This is a blog of ongoing projects starting with: 1) Antarctica -Dec. 2006 - February 2007 2) Work made from the experience 2008 3) Nevada Feb. - Oct. 2008

Friday, October 14, 2011

Vredefort - an impact Crater

As part of my research into the geology and archeology of the area I had asked to visit the Vredefort Impact Crater. 2 Billion years ago an asteroid hit the earth at this spot, creating a crater 300 km across with an impact dome in the middle 90 Km. across. This dome was formed by compressed rock coming back up again, like a rain drop does in a bowl of water. It was the remains of this dome which we were due to visit with the Geologist from Wits, Professor Roger Gibson. We spent the morning exclusively with him, looking at the rock at ground Zero, the various impact melt and fractured rock at the domes edge, and then after lunch joined a bigger tour looking at all the various rock, iron age settlements, Voer trekker farms, early gold mines, Boer war sites and ending up at a seam of melt rock, in the plains near the centre, called pseudotachylite on which were San petroglyphs of various animals and spirit creatures.

All the rocks on this hill at the edge of what was a dome have been lifted from the horizontal to the vertical and shattered.
On the edge of what was the dome looking at seams of pseudotachylite

We crossed the Vaal river and headed into the haze of the centre to what was ground zero and is now a shallow lake caused when an ice sheet, which was at some time responsible for eroding the dome, created a hollow. There are granite boulders strewn across the plain. These are some of the oldest rocks on earth, formed over 4 billion years ago when the earths crust solidified. These rocks formed the bedrock of the dome, ie the bottom of the impact. Everything else, including the asteroid was simply vaporised or hurled into the air. You would have had 4 K square blocks of rock hurled 40 k into the air. Everything beneath the surface was compressed and then sprung back up.

At the centre here, the impact of this 10 km. long flying piece of rock traveling at thousands of miles an hour was massive. The granite was heated, melted and fractured forming pockets of melt known as Granophyre in what had become a kind of plasticine rock. Its rapid cooling meant that this melted rock formed a kind of black glass, a tiny percentage of which is meteorite.
As the rock shattered it formed cracks which in grating together heated the rock to around 14,00 degrees, almost double the heat which will normally melt rock. The resulting liquid melt ran into all of the cracks, some of them several metres wide. this again is the fine grained, black pseudotachylite and is a conglomerate of all the melted rock around it. It takes this new form because it cools rapidly, so being unable to reform its original crystalline state.


pseudotachylite breccia within granite. Friction in the cracks, seconds after impact, melted the rock which ran into the cracks

After lunch we joined a bigger tour and headed for the old gold mines and Boer war sites.

Upended quartzite on the Vaal river which cuts through the dome. These layers of rock are ancient beaches.

searching for gold in the ancient river deposits.

Roger explaining the significance of the shatter patterns in the rock.

As the sun dropped lower in the sky we walked across flat farmland near the centre, headed for a small tree and a line of boulders on the horizon. This is a pseudotachylite fault line, eroded into a line of massive boulders. There is evidence here of massive destruction 2 billion years ago, but 10,000 years ago, San bushmen were using this place as a sacred rain making line. For some reason the rocks have attracted acid drips of water, either from trees, or from a since eroded porous rock. As a result these rocks have little depressions and cups within them which hold water. Rain brings the migrating animals onto fresh grass and so it is here on these rocks, which were once melted at 1,400 degrees centigrade, that the bushmen chipped images of animals and possibly a rain man.

Pseudotachylite fault line of melt boulders

Water cup rocks
Eland

Rhino

Hippo

Hyena

Wildebeest/Eland

Horned rain man?

As the sun sinks low we head back after a very long but extraordinary day, where we have looked at evidence of absolute destruction. A mountain sized rock traveling at 10 Km per second, blasts the Earth causing a fireball with temperatures up to 20,000 degrees centigrade and penetrating 40 Km into the bedrock, creating a 90 Km dome in a 300 Km crater. All this in about 10 minutes max. Early hominids will have lived in the area as they did in the Cradle 2 million years ago, and 10,000 years ago the San will have seen evidence in the rock of water and life. Perhaps the rocks allowed for the growth of trees which attract water and animals seeking shade. The San turned this site of destruction into a place of creation. Creation, destruction, creation, destruction, creation.

3 comments:

Elephant's Eye said...

That is a fascinating description of one of our heritage sites. There is a San rock painting in our Porterville mountains of a sailing ship.

Chris Drury said...

That would more than likely be Koi, the herders who came later.

Etxera Joan said...

I spent hours today going over this and the artworks on the site instead of doing my own artwork. I always go looking for the feeling of stuff; of rocks, trees, leaves and flowers and the smell of moss and rain in nature. But when I want to understand ways of doing it with art, I come to blogs/ sites such as these. And then i started to make something... thanks for the inspiration!