Vredefort Dome- A Stone from Heaven

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.

Vredefort Dome bears witness to the world’s greatest known single energy release event, which had devastating global effects including, according to some scientists, major evolutionary changes. It provides critical evidence of the Earth’s geological history and is crucial to understanding of the evolution of the planet.

Despite the importance of impact sites to the planet’s history, geological activity on the Earth’s surface has led to the disappearance of evidence from most of them, and Vredefort is the only example to provide a full geological profile of an astrobleme below the crater floor.

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 of 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 travelling at more than 10 km per second (36 000 km/h!).

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.

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 tine 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 per second, which means they would take 30 seconds to fly from Parys to Bloemfontein!), when they enter the atmosphere they start to burn.

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.  These asteroids also sometimes fall into the Earth's atmosphere, . An asteroid that hits the Earth is called a meteorite. 

 Imagine an asteorite the size of Table Mountain heading for Earth at a speed of 20km per second. Packing more punch than multiple nuclear bombs, it penetrates 17km deep into the Earth and leaves an impact crater so large it’s been declared a World Heritage Site.

The damage caused when a meteorite collides with the Earth is massive. The speed of a meteorite is so high that is explodes when intense heat of many thousands of degrees Celsius 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.

Vredefort Structure

Vredefort Dome - imagine an asteroid the size of Table Mountain heading for Earth at a speed of 20km per second. Packing more punch than multiple nuclear bombs, it penetrates 17km deep into the Earth and leaves an impact crater so large it’s been declared a World Heritage Site.

The Vredefort Structure can be considered to be a gigantic scar that was left behind when a huge meteor (estimated size 10km diameter) collided with the earth about 2023 million years ago.

The impact site, in modern day terms, would be some odd tens of kilometres south of Parys. The collision, at an estimated speed of 150 000 k/h, took place on an ancient surface about 17 km higher than the mentioned modern-day target point.

The reason for this was that at the time 17 km of layered sedimentary and volcanic formations were piled up upon what, in today’s terms, we refer to as the Parys granite.

 Although a crater with an estimated diameter in the order of 100 km and some odd tens of kilometres deep was formed on the ancient surface, it was destroyed by erosion over a period of millions of years.

How do we know that it was indeed an impact?

Scientists look for the following evidence that a geological structure is impact related:

• Crater rim higher than that of volcanic craters
• Older rock formations on top of younger formations
• Chemical changes in the rock only possible at pressures and temperature far exceeding those possible in Volcanism

The most compelling evidence: Shatter Cones

The Proof . . . all of above is found in abundance in the Vredefort Dome

SHATTER CONES

The abundant examples of Shatter Cones in the Dome irrevocably proofs the origins of this structure lying in a huge impact somewhere in the past

What are Shatter Cones?

• Shatter cones originate only from rocks that were suddenly exposed to extremely high temperatures (Thousands of Degrees Celsius) and pressure (several Giga-Pascal).
• The crystal structure of a rock is deformed into a conical shape with the base of cone orientated away from the source of impact 

  Interesting Facts

• The location of the Vredefort Dome is adjacent to the even more unusual Witwatersrand gold field, the largest gold deposit on earth. Is the presence of the goldfields in an arc shape around the Vredefort Dome related to the meteorite impact some 2000million years ago? What is well known is that there are very few geological features that aren’t related in some way to mineral resources.

• The South African gold deposits constitute by far the largest known deposits of gold in the world and are still the source of considerably more than half of the annual world production of newly mined gold. 

In its wake, pulverised rock powder rained down on the Earth for months, blotting out the sun. But, curiously, scientists speculate that the incident may have increased the planet's oxygen levels to the point of making life possible.

The cataclysmic event left behind the Vredefort Dome, a South African World HeritageSite.

Some 200-million years ago the Vaal River began flowing through the Vredefort Crater, attracting a passing parade of ancient San people, followed by the Sotho, the Tswana, Boer and Brit, and gold prospectors in search of personal fortunes.

Ironically, this site of past violence has been transformed over time into a tranquil sanctuary for body and soul where a plethora of leisure activities are enjoyed.

In the portion of the Vredefort Dome that has been declared a World Heritage Site, private landowners take their role as custodians seriously, guarding against overdevelopment.

They operate B&B establishments, conference centres, and activities that allow adventure seekers to test their bodies against the Vredefort Dome's challenging geographical features.

Visitors climb its peaks and abseil its rock faces, explore its mine tunnels and canoe the rapids of the Vaal. In between, they visit Iron and Stone Age sites, granite quarries, and shop for antiques in nearby Parys.

Origins of the  Vredefort “Dome” in pictures