The Vredefort Dome

AN EXPLANATION OF THE
VREDEFORT DOME
WORLD HERITAGE SITE
What is a World Heritage Site?
A World Heritage Site is a place that has special importance for the
people of the world and that is worthy of preservation and
protection for future generations. A site may be chosen because
important events in human history occurred there (such as at
Robben Island or Mapungubwe) or scientifically because it
contains special plants or animals or rocks, or is scenically
spectacular (such as the uKhahlamba/Drakensberg Park). Only
830 places around the world have so far been chosen as World
Heritage Sites. On 14 July, 2005, part of the Vredefort Dome was
declared South Africa's seventh World Heritage Site. This brochure
explains why the Vredefort Dome is such an important site.
Shooting stars and meteorite impacts
If we look up into the sky on a dark night we often see “shooting
stars”. These are bright streaks of light that move very fast across
the sky for only a few seconds before they disappear. They are not
real stars - the streaks of light are caused by tiny pieces of burning
rock, flying faster than bullets, that enter Earth's atmosphere from
Outer Space. Because they are moving so fast (more than 10 km in
1 second, which means they would take only 30 seconds to fly from
Parys to Bloemfontein!), when they enter the atmosphere they
start to burn. (This burning is caused by friction with the air. When
you rub your hands together very fast, they also get hot. To melt
rock, though, the temperature must reach more than 1000 °C!)
There are many, many millions of such small pieces of rock in Outer
Space, left over from when our Sun and the planets were formed.
Thousands of them become shooting stars every day. But among
them are also larger pieces of rock, ranging from football size to
some up to many kilometres across. These asteroids (see Figure 1)
also sometimes fall into the Earth's atmosphere, but not as often
as the smaller pieces. When they do, they are too big to burn up or
Prepared for the Free State Province Department of
Tourism, Environmental and Economic Affairs by R.L.
Gibson, School of Geosciences, University of the
Witwatersrand. Copyright R.L. Gibson, 12/2006
slow down and so they hit the ground at very high speed. (An
asteroid that hits the Earth is called a meteorite. Thousands of
small meteorites have already been found around the world.)
The damage caused when a meteorite collides with the Earth is
massive. The speed of a meteorite is so high that it explodes when
it hits the ground, blasting a giant hole (a crater) and causing
intense heat of many thousands of degrees Celcius that can even
melt rock. In this way, the meteorite itself is completely destroyed.
This is what happened in the Free State near Parys and Vredefort
millions of years ago. The explosion was so great that it was many
millions of times more powerful than the biggest atomic bomb ever
built on Earth. If it happened today it would kill almost all living
things on Earth, including most people.
3. The Vredefort Dome is only the central part of the impact
crater. It is called a dome because the rock layers were bent into
the shape of an upside-down bowl 90km across by the impact
(see Figures 2 and 4).
If we could look inside the Earth (imagine that it has been cut
open so that we can look at it from the side and see the different
layers of rock that make the Earth's crust, and even part of
Earth's mantle underneath), Figure 2 is a drawing that shows
what geologists think must have happened.
After the impact
In total, the Vredefort impact event probably lasted less than 10
minutes, although the melted rocks would have taken millions of
years to cool down and become solid again. For a time it would
have looked like the crater in Figure 3, from the Moon. Over many
hundreds of millions of years more, rivers eroded the sides of the
crater and most of the cooled melt-rock. This exposed the rocks
that once lay underneath the crater (see Figure 4). In the
updomed centre (the Vredefort Dome), these rocks still show
signs of shattering and melting from the impact (see Figure 5).
Visitors to the World Heritage Site
The Vredefort Dome represents a unique opportunity to celebrate
the history of our planet extending back thousands of millions of
years and to marvel at the devastating forces that have shaped our
world. Apart from the unusual rocks in the dome, the area also has
important historical, cultural and archaeological sites recording
human habitation over the past 200 000 years, together with a
rich scenic beauty.
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Whether it is scientific curiosity, historical or cultural interest,
ecotourism or adventure activities, the Vredefort Dome has
something special to offer each visitor. Welcome to our
wonderland!
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Figure 3 is a crater from the Moon showing what the Vredefort crater
would have looked like 2023 million years ago, before it was eroded.
The crater was so big that it would have stretched from Johannesburg
on one side to Welkom on the other side. The Vredefort Dome would
have lain under the hills in the middle of the crater. (Image courtesy of
NASA/JPL)
The geology of the Vredefort Dome
Today, the rocks in the Vredefort Dome are exposed in several
rings (see Figure 4). The oldest rocks that were buried deep
within the crust before the impact event are found in the centre.
These are mostly granite gneisses more than 3000 million years
old. Parys and Vredefort towns are built on these gneisses that
have been mined for their stone in many quarries. The hills of the
Vredefort mountainland are made of hard white quartzite rock,
with the valleys in-between made of softer shale. Some thin
layers containing a little gold were once mined from these rocks
near Venterskroon. Beyond the hills towards Potchefstroom and
Fochville lie softer lava and dolomite rocks that form flatter land.
All these rocks form part of the Vredefort Dome and are steeply
dipping, but to the south they are covered by horizontal shale
layers that only formed long after the impact and that contain
some coal.
Figure 1 is a photograph of a 33 km long giant asteroid called Eros in
Outer Space. It is full of impact craters caused by collisions with other
smaller asteroids. (Image courtesy of NASA/JPL-Caltech)
The Vredefort impact crater
The meteorite impact that happened in the Free State made a
crater that was about 300 km wide (from Johannesburg to
Welkom!). This is the biggest meteorite impact that geologists
have yet found on Earth and it is nearly twice as big as the impact
that killed the dinosaurs 65 million years ago. This makes it a site
of great importance for scientists. It is also the oldest impact
crater that has been found on Earth. It is mainly for these reasons
that it has been made a World Heritage Site.
Over many decades, geologists from South Africa and other parts
of the world have been studying the broken and melted rocks
around Parys and Vredefort to understand this Vredefort Impact
Event. This is what they have found:
1. The meteorite impact happened about 2023 million years ago,
at a time when there were no people or even animals or plants like
we see today. The only living thing was a type of algae, like the
green slime seen in dams today.
2. To make a crater 300 km wide, the meteorite must have been
about 10 km across (as big as a mountain) and traveling at more
than 10 km per second (36 000 km/h!).
Figure 2 shows how the Vredefort crater formed. The meteorite
exploded in a fireball as hot as 20 000 °C and brighter than the
Sun. This blasted rock, dust and melted rock into the air but also
pressed the rock underneath the crater violently downwards for a
short time. When this rock bounced back a few minutes later, it
formed the Vredefort Dome. The Vredefort Dome is only the
central part of a much bigger crater.
Where exactly is the World Heritage Site?
Only a small part of the Vredefort Dome along the Vaal River,
between the Parys - Potchefstroom and Vredefort Potchefstroom roads, has been declared a World Heritage Site
(see Figure 4). Because the site needs to be protected, people
living in this area need to get special permission to build any new
houses, roads and fences. However, the evidence of the impact
is spread in the rocks over a much wider area.
Figure 4 shows the upturned rings of different rock layers in the
Vredefort Dome, with the oldest in the centre. These rings are
covered in the South by younger coal-bearing rock layers that
formed long after the impact.
Figure 5 shows 3 signs of the Vredefort impact event
that are found in the rocks of the Vredefort Dome.
1. Vredefort Granophyre is the name given to the black
crystallized impact melt-rock that flowed into cracks in the rocks
underneath the crater (see Figure 2). It contains broken pieces
from many different rock types, as well as very small chemical
traces of the meteorite that caused the impact.
2. Shatter cones, which look like part of an ice-cream cone with
lines on it, were formed when the rock was cracked by the force of
the impact shock wave.
3. Black veins of another crystallized melt-rock called
pseudotachylite look like Granophyre impact melt-rock, but they
contain no meteorite traces and the rock fragments only come
from rocks that are found next to the veins. Geologists are still not
sure whether the pseudotachylite formed because of the intense
shock heat, or because large blocks of rock slid past each other
and were heated by friction when the dome was forming.
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