Overberg Geoscientists Group
1200 Mya
Pilanesberg Volcanic Complex
The Pilanesberg is probably best known as a National Park. However, it is the eroded remains of a massive Igneous intrusion which created an enormous mountain which has been eroded to leave behind the remnants of the core. A massive volcano once towered up to 7000m in this area.
The structure of the intrusion has been likened to the shape of the impact of a stone on a car windscreen. Cracks radiate outwards from the impact point, but close to the impact the cracks form concentric rings.
In this case of course, the impact was not external but from the interior of the earth and generated by magma trying to escape up through hard rock.
The radial cracks form dykes that can be traced for up to 100km away.
Molten magma also filled the concentric rings forming ‘ring dykes’ surrounding the central upwelling of lava forming the main volcano.
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As the lava cools, the surface crust collapses back into the chamber and forces more lava into the ring dykes.
Over time, a series of volcanic eruptions occurred. There were further outpourings of lava, craters collapsed, ring fracturing took place around the volcano and magma was squeezed into these fractures. The result is several "onion rings" of rocks of different ages. This process repeated a number of times, with slightly differing lavas that had formed from the underlying magma chamber. This gives rise to different rocks forming the successive rings. Altogether an Alkaline Ring Complex, the largest in the world.
Erosion over many millions of years has stripped away the mountain and the highest peak. What we see today is not so much a volcanic crater, but a cross section through the magma pipes that were located at great depth below the mountain's summit.
The lavas that were extruded out onto the surface to form the volcano were eroded faster than the concentric rings formed by the dykes. Thus, we are left with roughly circular ridges with lower flats between them.
Further reading:
Elburg, Marlina A., and R. Grant Cawthorn. "Source and evolution of the alkaline Pilanesberg Complex, South Africa." Chemical Geology 455 (2017): 148-165.
R. GRANT CAWTHORN; The geometry and emplacement of the Pilanesberg Complex, South Africa. Geological Magazine 2015;; 152 (5): 802–812. doi: https://doi.org/10.1017/S0016756814000764
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D J Mourant Aug 2023