Very early Joburg


Very early Joburg

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The City of Johannesburg, as we have come to know it, had its structural beginnings in the late 1800s, after the discovery of gold in the area. There had, obviously, been movements of people, and settlements on the land prior to that. However, it was the gold that brought the people in ever increasing numbers, and caused them to stay and begin to create more permanent structures.

But why the gold in such rich, deep and concentrated deposits? One of the answers is not just ‘Out of Africa,’ it is, in fact, out of this world!

Journey with me back in time. What we have come to know as the Witwatersrand Basin is a large area
of gold-bearing reefs that stretches from about 65km east of Johannesburg to 150km west, then swings southwest into the Free State. Over 3 billion years ago this was not dry land but a huge inland sea, fed with water and deposits, by a number of rivers. This sea appears to have dried up, as a result of volcanic activity, about 2,7 billion years ago.

A little bit later, a mere 2 billion years ago, a meteorite about the size of Table Mountain, rocketed into our atmosphere, at a speed of some 20km per second, and ended its journey abruptly in this Basin. That was not all! The force of the impact was similar to the explosion of 100s of nuclear weapons. It vapourised itself and about 70 cubic kilometres of rock, besides melting rock at a deeper level, in a hole that was originally about 100km wide and 40 km deep. The resultant cloud of debris is estimated as being 350 km wide, and the fine dust that flew outwards is believed to have hidden the sun over the entire planet. At the same time earthquakes of enormous power would have shaken the planet with devastating effect. The crater very soon collapsed to form one about 300km wide and 10 km deep.

At the centre of the impact is now what is known as The Vredefort Dome – which could be a so-called ‘bounce-back’ effect similar to that which occurs when a stone is dropped into water. This area, about 120 km south-west of Johannesburg, is now a World Heritage Site. [Guess who will be going there soon!] It is the oldest and largest such crater in the world.

Besides the effect that it had on the rock and earth, the impact caused the ‘bowl’ of the Basin to tilt, disrupted the Earth’s crust and buried the gold deposits, that had been formed earlier, up to several kilometres under the surface. The result was that they were protected from the elements that would have eroded and washed them away – and preserved to be found, billions of years later, by the founders of the city we know as Johannesburg.

We can be grateful firstly, that we were not around at the time, and secondly, that a legacy of so great a destructive event was such a rich one.

And then, to conclude the amazement of your guests at the next dinner party, you could casually mention that, according to Eric Rosenthal, Vredefort in the Northern Orange Free State was founded in 1877. It received its name from the peace that it brought to many farmers who were unhappy about the long journey to church in Kroonstad!

There is a Power beyond the grasp of man, that makes us pause in awe

Acknowledgments:
• The Joburg Book – ed Nechama Brodie
www.parys.co.za/dome/index.html
• Prof. Uwe Reimold (Wits)

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