Investigation of Vredefort Dome

Local Researchers

Back in South Africa, however, the developments in impact science seemed to pass us by. In 1916, S.J. Shand first described the curious melt-rock phenomenon he called ‘pseudotachylite’ but did not rule out volcanic origin for the rock.

The first detailed geological investigation of the dome, carried out in 1925, came to the conclusion that the dome had been caused by a gigantic volcanic gas explosion. In 1936, two researchers proposed that an impact had caused the dome, but this was largely ignored by the local researchers.

The stumbling block was the Dome structure itself. How could an impact from above cause the ground to rise up? Surely an internal explosion or intrusion was much more likely to form an uplift than an external impact.

Nevertheless, in 1947, a geologist named R.A. Daly picked up on the impact hypothesis and suggested that the release of pressure after the impact could have caused the rock layers to rebound and raise up the dome structure. The august godfather of South African geology, Alex du Toit, remained unconvinced and reported that the dome was created by tectonic processes.

The Argument Continued

For years, the argument continued. International opinion tended to weigh in on the side of the impact-school. But South African researchers continued to produce papers suggesting an internal (magmatic or tectonic) origin.

In the 1960s, a report from the Geological Survey of Ottawa made the distinction between two types of craters: those with a diameter measuring less than several kilometers, which tended to have a simple bowl shape, and those larger than several kilometers, which tended to have a central uplift of older rocks surrounded by a collar of younger layers.

This ‘complex’ crater pattern fitted Vredefort perfectly. Nevertheless, the local scientists kept to their course, taking heart in the work of international cryptoexplosion expert, Walter Bucher, who maintained that dome structures were caused by eruptions of hot gas which leaked through lines of weakness in the Earth’s crust.

Shock Metamorphics

Despite the reticence of local researchers, the understanding of shock metamorphics continued to develop. In 1968, an international conference on the subject was held, bringing together experts from many countries. This gave the impact theorists renewed impetus and, over the next few years, significant strides were made.

Field reports from terrestrial craters, lunar rock samples and laboratory test results continued to corroborate with each other, and the scales began to tip towards the impact hypothesis.

By the end of the 1970s, 120 impact structures had been identified in various parts of the world and a suite of characteristic rock forms common to many craters had been described. These rocks were collectively called ‘impactites’ – a word which brings to mind the members of an aerobics class, or people in a crowded lift.

By the 1980s, most international experts were convinced and foreign universities were teaching that the Vredefort Dome had been created by an impact event. But not in South Africa. Here, the die-hards (led by prominent geologist Louis Nicolaysen) maintained their faith in a cryptovolcanic origin for the dome.

At last, a conference was called to resolve the conflict. It was held in Parys in 1987 and experts from both camps addressed the meeting. The assembly failed to reach a final conclusion, however. Tellingly, a vote was taken at the end of the meeting and a slight majority of the participants indicated their support of an exogenic (external impact) origin.

On an Atomic Level

To cut a long story short, the next decade was filled with a flurry of reports from researchers in South Africa and around the world. The dome was probed and poked by geologists and, slowly, the evidence mounted. Coesite and Shishovite were found, micro-deformations in quartz crystals were identified and the precise nature of the metamorphism in the rocks was studied.

Finally, in 1997, Hughes Leroux managed to analyse some rock samples on an atomic level. His findings proved conclusively that the Vredefort Dome had suffered a colossal impact event which had deformed the rocks on a fundamental level. The sceptics had finally been defeated.

By David Fleminger