If an object the size of half a soccer field hit the earth at 4 000 kilometres per hour hundreds of thousands of years ago, what would you expect to find? The rather special Tswaing Meteorite Crater, some 40 kilometres north of Pretoria.
What’s special about it is that firstly, it is one of the best preserved meteorite craters in the world, and secondly, it’s very accessible – you can walk down into the crater, unlike a similar crater in Arizona in the United States, which gets so many visitors – 250 000 a year – that access to the bottom of the crater is no longer allowed.
The Tswaing Crater (Tswaing meaning “Place of Salt” in the Tswana language) gets 12 000 visitors a year, mostly Gautengers, coming to marvel at this wonder that hit the earth 220 000 years ago with an impact of about 100 atomic bombs of the type dropped on Hiroshima in 1945. It created a crater of just over one kilometre in diameter and sent up 60 metres of earth to form the present-day crater rim.
The crater is one of around 170 impact craters in the world and one of four known impact craters in South Africa. It has one of only four meteorite crater museums in the world. The others are in the US, Germany and France.
An extensive wetland system
The broader Tswaing Crater site covers some 2 000 hectares in the north-west corner of Gauteng, with over 220 000 people living in informal settlements in its immediate vicinity. Just east of the crater is the Soutpanspruit, feeding a rare and extensive wetland system, with a mini delta with many streams and islands.
At the point of impact – an area spanning three kilometres around the crater – all life forms and rock, as well as the meteorite itself, would have vaporised. A broader area of 1 000 square kilometres would have been flattened. A gigantic air blast of up to 1 000 kilometres per hour would have added to the destruction.
The whole site is covered in dense bushveld, and walking along the trail on the rim, one looks down into this marvellous phenomenon, with its small 100-metre diameter lake in the centre of the crater. Another three trails are being developed, one of them to the wetlands.
The lake, which is filled by a spring in its bowl and rainwater, once contained high concentrations of salt and soda ash. These were mined for 44 years, until 1956. The salt content has dropped, making it no longer viable to mine. Remains of the factory and the manager’s house still stand on the south outer side of the rim. The lake is three metres deep and supports bacterial growth only.
Drilling the crater
Drills have been put down into the crater by geologists trying to establish whether the crater was formed by a volcanic eruption or a meteorite, a controversy that has raged for some years. Because volcanic rock was found in the crater rim, it was thought for many years that Tswaing was the result of some kind of volcanic event.
In 1989 a drill was sent down 200 metres through the crater fill (sediment filling the crater over the years). Analysis of the drill core confirmed that the crater was indeed caused by a meteorite.
What was also revealed was the climatic conditions going back 200 000 years. Unlike the present climate of sub-tropical and dry-humid conditions, in the past there were alternating dry and wet periods, and at one point the site was a forest of yellowwood trees. The drill core also helped scientists determine the age of the Crater, at 220 000 years.
These drill sites now contribute to the water flow into the lake, and are still visible in the form of pipes protruding from the lake.
This analysis revealed too that several animals that lived at the time of the impact no longer exist. These include the giant wildebeest or hartebeest, a long-horned giant buffalo and a giant zebra. An early form of homo sapiens roamed the area, but would not have recorded their presence through rock art – the first rock art was recorded in South Africa 170 000 years later.
South Africa has another impact crater, about 200 kilometres south of the Tswaing Crater. The 250-300km wide Vredefort impact crater – formed by an asteroid some 10 kilometres in diameter hitting the earth – is located about 120km south-west of Johannesburg. Known as the Vredefort Dom,e the crater dates back 2 020 million years.
The significance of the Vredefort Dome is that when it hit the earth, the gold-bearing rocks of the Witwatersrand, which were deposited some 800 million years prior to the Vredefort impact, were covered with impact debris that protected them from erosion over the subsequent two billion years.
If you’re wondering whether it’s about time another meteorite ploughs into this part of the earth, it is estimated that such an event occurs every 7 000 years.
The Tswaing Crater Museum
The Tswaing Crater is managed by the Northern Flagship Institution, Gauteng Nature Conservation and the City of Tswane. The local community is involved in monitoring the fence around the perimeter of the site, and providing guides who have been trained to take visitors on tours of the site.
The Tswaing Forum was established in 1993, and one of its major aims is to establish “a non-aligned independent people’s project for the conservation and sustainable utilisation of the environmental (natural, cultural, human) resources of the Tswaing area”. This includes liaising with local communities, breeding indigenous domesticated animals and plants, and developing Tswaing as an ecotourism destination.