Pilanesberg Geology

The Pilanesberg's Calm Undulations

The Pilanesberg’s calm undulations, now the best-defined alkaline ring complex in the world and coated in savanna plains, tree-dotted slopes and riverine thickets, were once the scene of geological turmoil. Before it all happened, a mountain stood higher than Kilimanjaro: an enormous volcano-like edifice towering over the landscape. 

©Chris Daly

Then there were cycles of volcanic eruptions deep down where magma cooled before it reached the surface; eventually the centre of the mountain collapsed, forming a crater (now the Mankwe Dam). Millions of years of erosion have reduced the mountain to rolling plains and exposed the hard rock - the cooled magma - that is what we now know as the Pilanesberg.

The ring formation is best seen from the air, as a perfect circle of hills with a fracture line that cuts it cleanly in two. offsetting the two semicircles. In appearance it's very much like the Ngorongoro Crater in Tanzania. but technically it does not belong to the same geological family.

There's a story that when hotel mogul Sol Kerzner was looking for a place to build his great fantasy hotel complex. He flew over the Bojanala region, looked down to where the fault line creates a nook on the outer perimeter of the ring of hills and said something like: “That's where I'm going to build Sun City”. The inside of the ring complex is of course Pilanesberg Game Reserve - once the only national park of the short-lived Bophuthatswana homeland, but now a provincial park.

A Satellite's-Eye View

©Jacques Marais

Sometimes you need a satellite's-eye view to appreciate how things really are on the ground. Images captured from space have revealed many of earth's secrets, such as how the El Nino weather cycle works and the source and path of frontal storms, among others.

What they tell us of Pilanesberg, that you wouldn't easily be able to make out at ground level, is that the largest of the world's three alkaline ring complexes can be found there. Well, a satellite image would not tell you all that, but what it would show is a near perfect circular set of four concentric rings of hills, covering about 50 000 hectares the alkaline ring stuff would have to be done by geologists on the ground.

What Pilanesberg really is, is the base of possibly earth's oldest volcano. It now rises about 700 metres above the bushveld plain, but originally it would have risen to a massive 7 000 metres far higher than any volcano today. The near perfect circle is disturbed only by a geological fault, running right through the middle, that has slightly misaligned the two halves.

The Original Rock

©Shem Compion

During the time of volcanic activity when the Bushveld Igneous formations were laid down, molten lava bubbled up in the area, with a number of small volcanoes forming a cauldron of magma around a fault line. Repeated welling up and subsidence of the lava resulted in a series of ring dykes around the collapsed centre of the cauldron.

The original rock cover of these ring dykes has now been almost entirely eroded away and only fragments of the alkaline lavas and coarse breccias remain. However, the Pilanesberg still forms the largest known alkaline complex, rich in countless minerals and a geologist's playground among the volcanic and crystalline formations. Satellite photographs clearly show the circular cluster of hills, faulted diagonally along the diameter of the circle from south-east to north-west.

On the fault line squats Sun City, a large pleasure dome where sun worshippers loll on manicured lawns or in smoky casinos only a hillside away from the rugged African bush. From a black eagle's nest on a ledge at the reserve's boundary, the view to the north-east stretches across the flat Springbokvlakte, a broad expanse that is punctuated only by an occasional granite outcrop before terminating on the far horizon in a long band of indistinct rumples.

These represent the Waterberg, where the cool mountain streams must have seemed like oases to the Boer settlers who first crossed the hot plain to get there.

By David Bristow