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About THE VREDEFORT DOME
A BLAST – BILLIONS OF YEARS AGO
Some
2.023 billion years ago, something blew a huge hole in the
Earth’s surface near the present-day towns of Parys and Vredefort.
Three crater rings were formed, the outer one being being
the edge of the Witwatersrand and
State goldfields (from Johannesburg through
Klerksdorp to Welkom) some 280-odd km across.
The middle one can still be vaguely seen at the Grasmere
tollgate on the N1. What remains of the inner ring is the
semicircle of the Dome Bergland, mountains formed by
Witwatersrand rock strata that were capsized by the blast.
This is the world’s largest known blast crater and the
oldest known to exist, the Earth itself being about 4.5
billion years old.
The
Vredefort structure is the oldest known impact structure on
earth and the largest astrobleme (impact
site) on earth. The impact resulted in a
crater which was originally approx 300km in diameter, of which the
remaining 45x50 km Vredefort dome is merely an erosional
remnant. In July 2005 the Vredefort Dome was
recognised by the World Heritage Committee as a formation of
“outstanding universal value”.
VISIBLE SIGNS
Visitors often look
for "bits of meteorite" - but there
aren't any. However, many remaining signs of the blast are
to be found only in the "rock signatures": melted seams of
matter caused by great heat and friction; microscopic
fractures indicating that the rocks were shocked; and
semi-triangular rock fragments called shatter cones. There
are also the mountains around Parys and Vredefort,
representing what is left that is visible of the rings
caused by the blast.
CONTROVERSY - IMPACT OR ERUPTION?
The Vredefort Dome "Blast Zone"
is the subject of long-standing controversies about whether
it was caused by an asteroid impact or by a blast from
within the Earth's crust. The idea that
mass extinctions are caused by impacts from outer space has
been one of the best marketed pieces of popular science
but it is not the only explanation. More...
ASTEROID
THEORY
Today almost all experts agree that the Vredefort “anomaly”
– a structure of puzzling mountains around the Vaal – was
probably caused by an asteroid strike some 2.02 billion
years ago. The asteroid theory was only recently broadly
accepted by most, but not all, scholars.
This holds that a meteorite
(actually an asteroid about the size of Table Mountain)
caused the Vredefort structure. The evidence is indeed
persuasive and tours of the region will allow you to touch
and feel the rocks that felt the ancient blast. The presence
of the element iridium (one of the two heaviest elements,
iridum and osmium) in the rocks would probably prove that
the impact was caused by an extra-terrestrial body. But
according to a recent NASA/Los Alamos study no such
component has been identified. There is no obvious
terrestrial (or Earth) source of the widepsread iridium
found in the Earth's crust as a result of other impacts such
as Chixulub in Mexico.
New
scientific reasoning (not directly related to Vredefort but
certainly relevant to it) has thrown the impact explanation into
doubt yet again.
VERNESHOT THEORY
An older explanation is that the crater was caused by a
volcanic eruption. The signs in the rock would be the same
as for an impact:
the crater rings in the
landscape,
“melted glass”,
shatter cones, and microscopic lines in the
quartzite. The name given
to the revived volcanic-type explanation is "Verneshot"
after the science fiction author Jules Verne, who proposed
that spaceships could be shot to the moon using a mighty
cannon. The cannon principle was revived by scientists
at the Geomar earth science institute at
Kiel University to explain mass extinctions.
They speculate that a huge eruption of gas took place from
beneath the crust. This was not a volcanic event but a
release of pressure. If the Earth itself was responsible for the blast, the hole in
the crust that occurred at Vredefort may not have come from
outside but from within.
KAAPVAAL CRATON
Could such a blast
from within the earth happen on earth today? Don't hold your breath... These things take a bit of time to develop,
say a few million or billion years. But the huge lump of solid rock on which
South Africa largely sits, the Kaapvaal Craton, is certainly rising at a
relatively fast pace. Some students of the Vredefort phenomena believe that
the Blast Zone is a window into the craton and can tell us a lot about what
happened in the evolution of the Earth as well as what may happen in the
future. The so-called "African Superswell", which is a strange and so far
unaccountable swelling up of the Earth's surface (today) in our region of
the planet, apparently as a result of massive forces underneath the crust.
METEORITES, LIFE AND EXTINCTIONS
Because the
Vredefort blast happened so long ago it is highly
unlikely to have caused a mass extinction. But it may well
have changed the course of life's development.
Paradoxically, meteorite helped to give birth to complex new
life forms by raising the world’s oxygen levels.
It is considered possible that meteors and
comets may have created conditions for life on Earth to
develop in the first place.
The heaviest
meteor bombardment of Earth happened about 3,8 billion years
ago, around the same time that life on the planet is
believed to have started.
Geologists researching the crater left when the Haughton
meteor slammed into what is now Canada's Arctic 23 million
years ago found the impact created hydrothermal springs in
the cracked rock and other conditions that would have made
it easier for microbes to survive and evolve.
Most people think
of meteorites in terms of life's extinctions. A major meteorite that
hit at Chixulub, Yucatan, some 66 million years ago is
thought to have wiped out the dinosaurs. But at the at the
time of the Vredefort impact just on 2 billion years ago
there were no multicelled species to destroy. There were primitive one-celled forms
of life, and it is possible that the cataclysmic effect of
the blast mutated the DNA of these creatures, causing a
change in the direction of evolution. So-called
"stromatolites", which are beds of ancient algae, can be seen
in the rocks exposed in the Vredefort Dome, which pre-date
the dome. Ancient signs of life
have also been uncovered in the deep gold mines around
Klerksdorp (part of the Dome). As these signs of ancient
life are buried very deep in rock distorted by the blast, it is clear that life did
exist beforehand.
Some evolutionary
biologists speculate that the blast would have affected the
direction of evolution, not only near the blast itself but
in many parts of the planet because the cataclysm would have
spread devastation across the planet. At the same time, it
raised world oxygen levels.
HISTORY AND PREHISTORY
Apart from its scientific interest, the area is extremely
rich in history and prehistory, ranging from the 35 000 years of modern
Bushman occupation to battles between Boers, Blacks and Brits.
Amongst the artefacts we have recently found
are 7500-1200 year old stone scrapers of the Oakhurst Series. On BLAST!
excursions we can visit Boer War
forts and gold mines, Stone Age enclosures and rock art heritage.
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Contact Otters'
Haunt by phone/fax 056-818 1814 or 082-4758767 or
email
or visit us (phone first).
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