The hill rises up from the dry bushland plains, forming a 400 meter tall ridge with gaps. Pass through a gap and a new ridge appears, and another one behind it. In between are dry valleys. It is winter now and there has been no rain since early autumn: the rivers have dried up. After passing through several such ridges, the valley becomes wider and the hills are now further apart. There is a lake here, man-made, amidst the brown thorn bushes. The animals congregate to drink. Elephants abound, zebras herd and impalas jump. They are wary and with good reason: there are predators about, ranging from wild dogs to lions. The variety of animals in a smallish area already tells you that this is a wildlife reserve. Continue on your journey, and you will pass more ridges similar to the ones you saw before. Eventually the last range is reached and the plains suddenly open before you. The ridge behind you seems curved. You have traversed a circular, broken mountain.
The satellite reveals what is already evident from the ground. The ridges indeed form several circular, concentric ridges. Each ridge is perhaps a kilometer wide. The total extent is just under 25 kilometer, a distinct, isolated mountain on the plains. It is almost – but not quite – round. If you know whether to look, the peculiar Pilanesberg can be recognized even on coarse maps of southern Africa. It resembles the famous Eye of Africa, also best seen from space.
The Pilanesberg is the remnant of an old volcano. Each of the ridges seems to trace an apparent caldera, but one where the central hole is filled with another ridge and a smaller caldera. This nested volcano is like a Russian doll, each babushka containing a smaller one until there is no hole left. The structure is eroded, and the volcano is clearly deeply extinct. How old could it be? There has been no serious volcanism here since the break-up of Gondwana. But the Pilanesberg is much older than that. Its origin lies in deep time. The land itself is the elephant in the room: the African animals are living in a place that formed long before any animals existed, when Gondwana was a distant future and bacteria ruled the waves. This caldera is not just an ancient of days. It is an ancient of millennia. It formed a staggering 1.4 billion years ago. This is the oldest recognizable volcano in the world.
There is also human history here, near the border between South Africa, Zimbabwe and Botswana. The area was already frequented in the iron age, when it was a source of copper and iron. In the 1800’s a Tswana tribe lived here, the Bakgatla. They traced their origin to Chief Pilane, and the area is still named after him. The name Bakgatla refers to the local monkeys. In the Boer war the tribe sided with the English. That worked out relatively well for them: when in the 1920’s platinum was discovered, they remained the owners and received royalties from the mines. Part of the area was farmed, and the central lake was build for this. But farming was always marginal: even the ground water was barely useable, being saline and rich in fluorine. In the 1970’s it was decided to attempt a wildlife reserve in the area. There were fewer than 100 families living in the region: they moved out, and a reserve was created.
There is modernity here too, in a different kind of history. Sun City, on the southern edge of the Pilanesberg, must be one of the strangest holiday resorts in the world. It was developed in the (at that time) bantustan of Bophuthatswana. The bantustans were a series of semi-independent homelands for the black populations, created in apartheid-era South Africa, with as main aim to marginalise. The closest analogy is perhaps the native American reservations in the US. No other country ever recognized them, but this did not stop business from taking advantage. The Sun City holiday complex was build in the bantustan, away from the legal restrictions of South Africa. Gambling was illegal there, and Sun City therefore catered to a massive demand. It presented itself as ‘ the lost city’, a magical albeit imaginary place away from the real world. Here one could gamble, play golf on well-watered courses, swim in well-watered swimming pools, and enjoy world-class entertainment in the well-watered venues.
Nothing was real, of course. No lost city was ever like this. This luxury resort of the imagination, where people were invited to spend their wealth, was situated amidst the poverty of a dry and thirsty land. Sun City was created in the same year that the Pilanesberg became a game reserve and the animals were re-introduced. The Sun City holiday makers could take tours into an imagined Africa and see (but not shoot!) the ‘big five’, before their afternoon game of golf, an evening concert by Frank Sinatra, and an affordable flutter.
The reserve now covers over 50,000 hectares, and is connected via a corridor to Madikwe, a similar-sized reserve on the Botswana border. The reserve remains small for a true African ecology. When it started, there had been little life here: farming on poor soil and roaming animals in search of food do not go well together. The animals had to be imported from other reserves. It did not go without a hitch. Some 15 years after the initial wilding, rhinos started to die. They would be found badly mauled. Which animal could do that? It was a mystery, a Death on the Nile where every suspect had an alibi and lacked a motive. The wounds fitted elephant tusks, but elephants do not kill rhinos. But the wounds did not lie, and the elephants had gone rogue. It turned out this was unsupervised teenage behaviour.
Elephants live in family groups, ruled by the matriarch. Female children remain with the group, but the male children leave the group once they reach 15 years or so and begin to live solitary. This began to happen at Pilanesberg. It turned out that these hormonal young solitary elephants had been turning violent, and they were attacking and killing the innocent (in a way) and rather expensive rhinos. But such behaviour had never been seen elsewhere. Investigations began.
The outcome was that a mistake had been made when acquiring the early elephant groups. The groups were taking from other reserves as complete units. But no solitary males had been included, as these were too big to easily move. It turns out that the mature solitary males keep the teenagers in check. Their dominance suppresses hormone production in the teenagers who cannot compete with them, and this stops the violence of the youngsters. In the absence of such elders, the teenagers had run riot. Eventually, a few older male elephants were transported from the Kruger to Pilanesberg. The rhino killing immediately stopped.
Rewilding sometimes does what it says. The elephants in this picture-perfect reserve replayed their very own Lord of the Flies.
There are geological riches here too. There is platinum in those rocks, and mines that dig for it still surround the Pilanesberg. At one time there was a fluorite mine. But the most important value is a link to the past. The Pilanesberg connects us to a deep past, and gives us a glimpse of how this land was created.
South Africa is an ancient land. Pilanesberg lies on the northern part of The Kaapvaal craton, well over 3 billion years old. 2.7 billion years ago it joined up with the Zimbabwe craton, in one of the earliest documented continental collisions. It formed a Himalayan sized mountain chain to the north of Pilanesberg. The two cratons have remained attached ever since: although the juncture re-activated 2.0 billion years ago, there was to be no separation. Together, they became known as the Kalahari craton.
The next event of note was a big intrusion, 2.061 billion years ago. It involved both basaltic and rhyolitic volcanism, but more importantly a huge intrusion of basalt formed below the surface, called the Bushveld complex. The magma chamber was a staggering 400 kilometer wide: the heat had melted the entire bottom of the crust. Was this a failed attempt at rifting? Was it a plume? The extraordinary magma chamber (the largest layered intrusion in the world) very slowly cooled and solidified: this took over a million years. It left behind a rich legacy: the world-class deposits of chromium, vanadium, platinum, and some rarer elements turned the area into some of the richest real estate in the world. Here is where the world gets its platinum, including from the mines which still surround Pilanesberg. A second intrusion followed 2.049 billion years ago with the Phalaborwa complex. It formed a volcanic pipe which has now been excavated to a depth of more than 700 meters. It is mined for copper and phosphate.
More world-shattering events followed in this exciting 40 million years. 2.023 billion years ago a large asteroid hit the town of Vredefort – or at least where this town would be located in the distant future. It formed a crater 300 kilometers in diameter (the largest confirmed crater on Earth – some countries have all the luck), part of which still exists. The Pilanesberg area would have been destroyed by the magnitude-14 earthquake, and buried by the ejecta – the impact could have hurled blocks 4 kilometer in size 300 kilometers away. But the rocks underneath survived.
Now, activity calmed down and little noteworthy happened for 500 million years. Not entirely: there was a flood basalt 1.9 billion years ago but very little is known about it. The Congo craton came and joined Kaapvaal and Zimbabwe. A mountain range formed at the suture; it eroded, leaving a thick layer of sandstone sediment in the region. It is all just part of normal life for a craton. It seemed to become just another boring billion years.
But now, 1.4 billion years ago, there was something new. Rifting began in the area, roughly following the line of the Limpopo, the old collision between Zimbabwe and Kaapvaal. It came to nothing: eventually it ceased and the cratons survived. But it allowed magma to form, and this began to collect in the crust, looking for a way up and out. The magma was alkaline, rich in sodium and potassium. It formed a series of intrusions across the north of South Africa. The largest of these intrusions formed Pilanesberg, where on the surface a volcano began to grow. Some claim that the volcano reached 7 kilometers tall. That is based on the diameter of the Pilanesberg but the height seems strongly overstated. In any case, not enough has survived to decide on the original height. But what remains is impressive.
And there is more hiding underground. Surveys have shown the remains of a system of dikes, radiating out from Pilanesberg. The swarm of dikes extends to the south-southeast and to the northwest. It extends into southeastern Botswana as a 40-km wide band of anomalous magnetism. Pilanesberg is not exactly at the centre, but is about 20-km southwest of the centre of the band. To the southeast of Pilanesberg the band is less distinct but it broadens. One of the dikes runs under Pretoria, and a dike swarm south of Johannesberg may also be part of the same complex. The dike system extends 400 or even 500 km in total! (A dike swarm is called ‘giant’ if it is more than 300 km long.) The magnetic surveys have shown that the dikes often (but not always) have a magnetised outer layer but a non-magnetised core. This is explained as a dolerite dike which formed first, which was later intruded by syenite – the latter being non-magnetic.
The dikes have opposite magnetisation to that of the Pilanesberg complex itself. The emplacement may have happened over a period which included a magnetic reversal of the Earth’s magnetic field. The dike swarm and the Pilanesberg are clearly closely related and are part of the same event, but the precise relation is not yet deciphered.
Other alkaline intrusions
The Pilanesberg intrusion indeed did not stand on its own. There are several intrusions of similar chemistry known in the wider region, with ages between 1.4 and 1.35 billion years.
Just east of Pilanes is the Crocodile River complex, a series of 4 or 5 volcanic remnants separated along a north-south line. The largest is Kruidfontein, a 3 kilometer wide caldera with two concentric rings.
The Pienaars River complex lies 150 km east of Pilanesberg and north of Pretoria. It consists of a series of intrusions over a length of 70 kilometers, extending northwest to southeast. Northernmost is Elandskraal, a true 10-km wide volcano with remnants of lava and pyroclastics. It is invisible at the surface but was found from geomagnetic anomalies. (It is close to the Tswaing impact crater. Some places have all the luck.) To the south, Roodeplaat volcano is rather similar to Pilanesberg but half the size and like Elandskraal not visible on the surface. There is a rim of lavas, an inner region of pyroclastics, and a series of plugs and dykes containing various forms of syenite. Just beyond is Leeuwfontein, also containing syenite dikes and plugs. It is the remnant of a trachytic volcano. There are several further locations with small volcanic remnants. Spitskop, 300 km east of Pilanesberg, consists of a 5-km wide ring, with a separate small intrusion 20 kilometer to the northeast. Another one is at Stukpan, 200 km south, just west of Vredefort. Others likely remain hidden below the surface. But Pilanesberg was the largest of these intrusions.
The combination of Pilanesberg, the enormous dike swarm and the other intrusions with almost but not quite the same age (1.4-1.35 billlion years) shows that something happened that was more than just the formation of a nice volcano. One possibility is that we are looking at the remains of a hot spot trail. The trail would start in Pilanesberg, the oldest, followed by the Pienaars River complex 200 km east and possibly 10 milion years younger, and finally Spitskop another 100 km in that direction and another 10-50 milion years younger (it has a much more uncertain age). Stukpan to the south does not fit in with the sequence and may be slightly younger. One could point at the Yellowstone hot spot for comparison, with its trail of large calderas starting from the major eruption at Steen mountain. Steen Mountain is also the origin of a massive dike swarm and the source of the Columbia flood basalt. However, there is no evidence of a flood basalt related to Pilanesberg. There is also no evidence for mafic material in the intrusion, however the original source may have been an alkali-rich basalt.
The second possibility is a failed attempt at continental break-up, as was mentioned already above. The alkaline magmatic intrusions of southern Africa appear to be found along old terrane boundaries and younger extensional faults. Sutures where continents merge (terrane boundaries) remain areas of weakness. The dike swarm and intrusion would come from a weakness in the crust as rifting set in. The Pilanesberg dike swarm follows roughly the direction of the Limpopo suture. The dikes followed the direction of the rift, just as they do in Iceland, and the width of the dike swarm show the total extent by which the continent rifted. The rifting model explains the extraordinary length of the dike swarm, and the offset of Pilanesberg which may have come from the rifting – the intrusion drifted away from its original location with the rifting (let’s call that adrift with the rift). But eventually the rift failed to develop further and the continent survived to live another billion years.
The bottom line is that we can’t always know! It is too long ago. However, the rifting model seems attractive. It has similarity to the Great Rift Valley in modern Africa, which has also reached 50 kilometers in width. No continent carries more scars from rifting than Africa. These are the wrinkles of age, where the magma in the dikes acts as the botox of the craton.
The rings of Pilanesberg
But how did the structure of the Pilanesberg form? It is clear that much of what we see formed underground, and that we are looking at the original plumbing, the skeleton of the volcano. The ridges were faults, filled with magma which hardened over time.
The idea is that the underground intrusion had the shape of a cone. The picture shows the general idea. The ring fault has a cone shape, and the area around it subsided. This structure is inferred from gravity measurements. The cone-like fault developed from the pressure of a central magma chamber. As the chamber grows it pushes the land up and to the side. This allows cracks to form which radiate out from the magma chamber.
The cracks filled with magma, while the centre erupted and collapsed. What is left is a set of concentric cones. This process appears to have happened several times, because the ridges have different chemical compositions. It may have taken one million years or more. Eventually the intrusion ended, and the volcano died. Over time the surface eroded away. The lava and tuff that had erupted were more susceptible to erosion than the hardened ring dikes, and therefore eroded faster. What is left are the ring dikes, reaching several hundred meters above the rest of the surface. We see the faults where once the crust gave way to the pressure from the magma. It is a rare glimpse of what a volcano may look like several kilometers underground, a piece of art created 1.4 billion years ago. What a wonderful world we live in.
Here is a cross section from Sun City to the lake (not quite at the centre), and out along a different direction. Every ridge is named by its main composition. The outer ridge, on both sides, consists of red syenite. Other ridges are white foyaite, green foyaite and Ledig foyaite. In between are volcanic rocks: pyroclastic tuff and lava. At the centre of the volcano, there are parallel bands of red syenite and red foyaite. The alkaline rocks are rich in fluorine, light rare earths, strontium, thorium, and uranium.
Some of the tuff is radioactive (only mildly!), with an unusually high uranium and thorium content. This came towards the end of the life of the volcano, when heated water penetrate the tuff layers, carrying the radioactive oxides. The Ledig foyaite also has some uranium aded to it, though not in mineable quantities. The red in the red syenite comes from ironoxide, also carried in by water.
From a previous post on igneous rocks you may remember that dolerite (found in the dike swarms) is a medium-grained basalt, commonly found in dikes . Dikes cool faster than magma chambers, and therefore the grins have less time to grow. Magma chambers form gabbro (coarse grained) as they cool. This was the original intrusion.
Later, the intrusion became more alkaline, with potassium replacing the calcium. This formed an alkali feldspar: syenite, a kind of granite with more potassium. (I am waiting now for a true chemist to jump in and correct me!). If it erupts on the surface, it become trachyte – found in the remnants of the surface eruption in this region. Foyaite is also a syenite, but it contains less silica (and no quartz) and instead contains other, rarer elements and minerals. It is found mainly in connection with continental rifting.
It is amazing that a volcano almost 1.5 billion years old has survived until the current day. In fact, it is incredibly well preserved, with parts appearing just as they had formed. Erosion has been kind to it. But why? It probably formed only a little above sea level: erosion is slower at low altitude. The area may often have been dry, being at the centre of a continent. But that seems insufficient to explain the oldest volcano in the world.
Pilanesberg formed in a failed attempt to break up southern Africa. But now came the era of the supercontinents: 200 million years later, Rodinia began to assemble. During this event, a flood basalt covered part of the region in lava: it is called the Umkondo Large Igneous Province and it covered 20 million square kilometers with lava. The remnants are found even in Antarctica. For a while, the layers of lava may have protected the original surface against the slow ravage of time. A continental collision occurred to the south of the Kaapvaal craton. This formed mountain belts (part of the Kibaran belts) with extensive folding in the Northern Cape but it did not affect Pilanesberg. Rodinia broke up again, but only partially and the Kaapval craton remained in the interior of a fair-sized segment. Later, Gondwana assembled with activity all around the edges of Kaapvaal. The famous granites around Cape Town (Pearl Rock, Boulders Beach, but not the even more famous Table Mountain) formed at this time. New mountains ranges formed, and these quickly started to erode again, leaving their sediments across the land. The ancient Kaapvaal craton found itself surrounded by all this activity. But Pilanesberg never took part: enclosed by mountain ranges, as if in a fortified estate, it remained unaffected. 500 million year ago a rift developed along the southern Cape, and the sea came in. Deep sandstone sediments formed, including what later would become the most scenic mountain in the world (in my humble opinion), Table Mountain. Drive along Chapman’s Peak (take your time: you’ll be stopping at every bend, and there are a lot), and you will follow the contact between the granite below and the sandstone above. But the sea did not reach Pilanesberg, which still quietly slept while the world around changed and changed again.
Now, finally, it did begin to take part in events. A mountain ranged formed north of South Africa, covered by a thick ice sheet: the continent at this time was situated near the south pole. The ice descended to a shallow sea which covered the Karoo. This must have affected Pilanesberg which lay in between. The area may have been the recipient of sediment brought by the descending glaciers, but no such sediment is known in the area (there is some a bit further south). It has been suggested that this deep glacier instead eroded the rocks and created the current structure, with the hard ridges sticking out above the softer volcanic rocks and the surrounding area. Now the continent drifted away from the pole, towards much warmer regions. The ice disappeared and a delta formed along the northern mountain range. Sediment now covered the region again. Later the climate dried and the region became a deep desert, a sea of sand. And 180 million years ago the unthinkable happened: South Africa broke up, along a line not too far from Pilanesberg. An immense flood basalt formed, and the sea of sand became a sea of lava which covered much of South Africa. The highlands of Lesotho still show this deep pile of lava. After the sand, now kilometer-deep lava also covered Pilanesberg. This became its final saving grace. Over the 180 million year since, erosion took away both the lava and the sand that covered Pilanesberg. Some 30 million years ago, the erosion had removed the new (compared to the age of Pilanes) surface down to the level of the top of the ridges. This was also the time the African bulge formed, and the land began to rise. Did this increase the rate of erosion? The sediment that had filled in the valleys and covered the surrounding land was removed by rivers. And so the Pilanesberg re-appeared, looking like it did after the age of ice 300 million years ago. It has left us just enough. After the many years of quiet, and the years of being buried, the ridges appeared on the surface. And so, after almost one and a half billion years, while even the Moon had grown old, the Pilanesberg we know took shape.
One more thing had happened in the mean time. Around 90 million years ago a different type of eruption occurred within the Pilanesberg. It happened in different places across South Africa: Kimberlite. These are eruptions from the deepest of the deep, 200 kilometers below. Sometimes, but far from always, they bring up diamonds. One of these kimberlite eruption occurred at Pilanesberg, in between the eastern ridges, and it added a new type of volcanics as a small pockmark to this ancient of days.
Erosion is still occurring: it cannot be stopped. Weathering breaks down even the hard syenite of the ridges, and rain and gravity takes them to the valleys. A soil formed, and this soil attracted the farmers. But such an old land has little fertility left, and some of the local chemistry was positively unhealthy. The farmers reached the limit of what was possible and decided that enough was enough. The land was returned to a semblance of wilderness, so that we could see what Africa, our old home, could still be like.
Against all the temptations of Sun City, still people come to see Pilanesberg. Instead of a lost city, they find a lost world where hyenas laugh, monkeys play and bushbuck hide. And all around them is an ancient heritage. They live within an excavated volcano that formed when our world was young.
Albert, February 2022