Focus on Ficksburg, Free State, South Africa

By Proplist Potchefstroom

Vredefort crater is the largest verified impact crater on Earth. It is located in the Free State Province of South Africa, and named after the town of Vredefort, which is situated near its centre. The site is also referred to as the Vredefort dome or Vredefort impact structure. In 2005, the Vredefort Dome was added to the list of UNESCO World Heritage Sites for its geologic interest.

The asteroid that hit Vredefort is one of the largest to ever impact Earth, estimated at over 10km (6 miles) wide. It is believed by many that the original size of the impact structure could have been 250km in diameter, or possibly larger. This makes Vredefort the largest known impact structure on Earth.

More information on the Vredefort Dome;

What Created the Vredefort Dome?

The Magaliesberg - Witwatersrand feature is the result of natural upliftment from below of sedimentary sandstone layers - what was once the bottom of a sea - so that the Magaliesberg rock layers slope down to the north, while the Witwatersrand rock layers slope down to the south. The "Johannesburg Dome" in the center this feature is occupied by the archean granitic crust, some 3000 to 3400 million years old.

The same rock layers seen in the Witwatersrand are found in the Vredefort dome hills, but here they are found standing nearly vertically - the result of extreme upliftment.

Evidence has been found by geologists that the cause of this upliftment was an extreme impact event, caused by an asteroid some 10 kilometres in diameter. The ring of hills we see now are the eroded remains of a dome created by the rebound of the rock below the impact site after the asteroid hit. The original crater, now eroded away, is estimated to have been 300 kilometres in diameter. Some 70 cubic kilometres of rock would have been vaporised in the impact.

The Earth Heaved and Melted...

Within the ring of hills at Vredefort is found granitic gneiss rock - as in the Johannesburg Dome - which is dated at some 3200 million years old. Here a quarry at Leeuwkop near Parys reveals one of the indicators of the impact. The image above left shows the cut surface of the rock. The pale and medium greys on the left are the natural colours of the granite. But from the centre to the right edge is a wide dark grey band containing large and small fragments of the granite. This is "pseudotachylitic brecchia" - the dark grey is granite that was melted by the impact and flowed, carrying chunks of unmelted granite within the melt. The vertical face seen here is two to three metres in height.

The image above right is of a nearly horizontal exposure of pseudotachylite at the quarry. The width of the band is about one metre. Similar rock is found at elsewhere, in geological faults where rocks move against each other and melt at the interface, but this produces a melt band that is centimetres rather than metres wide as seen here.

The basement granite exposed within the ring of hills is estimated to have been seven to ten kilometres beneath the surface when the impact occurred, which is dated at 2000 million years ago. The date was established from zircon crystals found in the pseudotachylite and granophyre (below), and, more precisely, is 2023 +- 5 million years. All the covering rock has since been removed by erosion. Later the south-east part was covered by much younger Karroo rock formations.


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