Anatomy of an apocalypse
Roughly 2.023 billion years ago, during the Palaeoproterozoic Era, an asteroid between 10 and 15 kilometers in diameter struck the Earth. Traveling at a velocity of up to 25 kilometers per second, the impact released an amount of energy equivalent to billions of nuclear bombs. The collision vaporized the asteroid and tens of cubic kilometers of the Earth's crust, creating a transient crater initially 100 kilometers wide and 40 kilometers deep.
Almost immediately, the crater floor rebounded and the walls collapsed inward, forming the final, multi-ringed structure estimated to be up to 300 kilometers in diameter. This makes Vredefort the largest and oldest confirmed impact structure on the planet. The force of the impact was so extreme that it overturned a 25-kilometer-thick section of the crust, bringing ancient rocks from deep within the Earth to the surface. Today, after two billion years of erosion, the original crater is gone. What remains is the deeply eroded central uplift of the structure, known as the Vredefort Dome, which is about 90 kilometers across.
Geology forged in catastrophe
The evidence for this colossal impact is written directly into the rocks. The immense pressure and heat of the event created unique geological markers that are definitive proof of a hypervelocity impact. One examples is the presence of shatter cones—conical fractures in rocks with distinctive, horse-tail-like striations that point towards the center of the impact. These features are found in various rock types throughout the area.
Another important piece of evidence is pseudotachylyte, a dark, glassy rock that formed when the intense friction from the impact melted the surrounding granite. These veins of "frictionite" are found throughout the Vredefort Dome. On a microscopic level, minerals like quartz show planar deformation features, which are parallel lines of shock-induced damage that can only be formed by the extreme pressures of an impact event. Geologists also find high-pressure forms of silica, like coesite and stishovite, which are further proof of the cataclysmic pressures involved. In 2005, UNESCO recognized the area's unique geology by designating the Vredefort Dome a World Heritage Site.