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INTRODUCTION
A VERY OLD CRATER
Scientific debates continue to swirl
around Vredefort Dome
The pages
in this section may repeat what appears elsewhere on the
website but in more detail and with greater emphasis on the
scientific facts and debates.
Some 2.03 billion years ago, something blew a huge hole in
the Earth’s surface near the present-day town of Vredefort
in central South Africa. Three crater rings were formed, the
outer one being being the edge of the Witwatersrand and Free
State goldfields (from Johannesburg through Klerksdorp to
Welkom) some 300-odd km across. The middle one is marked by
the Gatsrand, a wide ridge of hills stretching from
Potchefstroom in a northeasterly direction, and it can be
seen at the Grasmere tollgate on the N1. What remains in the
middle is the semicircle of the Dome Bergland, the so-called
collar of the Dome core. This collar consists ofmountains
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. Despite its size and the fact that it has
been intensively studied, the crater remains controversial
in all sorts of ways. For example, reasons for the
concentration of gold in the Witwatersrand, encircling the
Vredefort Dome in the north and west in a so-called Arc of
Gold, remain a subject of scientific dispute. Even the size
of the crater is not widely agreed. The
Unesco Declaration that identified the Vredefort Dome in
2005 gave its diameter as 190km. This may represent the
Dome's eroded surface today, but some scientists say it is
250km across and the original surface of the crater could
have been as much as 360km across.
Controversy does not stop with the science.
Historically, this area has seen more chaos and carnage than
most other parts of South Africa because it lies at the
heart of South Africa and thus attracts racial and cultural
disagreement over who did what to whom and how history
should be interpreted. Sharpeville, for instance, the scene
of a passbook protest in 1960 which resulted in the massacre
of PAC supporters, lies near the Vaal River outside
Vereeniging. The name of the city means “unity” referring to
the settlement between white Afrikaners and the
English-speakers who formed the white-dominated Union of
South Africa in 1910. About the only thing everyone agrees
on is that the uniquely beautiful and meaningful natural
environment should be conserved.
Blast or Belch?
Scientists differ about the cause and consequences of the
blast.
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The overwhelming consensus today is that
it occurred when a wandering asteroid struck the Earth.
This lump of space rock could have been the size of
Table Mountain or larger, some 11-15km across,
travelling at 20-30km a second. It exploded with the
force of trillions of H-bombs and immediately melted the
crust of the Earth down to about 50km. The hole
spat meteorites into space and scattered debris over the
earth, darkening the atmosphere for years. Molten rock
ballooned upwards from within the crust and this upheaval
dome, together with infalling debris, plugged the hole. This
is the origin of the “dome” of granite that exists in the
centre of the blast zone, now some 30-60km across in parts.
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Another theory is that the crater was caused by a blast from
inside the Earth itself. For most of the 20th century this
was the explanation
offered for the existence of the Vredefort Dome. But it was
always controversial. Recently,
researchers have suggested that some large craters – and
Vredefort could be one of them – were caused by “Verneshots”
as the Earth exhaled a mix of gases, spitting rock into
space. Like the meteorite hypothesis, this explanation may
account for the strange signatures in the rock – pseudotachylite seams shatter cones
(or triangular shards of rock like broken glass from a car’s
windscreen), and shock metamorphosis (or tiny markings in
quartzite indicating that the rock has been shocked).
World Heritage Site
In 2005 the Vredefort Dome World Heritage
Site (WHS) was designated by Unesco.The Dome was previously
listed as a National Heritage and it includes several
National Monuments such as the Wild Olive Tree Forest at
Koppieskraal, and Liebenbergskop where a family of
Voortrekkers died at the hands of the Matabele. Vredefort
Dome was declared primarily for its geological interest but
its ecological, archaeological and historical features add
to its value. And its source of main appeal to the general
public tend to be the
activities, from outdoor adventure to antiques and
artwork.
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UNESCO DECLARATION,
2005, Durban
The Vredefort Dome, approximately
120km to the south and west of Johannesburg, is
a representative part of a larger meteorite
impact structure, or astrobleme. Dating back
2,023 million years, it is the oldest astrobleme
found on earth so far. With a radius of 190km,
it is also the largest and the most deeply
eroded. Vredefort Dome bears witness to the
world’s greatest single known energy release
event, which caused devastating global change,
including, according to some scientists, major
evolutionary changes. It provides critical
evidence of the Earth’s geological history and
is crucial to our understanding of the evolution
of the planet. Meteorite impact has played a
significant part in the geological history of
the Earth, but geological activity on the
Earth’s surface has led to the disappearance of
evidence from most impact sites. Vredefort’s
structure provides the only structurally intact
exposure of the basement below the crater floor
and is, in this regard, unique. |
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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.