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BullsEye
Earth!
Graeme Addison tells why
the little-known Vredefort impact zone—site of the world’s
oldest and biggest meteorite strike—is being proposed as a
World Heritage site.
Fears that a wandering
asteroid or comet could cross earth’s path and destroy life
as we know it are not unfounded—it’s nearly happened a
couple of times in recent years. But bombardment by
meteorites (meteors that actually hit the ground) has also
benefited mankind. What most people do not know is that the
world’s oldest and largest impact crater sits in the heart
of South Africa, centred on the sleepy Free State dorp of
Vredefort. And it is this “astrobleme” (eroded crater) that
South Africa has to thank for first preserving, then
exposing, the mineral riches of its gold-bearing reefs.
The hit occurred some
2.02 billion years ago, before multi-celled life evolved on
this planet. It forced the rocks of the Witwatersrand basin
deeply downwards, protecting them from erosion, so that
finally when prospectors arrived on the scene in the 1870’s
they were fortunate to find traces of gold on one of the
upturned edges of the basin, which had by now broken the
surface. Johannesburg, the mining and financial powerhouse
of Africa, was born.
ASTEROID
The Vredefort Ring is one
of the largest natural disaster features on earth and has
been photographed by Nasa astronauts in the Space Shuttle.
It’s a regular BullsEye seen from space, but you don’t have
to put down $20million like Mark Shuttleworth to see the
crater for yourself. A short drive south of Johannesburg on
the N1 takes you over the Grasmere ridge which is one of the
concentric rings that registered the force of the impact,
like ripples spreading from a stone dropped in a puddle.
Closer to the Vaal river
the ridges get higher, and overlooking the river itself in
the vicinity of Parys are several hump-backed bush-clad
mountains known as Die Bergland. Whitewater rafters who run
the rapids of Parys, and mountain-bikers tackling the
championship courses of Bergland have the meteorite to thank
for turning what would otherwise be flat mielielands into
rugged hills and tumbling rapids. Because the meteorite at
an angle from the southeast it pushed up the northwestern
edges rather like a crumpled carpet, and along these
northwestern edges today are the mines of the central and
western Witwatersrand. But the circle extends right around
to the south where other gold mines operate at Welkom.
Of the asteroid itself
there is little or no trace: it probably vaporized, but is
likely to have been a wandering asteroid at least the size
of Table Mountain. Drawn in by the earth’s gravity, it
blasted the surface with a detonation equivalent to millions
of nuclear bombs. The crust down to about 25km melted and a
mass of magma welled up, forming a vast granite bubble or
dome (so the place is widely known as the Vredefort Dome).
The existing ridges are
no more than the stumps of the once mighty dome perhaps 7km
high—nearly as high as Everest today—that ballooned upwards
from the cauldron and immediately collapsed. Today a small
pan near Vredefort called Die Inlandsee is much favoured by
passing flocks of flamingos, and this is the dead centre of
the impact zone that has now appeared at the surface due to
aeons of erosion.
AGED WATERCOURSES
It is arguable (but
speculative) that the generally placid Vaal River is one of
the world’s oldest watercourses, if not the oldest. In the
era when the Witwatersrand basin was formed the
supercontinent of Gondwanaland consisted of Southern Africa,
India, Madagascar, Australia, Antarctica and South America.
In the middle was a vast flat lake where sand and pebbles
settled down in strata, carrying gold and other minerals in
powdery grains. As Gondwanaland split apart like a mighty
jigsaw all that was left was Southern Africa with its
central basin, drained by rivers being the predecessors of
the Orange-Vaal watershed.
The Vredefort crater,
estimated at 300km in diameter, is one of three known
massive impact sites amongst many hundreds or even thousands
of meteorite blemishes on the earth. The other two really
large sites are at Sudbury in Ontario, Canada, and Chicxulub
on Mexico’s Yucatan Peninsula. The Sudbury area has the
dubious record of being the target area of two major hits:
the first, almost as ancient as Vredefort, causing a
200km-wide crater, and the second a smaller one just 37
million years ago.
The most celebrated
crater is Chicxulub (pronounced CHEEK-shoe-lube), for this
impact was almost certainly the cause of the extinction of
the dinosaurs about 65 million years ago, a catastrophe that
extinguished up to two-thirds of all animal and plant
species and could easily happen again unless we keep our
wits about us. Today the crater extends into the Caribbean
Sea across a diameter of 180km. Oil prospectors first
spotted it as a rather peculiar irregularity in the rock
formations, which are characterized by typical “shatter
cones” (triangular shapes) caused by intense shocking of the
rock. Shatter cones are found in profusion on the remains of
the Vredefort impact too.
There was a period about
four billion years ago when the earth, like the Moon, was
subjected to a steady bombardment of asteroids and comets
that pocked the land extensively. The fact is that asteroids
or comets can and do hit the earth and if we should ever
have another one this size it would certainly wipe out all
life. Task forces have been appointed both in Britain and
the United States to track down near-earth objects and lay
plans to neutralise them if possible. Nasa has a substantial
programme and has been set a goal by Congress to detect at
least 90 percent of all near-earth objects with a diameter
greater than one km within 10 years.
Up to 10000 meteorites
hit the earth every year, but they are generally no larger
than a grapefruit and often as small as a pea, so they go
unnoticed. A larger impact occurs perhaps once in 100 000
years, a very large one every couple of million years. The
probability of a gargantuan strike like Vredefort is remote,
but probabilities are not predictions and one could threaten
at any time. The effects of the earth being hit by an
asteroid bigger than one kilometre in size would be
devastating, with a blast and dustcloud likely to cause
climate change and rapidly cut off food supplies. Millions
of human beings could die—if not all of us, depending on the
severity of the event.
Everything you have seen
on the wide screen in Hollywood films like “Deep Impact”
could come true, so it is no wonder plans are being laid to
launch nuclear warheads into space to deflect any menacing
lumps of rock or ice. But don’t breathe easier just yet.
Several large space bodies have passed close the earth in
recent years without anyone noticing until it was too late.
As for the Vredefort
Dome, it happened so long ago that only the barest traces
remain. We do not know what effect it had on the very simple
single-celled lifeforms that seem to have been evolving at
the time. Its importance as the oldest and biggest known
impact zone, and the golden heritage it has left us, are
ample justification for the effort now being made through
the SA Department of Environmental Affairs and Tourism to
have the Vredefort Dome declared a World Heritage Site by
Unesco.
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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.