03/08/2024
NOT A DOME BUT A BASIN
One of the mistakes many visitors make when looking for the Vredefort Dome is to look for a dome-shaped rock at Vredefort. There is one - with the Gateway Information Centre under reconstruction on top of it - but the Dome is not a visible Dome and it's not at Vredefort.
It is a basin. Look at the photos of a saucer and you get the idea. The middle ring of the saucer is the Dome itself, surrounded by a set of rings, the first of which is the range of mountains called the Dome Bergland.
A very good slow-mo shot of a rock being hurled into water shows how the Dome and its surrounding crater were formed. This video was shot by Matt, a visitor to our place yesterday for the Dome Self-Drive Briefing. In the briefing I always explain and demonstrate the formation of this mighty crater some two billion years ago.
The surface of the river behaves like the surface of the earth when a meteorite hits it. Like a liquid, the rocks part and the fireball rock plunges into the crust. It throws up ejecta (droplets) which either fall back to earth or burst into space to become meteorites themselves - perhaps landing on Mars or other planets. Meanwhile the rings of the large crater radiate outwards. In the centre, a flat patch on the water surrounded by bubbles marks the Dome Core with its Collar of mountains.
The term "Vredefort Dome" is a misnomer. What geologists call a dome is a vast plug of granite in the surface of the earth, between 50-70 km deep and about 50-60km across. This is the central Core of the enormous impact crater which today stretches from Johannesburg to Welkom, covering much of the Highveld. This is by far the largest and best preserved visible asteroid "star scar" on the face of our planet. It is by no means the biggest ever but it is still clearly seen (from space! - it's too big to see it all from any point on the surface).
The story of the Dome has been reconstructed from the available evidence found by generations of geologists and supplemented by deduction. It's a detective novel: we have the body but Whodunnit and how was it done? Rocks that we still find today suggest some of the story.
The pictures here, also taken by Matt, show friction melt-rock consisting of a black glassy substance with embedded lumps of rounded granite scattered through it. These rocks are rare in the world but plentiful in the Dome, and are also found (much smaller) in other impact craters. Their formal name is pseudotachylite or false volcanic glass, first named in 1916. It was the identification of such a sample that really started the debate over what the landscape reflects. If it wasn't a volcano, what was it?
The answer is partly disclosed by the other rock sample. Curiously named chocolate tablet breccia (chocolate for the colour, tablet for the small roundish bits like aspirins set in the matrix, and breccia for rock made of many pieces) the rock shows that a mighty shock travelled through the terrain. The original chert, a very hard rock, could not handle the shock wave and simply exploded, with the bits reforming as breccia.
It takes imagination, science, chemistry and concepts of rock mechanics to figure all this out. Geologists are good at doing thism ending with informed guesses. They will often suggest alternative explanations in case later evidence shows something else.
A joke about geologists hedging their bets is that AngloAmerican advertised for a one-armed geologist - so that he couldn't say one the hand this, on the other hand that...
