Catastrophic Geology: Case 14 - Vredefort Impact Crater

In the area southwest of Johannesburg, South Africa, is evidence of the largest impact crater on the planet.  Although the origin of the Vredefort structure has been uncertain for many years, until the 1990's when consensus was reached.  The impact feature is only visible from geology mapping information and satellite imagery.  The crater has a diameter of 50 miles !  An outer ring is 200 miles in diameter !  A prevailing model is that a 10 km diameter asteroid within the overall asteroid shower of the great catastrophe, punched into the earth, and rebounding rocks thrust up and out forming the inner ring composed of hills of metamorphosed rock (gold bearing quartz and conglomerate rock).  As reported by Tas Walker (geologist):

"The area inside the ring is granitic rock which forms the basement of the area.  Before the granite rebounded, it sat more than 20 km beneath the surface.  The force of the impact deeply fractured the granite, and partly melted it.  Some magma was rich in iron and magnesium and filled the cracks, enveloping chunks (clasts) of broken granite, and solidifying into a black, glassy rock called pseudotachylite (soo-do-tacky-lite).  Because it has been found at a number of impact sites around the world it has been taken as evidence for the asteroid [shower] impact.  Leeuwkop Quarry near Parys shows magnificient exposures of pseudotachylite enveloping large, rounded chunks of granite."

"Other magma was rich in silica and aluminum.  This flowed into other cracks forming a light colored rock called granophyre, which is exposed as a number of long dykes in the area.  These dykes and the pattern they form are also evidence for the impact."

Widespread and deep geologic changes occurred within minutes.  Not "millions" of years.  The asteroid penetrated sedimentary deposits (water borne deposits) and the underlying bedrock.  Subsequent rapid sedimentation (the Karro Supergroup geologic formation) occurred after the impact.  Tremendous erosion then occurred as the water level receded, forming the Great African planation surface and exposing the Vredefort dome.  The patterns of drainage erosion around the dome through the tilted ring of mountains are evidence of receding water, not slow erosion along drainage valleys.