mineral wealth of the Bushveld intrusion is east of Johannesburg, and of course the infamous gold mines are near Johannesburg. How did one country end up with so much treasure?

Craters

Not every hole is volcanic. 40 km north of Pretoria lies the Tswaing crater. It is just over 1km in diameter, with a saline lake at the centre – the name ‘tswaing’ means ‘salt place’ in the Tswana language. Much of the salt was mined and the lake is not as saline as it used to be. Initially the crater was thought to be volcanic, but drilling done in 1989 found no evidence of any volcanic activity. I would guess they were hoping for a diamond find! Instead, shattered rock indicated that this was an impact site. Tswaing is among the most recognisable meteor craters in the world. The crater is 220 thousand years old (ten times younger than Mrs. Ples) and was made by a meteor some 50 meters across.

Meteors come in all sizes. Small ones burn up in the atmosphere and cause fire balls. Larger ones reach the ground and do damage but may not leave a large or long-lived crater. When found on the ground, a meteor is called a meteorite. The first one known from Africa was found in 1793 at the Cape of Good Hope. It was a 140 kg block of iron – nowadays most of it is kept by the National History Museum of Vienna. A much larger meteorite is on display in Namibia, the 50-ton Hoba (iron) meteorite. The display is built around it as it was too large to move. The Chelyabinsk meteor was 17 thousand ton, but it fell apart in the air and all parts found on the ground were small. The only crater was formed within a lake, under water. The Tswaing crater was formed by a 30 thousand ton meteor. The rock hit the ground at its enormous speed, came to a halt, the kinetic energy melted and vaporized the rock and caused a big explosion underground – hence the crater, and hence the lack of any remaining Tswaing meteorite.

There is another meteor crater in South Africa. It is in the northwest of the country in the Kalahari desert, is 145 million years old, and is far larger than Tswaing: the Morokweng impact structure is 75 km in diameter. (Some have argued for a much larger size but drilling has not confirmed this.) It is similar in size to the Chesapeake Bay impact structure. But nothing is visible on the surface: it was found from gravity surveys which revealed this buried structure underground. Impact craters don’t last – see them while they are young. How many impact craters will have been lost to time in a land more than 2 billion years old? Did good old Mrs. Ples see ones that we no longer know about?

But there is an impact crater near Johannesburg that is both older and larger than Morokweng. And it is visible above ground, if you know where to look. You do need to zoom out – it is too large to be recognizable from a view from the ground. And Mrs. Ples lived on its edge.

Vredefort

It is a small town some 120 km south-southwest of Johannesburg. It feels far from anywhere. This is a farming region and there are grain silos on the edge of the town. Vredefort’s main claim to fame is as the site of one of the concentration camps used during the second Boer war – something that would now be seen as a war crime. There isn’t much to do and don’t expect niceties such as cafes. There is a larger town, Parys, 15 km away with a bit more activity. It is a peaceful town: ‘vrede’ means peace. The name reportedly comes from a major dispute between two Boer regions, which was resolved at a meeting over coffee and cake at a local farm. (Other versions mention brandy.)

But Vredefort is also near ground zero of one the most devastating events to hit the Earth since life began. It was an accident.

Some 10 km from Vredefort is a small range of hills, forming a distant an arc around the town. The hills consists of several aligned structures, a bit like Pilanesberg. On top of some of the hills are lines of rocky structures, which were interpreted as a ring dike. The plains inside the arc contain granite, which should have been much deeper down. This uplifted granite is called the Vredefort Dome – although it is a dome that is entirely underground. It is an extraordinary structure, and even early on (1930’s) some papers argued that this was caused by an impact. But geologists are down-to-earth people and frown at extraterrestrial speculation. The dominant geologist in South Africa, Du Toit, was among those coming out against it. A paper as late as 1988 called suggestions of an impact as ‘somewhat premature’. The Vredefort Dome was seen as a prime example of a series of such domes around the Witwatersrand. A magmatic intrusion from the large ring dike was considered, with perhaps a large hidden batholith. The age was the only aspect all geologists were agreed on: the structure had formed some 2 billion years ago.

The dispute may seem surprising to us, as the evidence for an impact origin was actually quite strong. But remember, up to the 19^th century it was not known that stones could fall from the sky. Then in 1803, a meteorite broke up over L’Aigle in Normandy and showered the town in some 3000 fragments. Jean-Baptiste Biot investigated and found that the physical and oral evidence agreed: these stones had fallen. But to go from there to the idea that those stones from the sky could create geology remained a step too far. There was no need to call upon the heavens to do things the Earth was perfectly capable of itself. There were enough other processes that could cause ring-like structures: volcanoes, intrusions, erosion. Even the obvious impact craters on the Moon were long attributed to volcanism.

The scientific dispute blew up in 1980 when Luis Walter Alvarez and co-authors argued that the iridium anomaly of the time of the dinosaur extinction had an extraterrestrial explanation. He was already a Nobel-prize winning scientist who had worked on the Manhattan project, but he was not a geologist. It was seen as an unwelcome intrusion in the field and the arguments became emotional, personal and at times vitriolic. The argument about continental drift seemed little more than a minor disagreement in comparison.

The discovery of the smoking gun in 1990, when the predicted crater was discovered, tilted the discussion towards acceptance, but this was after Alvarez had died. The discussion now became about what had made the dinosaurs susceptible to extinction by impact. This is of course a valid question, but the strength of the impact hypothesis is such that any other cause was now seen as secondary to the extraterrestrial killer blow. It showed us that extinction events can happen to anyone, anywhere and at any time. But emotions still run high. A recent as the 1990’s, impacts were rarely covered in geology textbooks.

It was against this background that the discussion took place whether or not the Vredefort dome was an impact structure. The defining characteristic that identifies a structure as originating from an impact is the presence of shocked minerals. A falling mountain that impacts with a speed of over 10 kilometers per second creates extreme pressure. This gives rise to minerals such as stishovite, an extremely hard form of quartz that has only ever been found in meteor craters. Planes of deformation inside minerals are another telltale sign of massive shocks. Shatter cones are also an important sign of impacts. They are rocks with lines that form a cone shape, with the top of the cone shape pointing towards the centre of the impact. They require high shock pressure, but not as high as some other minerals: they can also form in nuclear explosions (something else not commonly considered in geology). Shattecones are widespread in Vredefort.

All this evidence for Vredefort had been known by 1978, and in hindsight it was compelling. But geologists take their time – after all, they work on geological time scales. And there was a commercial angle. The gold fields of the Witwatersrand also ran in an arc around Vredefort, albeit much further out than the Vredefort dome. When money talks, scientists become cautious and are wont to stay away from disputes.

But the data spoke loud and clear, and by the mid-1990’s the Vredefort dome was largely accepted as an impact structure, with an age of 2.023 billion years, give or take a few million years. The accuracy of the dating is remarkable. Clearly, the land itself must be even older, and indeed it is: the basement rock here is 3.2 billion years old. The Bushveld complex which gave rise to the mineral riches and lies east of (and not within) the impact structure is older, and lived through the impact. So did the gold deposits.

The gold mines are spread over a region called the Witwatersrand Basin. The Witwatersrand is a 200-meter high plateau, 100 km long and 40 km wide, which runs from a bit northeast of Johannesburg to the southwest. The name is used both for the plateau itself and for the steep escarpment on the north side. The Witwatersrand Basin lies to the south of this and is a larger region: this is a sedimentary basin which was a shallow sea around 3 billion years ago, when the gold became deposited. The Cradle of Humankind lies within the basin. The Vredefort Dome is at the centre of this basin, although the two were not related: the impact came many hundreds of millions of years later. The gold mines are largely along the Witwatersrand at the edge of the basin,

Size

The Vredefort Dome is an uplifted area, 50 km across. (Erosion has long removed the uplift, so now it is an area where the deeper granite rocks have come to the surface.) Around this dome is a collar with a diameter of 80 km, consisting of younger rocks: sediments and lavas. This would seem to be the limit of the original crater. But there are larger concentric features as illustrated in the figure. How large really was the original crater?

To answer this, we must look at how impact craters, especially very large ones, form. But first, let’s have a look at the region.

The arc of hills around Vredefort look remarkable in the images. They consist of sandstones, not surprising in a sedimentary region. But where in the general region the layers are approximately horizontal (although there is a bit of a tilt), in the hills the layers are nearly vertical. Originally this was several kilometers below ground, by the way. Everything higher up, including the crater wall and bottom, have been eroded away. Something upended these rocks and only these rocks, while deep underground.

There are quarries near Parys, within the uplifted dome. They reveal granite – this is after all uplifted granite. The image below shows a side of the quarry. The light grey granite on the left is the original base rock which formed 3.2 billion years ago and was tens of kilometers underground. At the centre and to the right is a different band of rock where the granite is broken and surrounded by a dark, smooth rock. The second picture shows a close-up of this. The pictures are from Mike Gaylard.

The black mass is glass. It is called pseudotachylite – the ‘pseudo’ part is because this should be a volcanic rock but Vredefort is not volcanic. The pseudotachylite formed when the shock from the impact pass through the granite rock, and melted it. The shock melt formed the dark rock; the granite blocks are the broken-up granite that fell into the melt. All this happened deep underground.

There is another melt type. When the crater formed, it became filled with lava which formed a melt sheet. This sheet is long gone: it has completely eroded away. But immediately after the impact, the ground was deeply fractured and the melt flowed down into those cracks. This formed deep dikes and those can still be seen. This is called a granophyre, and if you have never heard of that, join the crowd since it is rather rare. Some of the granophyre has osmium/rhenium isotopic ratios which appear to come from the object that created Vredefort, so the granophyre is in part extraterrestrial. The pseudotachylite is pure terrestrial granite.

How large was the crater? The granophyre is found only close to the impact centre, out to the ring of hills, within around 25 km of the centre. The ‘planar deformation features’ are seen as far as 45 km from the centre. And the shattercones are found out to 90 km, which is not far from Johannesburg. This has been taken as the size of the original crater, giving it a diameter of 180 km.

Again, it should be pointed out that this is not the crater itself: there has been between 6 and 10 kilometers of erosion. We are looking deep underneath the original crater. This is the reason Vredefort is called an impact structure and not a crater, as the crater is long gone. That makes it hard to define where the original crater ring was.

Shape

Craters come in four different types, depending on size.

A crater up to a few km is called ‘simple’. It has a bowl shape and an outer ring and that is it. Tswaing is of this type.

Between a few and perhaps 20 km the crater is called ‘complex’. It has a shallower, flat bottom rather than a bowl, and has a central rise caused by rebound after the impact.

Eventually, the central peak becomes a central ring. The crater is now called a peak ring crater.

The larger the crater, the larger the central ring. Eventually this ring approaches the outer ring and the crater is now called a multi-ring crater. The diameters where the craters change type depends on the local gravity (it is different on the Moon and on Earth and also depends on the type of surface (rock or mud, for instance.)

(Strictly speaking there is a 5^th type which is the first. These are the pit craters that are created when dust particles hit a surface. They only form in vacuum and have sizes of millimeters or less.)

There are only three multi-ring craters known on Earth: Chicxulub, Sudbury and Vredefort. In all these cases, it is hard to know whether the outermost mapped ring is or is not the outer ring! For Vredefort, it appears not to be. The true diameter of the original crater is thought to be 250-280 km. That makes it the largest one on Earth, just beating Sudbury. There is a fragment of an arc of the granite base rock found far to the northwest – if that is from the original crater rim, then it reached 300 km diameter.

Why is there only half a structure left? The structure is visible in the north, but the southern half is not seen. Actually it is seen in gravity measurements, but it is buried. The reason is the Karoo flood basalt (already mentioned): it covered the southern half, and the weight of the lava depressed the region. Just wait a few hundred million years. By then, the northern half will be eroded away but the southern half should appear.

What kind of impactor could it have been? A useful rule of thumb is that the crater is 20 times the diameter of the object that caused it. That is not a law of physics: it depends on what kind of surface was hit. But it gives a reasonable idea. That makes the Vredefort rock 10-15 km diameter. It was bigger than Mount Everest. Imagine Mount Everest falling down on your favourite location on Earth. It was like that.

Impact

It was worse than that. The mountain came down with a speed of some 15 kilometers per second. It passed through the atmosphere in mere seconds. Earlier, while in space, it would have been visible by reflecting sunlight. At a distance of 3000 km, it would already have been as bright as the moon. And well before it would have been easily visible at night to the naked eye – at least if it didn’t come in from the direction of the Sun. Even at the distance of the Moon, it could have been the brightest star in the night sky. That may have been one to a few days before the impact.

Asteroids tend to move on orbits that are not much faster or slower than the Earth, so the relative velocity was likely small, a few km/s. (Ok, that is fast for geology, compared to, say, continental drift.) As it moved closer, it came under the influence of Earth’s gravity and started to move faster and faster, falling down. At a distance of 3000 km, impact was only minutes away. At 200 km it began to feel the atmosphere and by 100 km it had formed a bright fireball, burning the ground below just from the radiation. Along a track of several hundred kilometres, everything was killed well before impact – or at least it would have, had there been any life there. Less than 10 seconds later, it hit. It was not heard: the impact was before the sound wave arrived because the asteroid came in supersonically, and to the unlucky observer the asteroid would have been seen first, with the sonic boom arriving a minute later being the least of their worries. (The is true even for the Chelyabinsk meteor where most injuries came from the atmospheric sonic boom arriving a minute later. If you see a large meteor, stay away from windows for the next two minutes.) (But don’t worry about this if it is an asteroid.) If the observer was able to continue observing, they could have heard the asteroid safely moving away from them – because the sound from the more distant, earlier parts of the trajectory arrived later.

The impact was extremely energetic. The shaking was equivalent to an earthquake magnitude 14. (I don’t know what that feels like either. Very roughly, at a distance of 10 km, objects would be thrown into the air with a peak acceleration of 10g.) At a distance of 300 km, the earthquake would arrive after roughly a minute. Ballistic rocks thrown out by the impact arrived shortly after. The atmospheric shock wave arrived after 5 minutes. But the radiation from the rising fireball of the impact got their first, moving at the speed of light.

The underground explosion blew a rapidly growing hole in the Earth. The explosion occurred kilometers below ground but it did not penetrate into the mantle. The rebound was from granite from deeper levels in the crust, not mantle material. That would take an impact even larger than Vredefort.

The approximate sequence of events is shown below. This is taken from an old simulation of a large impact (not as large as this) hitting Berlin. (It was made by German scientists eager not to endanger the rest of the world.)

20 seconds after impact, the hole is growing and ejecta are spewing out from the side of the growing crater. The expansion continues for two minutes. By that time, the ground at the centre of the crater, which was pushed down by the force of the explosion, is rebounding and just like a drop of water falling in a bath, creates a central bulge. In a complex crater, the rebounding bulge solidifies at this point to form a central peak, but in a very large impact this rebound itself falls down again and in a second rebound will create a ring around it – the inner ring of what will be a multi-ring crater. The crater at this point may still be expanding.

As the deep crust is pushed down, it will in return push the ground much further out upwards. This combination creates a central bulge (the Vredefort dome), the deep crater around it, and around that a higher rim from this outer uplift.

Matter that is evacuated from the crater flies through the air. Much of it is fragmented rocks, although ‘fragmented’ should be taken with a grain of salt. Rocks that themselves would be decent-sized hills are thrown hundreds of kilometers distance. They are joined by much smaller fragments, down to dust. It creates a blanket around the crater which even 500 kilometers away is still 100 meters thick. Other flying material comes from the melt caused by the asteroid. These droplets rain down on top of the debris layer as minute glass-like spherules. They come in two forms, fragments that are ejected in ballistic trajectories and fall back down from space over several hours, and ones that drift in the atmosphere and are carried by wind – lots of wind after such an impact. The ballistic droplets may fall down thousand of kilometers away. The entire world was hit by a Noachian deluge of rock and glass.

But perhaps most damaging was the heat released by the impact. The spherule rain would have heated the upper atmosphere to a temperature of 1000 K. The radiation from this would be as strong as 10 times the radiation of the midday sun, comparable to an oven. The air temperatures, especially in dry air, are less affected by this one-hour heat pulse, but any surface exposed to the sky would be baked.

Remains of the day

These events happened long ago. Erosion has removed the rim, the crater with its melt sheet and the inner ring, as well as all ejecta. What we now see is the granite dome, the bit of the uplift which stayed below ground and which is surrounded by younger material (where the ground was flexed down), and another outer ring where deeper material has come up. But what was at ground level 2 billion years ago is long gone. Don’t expect to find spherules here!

But millimeter-sized spherules from Vredefort have been found, just not in South Africa. They were found in a 10-cm thick layer in Karelia, north Russia, with spherule shapes indicating that some came in from space rather than wind. Both the age and the composition point at an origin in the Vredefort impact: the iridium, platinum and various isotopes indicate that they are from an impact and are not volcanic. Russia is a world away, 10,000 km from Vredefort! But of course, the world was different in those days and continents were not where they are now. Models of the impact indicate that a 10-cm debris layer could have formed as far as 2500 km from Vredefort. Clearly, Russia (or better, the fenno-scandinavian shield) was much closer to South Africa in those days, but this is still a long way apart.

Spherules have also been found on the southwest coast of Greenland. Their date is more uncertain and they could come from either Vredefort or Sudbury. But the chemical composition is different from Sudbury and points at Vredefort. The layer containing the spherules is one meter thick and extends over 25 km. The layer appears to have been deposited below water, by high-energy waves or a debris flow which also deposited rocks up to 1 meter in size. It is tempting to associate this with a tsunami induced by the impact. The Vredefort impact itself was on land, but the pressure wave would still have whipped up the oceans into a frenzy. However, this is speculative. The region can also have been destabilized by the massive earthquakes caused by the impact which would have rocked the entire world. The Earth would have rung like a bell for weeks.

The layers below and above the spherule layer are a bit different from each other, indicating a change in the ocean chemistry. This could have been caused by a lot of continental debris entering the ocean but this also remains speculative.

Life

We know very little about the impact on the rest of the world. Nowadays, such an event would be devastating worldwide, as was the case of the Chicxulub event which was smaller than Vredefort. Imagine the entire world on fire! But 2 billion years ago, the continents were lifeless and without life there can be no fire. The dust from the impact would have dimmed the planet for anywhere from a decade to a century. Sulphur may also have been released as the impact hit an old sedimentary basin, though deeply buried at the time, causing additional cooling. This was counteracted by the sheer heat from the impact. But the oceans are more resilient than the land and take much longer to cool down. There is no evidence for a mass extinction at the time, but this means little, seeing how limited the fossil record from this age is.

We do know that life survived. The Earth was devastated, but not destroyed. There would have been a difficult decade for bacteria, algae and other single-cell life forms, but they pulled through. How many did not make it, we may never know. Stromatolites would have been destroyed but they re-established themselves. But if any multi-cellular life forms had developed (something we don’t know but possible), those did not survive and they had to re-evolve.

Gold of Humankind

South Africa is an amazing country. Within a relatively small area it has the Bushveld with the richest mineral deposits in the world, the gold fields of the Witwatersrand responsible for 30% of the gold mined in the world, and the largest impact structure known on Earth. And of course there is the Cradle of Humankind with the remains of ancient Mrs. Ples and 30% of all other known hominid fossils. Is there any relation between these?

Mrs. Ples and Vredefort are 2 billion years apart, so this coincidence seems coincidental. That Mrs. Ples was aware of the gold and platinum also seems unlikely. The Bushveld intrusion happened shortly (50 million years or so) before the Vredefort impact but we know that the two were unrelated. The intrusion is outside of the original crater but some dikes intrude into the region. Those dikes show the shock impacts, meaning that they were in situ and had solidified at the time of the impact. The orientation of the magnetism in the Bushveld intrusion also shows that it predates the asteroid. So this was coincidental. It is just that some countries have all the luck.

But how about the gold? Why are all the goldmines located in a semicircle around the Vredefort dome? And it is indeed all the goldmines: a third of the world’s gold comes from the Vredefort crater ring. The map below shows the goldmines in blue. How can this be accidental? The gold itself is solidly terrestrial, though. It was put done many hundreds of millions of year before the impact. Do asteroids hunt for gold too?

The answer is a bit more down to earth. Remember that some of the regions were pushed down by the impact, and some regions came up. The outer rim and central dome came up, and the main crater went down.

This image shows the effect, The reddish colour depicts the deep granite base rock. The yellow layer is the one that contains the gold. This layer was bend upward near the outer edge of the crater. The black dashed line shows the current surface level. The gold mines are located where the yellow layer approaches the current surface. There is probably much more gold elsewhere in the layer, but it is deep underground and we can neither find it nor mine it. It became minable because of the uplift by the Vredefort impact, the 6 to 10 km of erosion in the two billion years since – and because of other accidents such as the fact that the Karoo flood basalt left half the impact structure uncovered.

You may note that the yellow layer also reaches the surface near the centre, immediately around the Vredefort dome. And indeed, there are some gold mines in the hills around Vredefort. But the gold veins are much more broken up here and are difficult to find and follow underground, so the mining was abandoned here a century ago.

The twist at the end

There is a final twist. The gold deposits are located in a layer of sediment that formed in shallow water. The sediment is not just gold: it is mainly limestone deposited in a shallow warm ocean, as is found in many places around the world. Limestone dissolves in rain and forms caves. The hominid fossils of the Cradle of Humankind were found in those caves.

Hominids -us included- have always been attracted to shallow caves. Just look at how we build our houses which resemble large, airy caves with a view of the outside so we can see the neighbours and other threats. The Cradle of Humankind is there because of the uplift of this sedimentary layer by the Vredefort impact, and the fossils survived because they were protected inside those caves.

There is a relation after all: The Vredefort impact built us our first homes. Caves as safe as houses.

Legacy

The Vredefort impact has left a lasting legacy in South Africa. The impact shaped the country and made much of its wealth accessible. It may not be immediately obvious on the ground and it takes effort to see the signs of the event, but it is well worth that effort. This designated world heritage site is one of the wonders of the world, albeit a scary one.

If an event like this were to re-occur, humanity would go the way of the dinosaurs, and would do so in the way of the dinosaurs. Vredefort helped create us, but it also shows us our fragility.

Albert, June 2025

Normally there would now be a set of papers or reading list. Instead, there is a link to a youtube presentation by one of the main researchers working in the field (literally so, in this case). You will find a good overlap with the post, and for good reason. You may even recognise some of the figures.