A scar two billion years old
Approximately 2.023 billion years ago, an asteroid estimated to be 10 to 15 kilometers in diameter struck this location. The impact created the Vredefort Crater, which had an initial estimated diameter of up to 300 kilometers, making it the largest known impact structure on Earth. The original crater has long since eroded away, but what remains is the deep root of the impact structure, known as the Vredefort Dome. This dome, about 90 kilometers across, is the central uplift that formed when the Earth's crust rebounded after the collision.
The force of the impact was immense, releasing more energy than trillions of nuclear bombs. It blasted away kilometers of rock and soil, and the rebound that followed pushed up rock layers that were once 25 kilometers below the surface. This deep cross-section of the Earth's crust is now exposed for study. The southeastern portion of the structure is not visible, as it is covered by younger sediments from the Karoo Supergroup. The visible portion forms a distinctive semicircular pattern of ridges easily identifiable from satellite imagery.
Reading the rocks
The Vredefort Monocline is one of the most striking geological features of the impact. A monocline is a step-like fold in rock layers. Here, the unimaginable force of the impact took horizontal sedimentary layers from the Witwatersrand Supergroup and tilted them, in some places completely overturning them to a vertical or near-vertical orientation. These prominent ridges of hard, erosion-resistant quartzite now form an arc of hills around the crater's center.
Geologists have found definitive evidence of the impact throughout the dome. One indicator is the presence of shatter cones, which are conical fracture patterns in rocks created by the high-pressure shockwave of an impact. Another is pseudotachylyte, a dark, glassy rock that formed when the intense friction from the impact melted the country rock. These veins of melted rock are found throughout the area, sometimes containing fragments of the surrounding granite. The presence of these features, along with microscopic shock-deformed minerals like quartz, confirmed the structure's impact origin after it was initially thought to be volcanic.