By Graeme Addison
Legend has it that when two people get together and er... bond, the Earth will move – at least in a metaphorical sense. Likewise, it takes two heavenly bodies, an impactor and a target, to come together with Earth-shattering force to form a crater. There’s nothing dreamlike about this: it happens, frequently, throughout the solar system. Impact catastrophes are routine.
Just over two-billion years ago, a chunk of asteroid at least the size of Table Mountain struck the landmass that is now South Africa. It hurtled in at a speed in excess of 55 000km/h, or about 160 times the speed of Shumacher’s Ferrari in full cry.
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Welcome to the realm of cosmic uncertainty and sudden impacts. More specifically, an impact that changed the face of primeval South Africa.
The world-famous Vredefort Dome – centred on a tiny northern Free State dorp and now billed as the oldest and biggest asteroid impact site on the planet – was finally accepted as a blast site by the majority of scientists only in the mid-1990s. It is now being proposed as a World Heritage Site (South Africa’s fifth, after Lake St Lucia, Robben Island, Sterkfontein Caves and the Drakensberg).
Although a great deal is now known about crater formation, much remains speculative
Despite its obvious significance, there is still much speculation about Vredefort. Today leading geologists are disputing whether the impactor set the Earth’s crust on edge or not – whether there is a shear zone as if a mighty fist had punched a hole in the crust, leaving shards of it standing upright. Yes, say the crust-on-edge supporters; but there are others who say there is no real discontinuity in the underlying rock formations.
Why does it matter? Well, knowing how impacts have shaped the Earth’s crust could explain many things that remain puzzling, and perhaps aid in deep-level geological exploration.
Defending Earth against these roving destructors is becoming a political issue
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