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MORE... FREQUENTLY ASKED QUESTIONS
Why is the gold here?
The reason that gold lies in a large
semi-circle from Johannesburg to Welkom is simply this:
whatever happened to create the Vredefort Dome was also
responsible for dipping the gold-bearing strata of the Wits
Basin deep underground. The phenomenal wealth of South
Africa and the Dome are intimately connected. The
Witwatersrand Basin was formed about 2.7 billion years ago
as an ancient lake created by high-energy rivers (that is
steep torrents) coming off high mountains which must have
contained gold. Where exactly the gold came from originally
is a matter of speculation – it was probably formed along
with the other elements of the earth in burning stars that
exploded into dust and re-congealed as planets of our solar
system (like all solar systems). The gold carried down by
the rivers was “placed” in deltas around the flat,
overlapping strata of the lake bed. This placer gold formed
veins of the precious metal embedded in what the early Wits
miners called “banket” because it resembled Christmas
pudding with large and small pebbles surrounded by dark, finer material with the
gold glittering inside it.
Why say the Wits strata were
"capsized"?
The Vredefort impact happened almost in the middle of the Wits Basin. From the
site of the impact, ripples of molten rock spread outwards rapidly, forming the
Grasmere ring and, beyond that, the Wits-Klerksdorp-Welkom ring, which can be
seen from viewpoints on the Dome Bergland. Immediately following the blast, the
flat-lying strata of the Wits basin were thrown sideways and upwards, dipping
deep into the crust. Around what is now Parys-Vredefort, the inner Collar strata
were thrown so violently that they actually capsized. As a result, the oldest
layers are closest to the centre, and the youngest are further out. This is the
reverse order of the same strata on the Witwatersrand where the youngest are on
the inside and the oldest on the outside. Keep in mind that when the
Witwatersrand was discovered, no-one had any idea why the gold was there or what
its relationship was to the Vredefort structure. Indeed, as recently as the
1950-60s, when the Free State mines were opened up, no-one really knew what had
caused the Vredefort structure (the general theory was volcanic). There was
little suspicion that the two phenomena were linked.
Why is the gold concentrated in such rich seams?
Much controversy surrounds this question. One school of thought maintains that
the gold was already very rich as a result of some unique concentration of it in
the early mountains and rivers; then heat of the blast may have concentrated it
further. Another school says that this cannot be the explanation – placer gold
does not concentrate in such rich bands. In needed further hydrothermal (hot
water) action under the crust, over millions of years following the blast, to
run the heavier gold dust together into the extremely concentrated gold seams we
mine underground today.
What is the Kaapvaal Craton?
The Dome is still visible because very little has happened geologically to
disturb the surface of the earth in this region since the blast occurred. The
rings of the Vredefort blast are found on the surface of a vast plate of ancient
rock called a Craton (nothing to do with a crater). This Kaapvaal Craton
consists of Archaen granites – the very oldest rocks – which have remained more
or less in one huge plate since the earth’s crust stabilized 3.7 billion years
ago. What makes the Vredefort dome so interesting is that it provides a “window”
into the Kaapvaal Craton: we can look deep into the earth’s crustal structure
because it has been punctured here. Nuclear geologists working at Wits
University have probed the crater and drawn fascinating cross-sectional maps of
its beds of rock, extending downwards for 30-50km.
What is the African Superswell?
Today this part of the continent is rising out of the surrounding oceans
comparatively rapidly, in a phenomenon known as the African Superswell – and who
knows? – we may be in for another cataclysmic blast, this time from under the
earth’s crust. The African Superswell is a phenomenal build-up of forces
underneath the Kaapvaal Craton. Such a blast would be a Verneshot, and it is
possible, though unproven, that the African Superswell signifies some such
pressure building up underneath us.
Why do skeptics doubt the meteorite theory?
One logical reason for doubting the meteorite explanation for the Dome’s
existence is that the hole in the crust occurred in the middle of the Wits
Basin, which itself lies in the middle of the Kaapvaal Craton. Meantime, these
features lay in the middle of Gondwanaland, the super-continent that has
subsequently broken up to form India, Australia, Madagascar, Antarctica and
South America. Africa is pushing all these continents away. It just seems too
coincidental that in the middle of the middle of the middle is an asteroid
strike! Did the asteroid draw a bead on a bullseye? But many scientists contend
that the bullseye is, in fact, a coincidence.
Is another such blast likely?
Don’t worry, a Verneshot from under the Kaapvaal Craton is unlikely to happen in
our lifetimes, or anytime soon… say not for many more millions of years. But do
worry about this: much more likely than a Verneshot is the approach of another
killer asteroid, one of which about the size of the presumed Vredefort asteroid,
> one could collide with the earth in 2117! In fact, one passed us very closely
just a few years ago around Christmas time, coming closer than the Moon. Some
Christmas that would have been – and some wandering star for wise men to follow!
Because the solar system is full of wandering bodies, and many of the smaller
ones do in fact strike the earth – the chances of a big meteorite hitting the
earth are fairly high. Once in a million years there may be a blast the size of
Tswaing (1km crater north of Pretoria), and once in 10 million years something
intermediate between Tswaing and Vredefort. Once in a 100 million years there
could be an extinction event caused by a massive asteroid… so don’t start
counting the days.
Is there evidence of giant volcanic eruptions in this area?
Yes, the so-called Ventersdorp lavas, which surround the Collar to the north,
are evidence of mighty lava flows. Further away is the extraordinary Pilanesberg
crater, a feature known as a “volcanic shield” which took the form of a flat,
large-scale eruption leaving us with the mountains now hiding Sun City, its gold
course and adjoining game reserve. Even further away, the Drakensberg-Maloti
mountain massif is the product of lava outflows some 100 or more million years
ago which covered the entire region of Lesotho and beyond, and has been eroding
into the “barrier of spears” (Drakensberg escarpment) that is now a world
heritage site.
Why can only part of the Dome be seen?
The mountains of the Collar represent the very eroded remnants – like the roots
of a rotten tooth – signifying where the inner ring of the crater once existed.
In fact, the crest of the ring has long ago been scraped away and a depth of
about 7-17km of crater rock has been removed. The Vredefort Structure as a whole
was buried, for many hundreds of millennia, under the sedimentary rocks of the
Karoo basin and was only comparatively recently re-exposed as the Vaal River did
its erosional work. The Ring appears to have tilted so that what we can see is
only the northwestern part of it: the rest is still buried under the Karoo
plains.
What is the Karoo basin?
The Karoo basin (which includes the modern Karoo semi-desert) – was another
giant lake that succeeded the Witwatersrand basin in the past 500-600 million
years – covered most of central Southern Africa with new strata known as the
Karoo Supergroup as much as 10-15 km deep. This soft layer of stone, comprising
sedimentary rocks (sandstones), as well as desert sands, shales, tropical forest
detritus (coals) and glacial moraines, tells the story of major changes in
climate and locality as the continent drifted across zones of the earth’s
surface. The Karoo basin was the home to dinosaurs and many forms of aquatic
life, and also hosted some of the earliest mammals. Traces of them are found in
the rocks (visit Graaff Reinet museum).
Any more questions?
Get
hold of me. |
I'm
a writer of popular science and technology books and
articles, a former Professor of Communication whose
hobby is now "backyard astronomy" with history thrown
in! I'm a keen mountain-biker and kayaker, so the
Dome and Vaal River are my playground.
I'm
a former teacher who has worked with Graeme on many
aspects of science & local history research, and I have
got to know the Dome by preparing the maps and data used
in our various presentations. My passions are mountain
running and education. I manage Otters' Haunt.