Tech Track: The Vredefort Dome is a crater caused by an impact of an asteroid
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By Kelvin Kemm
An article was published in May in the international journal Geology reported the discovery of striking green-black rocks in the Vredefort Crater or Dome in South Africa.
The massive Vredefort structure was always referred to as the Vredefort Dome, but after many years of scientific debate it was established that it is a crater caused by an impact from an asteroid from outer space.
In fact it is the largest meteorite impact crater on Earth. The world's 8th largest meteorite impact crater is also in South Africa. It is the Morokweng Crater in the Kalahari Desert.
It was formed 145 million years ago and it delivered a surprise when geologists drilled into it and found pieces of the original meteorite still there.
This was surprising because usually meteoroids strike the Earth at such a great speed that there is a massive explosion ensuring that nothing is left of the original meteorite.
On March 7, a meteor lit up the night sky over Vermont in the US producing sonic booms. But the meteorite was estimated to be only about 4.5kg. On March 20 another one lit up the daylight sky over the UK and was also seen in northern France. It too produced sonic booms, but is thought to have been maybe one metre in size.
So now imagine the impact produced by a meteorite 10km in diameter. That is the estimated size of the meteorite which formed the Vredefort Crater. What happened was that this giant rock slammed into the Earth 2.02 billion years ago, releasing so much energy that it melted the rock deep down.
This molten rock then oozed like toothpaste to the surface where it formed a lake. This cooled to form the layered rock dome from where the feature got its name, before it was identified as an impact crater.
These obviously prehistoric molten magmas caused a great deal of debate among geologists. One argument was that the molten rock came from the impact of the meteorite. Another argument was that the meteorite cracked the ground and later molten rock produced from volcanic lava flowed into the cracks.
But now two researchers, Lisa Cupelli, and Desmond Moser, believe that they have proven that the melt came from the original impact. They state that the green-black rocks that they have now found in the crater are from the original magma sea that once filled the huge hole.
Yes, it was huge! It is calculated that the original crater was some 300km in diameter, and that the hole was 10 times deeper than the Blyde River Canyon.
That is a big hole. There would have been a big splash of molten magma as the meteorite hit. Beads of molten rock were blasted high into the sky. What is mind-boggling is that some of these molten beads from Vredefort have been found in Karelia in Russia.
What geologist Moser said was: "What Vredefort teaches us is that we haven't been looking with the right set of eyes at some of these ancient rocks."
As a nuclear physicist I can agree with that statement.
With very large intense explosions like that, many of the resulting effects are totally counter-intuitive. This was found to be the case with nuclear weapons tests conducted during decades after the Hiroshima nuclear bomb. For example, some buildings at some distance from a nuclear blast popped outwards from the blast and not inwards as many explosives people expected.
That was because a nuclear blast delivers a very fast high-pressure strike followed by low pressure vacuum, so to speak. The high-pressure pulse “broke” the building, but the sudden following vacuum “sucked it outwards”.
There were other odd effects discovered which resulted from the intense heat and light, delivered in a very short pulse. So with these large meteorite strikes it is necessary to calculate the real physics, and not rely on a gut-feel guidance initially.
Cupelli, the leader of the new study, says that the unusual zircon rocks must have crystallized at temperatures between 1 300 and 1 700°C, which indicates that they were formed from the impact.
The study has sparked academic interest and other researchers are now showing more interest in Vredefort.
Dr Kelvin Kemm is a nuclear physicist and chief executive of Stratek Business Strategy Consultants.
*The views expressed here are not necessarily those of IOL or of title sites.
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