Impact Studies

If you could hear what the rocks are telling you, you’d be blown away.  Nothing has a more impressive story to tell than the boulders seen here embedded in a black glassy matrix in the soaring cliff at Leeukop. The vertical quarry face is the best exposure in the world of friction melt-rock produced by a meteorite impact. It’s specially designated within the Vredefort Dome World Heritage Site – South Africa’s Mount Rushmore, if you will, though instead of the busts of US Presidents we see how a flaming rock blasted the landscape into dramatic forms.

Pseudotachylite

Formations like this are known as pseudotachylite breccias – pseudo (false) tachylite (volcanic glass) breccias (rock fragments cemented together). The phenomenon was named in 1916 by the British geologist James Shand. He wrote a description rather quaintly and mysteriously entitled “The Pseudotachylyte of Parijs (Orange Free State), and its Relation to ‘Trap-Shotten Gneiss’ and ‘Flinty Crush-Rock’”.

Not to get too technical, the term Trap-Shotten refers to a rock injected with some other rock in molten form. Shand, however, could not accurately explain how such crazy patterns were produced at Vredefort. He certainly did not imagine that a rock from space had plunged to Earth. In his time geologists did not believe that our planet had impact craters.

Astrogeologist

The father of Impact Studies was the American geologist Eugene Shoemaker (1928-97).  An astrogeologist, Eugene Shoemaker was the first person to be buried on the Moon, soon after his death in 1998. As a tribute to his role in launching impact studies, the NASA space probe Lunar Prospector carried his ashes and crashed into the surface of the moon.

Shoemaker, together with his wife Caroline, identified dozens of impact craters on various continents. He made it his life’s quest to show Earth scientists that our planet has been the target of many impacts. Before him, geologists were doubtful and in fact most rejected the impact theory as improbable.

Near Earth Objects

Why study impact cratering? Meteorites contain a record of our solar system’s history going back some 4.6 billion years. Our sun is about 5 billion years old  , and the Vredefort crater is dated at 2,023 billion – close to half the age of the Earth. Because it is so old and so eroded, it is seen as a “window” into the deep structure of a crater.  Such large  “astroblemes” (star scars) can lead to major extinctions and alter the course of life on Earth. Estimating the power of these giant impactors and tracing their origins is vital for our own survival.

The most important contribution of the Shoemakers was drawing attention to the impact cratering process throughout the early Solar System. This led to pioneering surveys, by NASA amongst others, seeking to identify dangerous of Near-Earth Objects (NEOs). It took Shoemaker a lifetime to prove that the Earth has been pounded just like other planets (and our Moon) by stray asteroids and comets. Earth did not enjoy a charmed existence.

As a doctoral student in the 1950s Shoemaker identified a crater in Arizona as impact produced.  Employed by the US Geological Survey, he was sent to identify uranium deposits in Colorado and Utah. While studying the volcanic rocks of the southwest he became convinced that Meteor Crater, Arizona, was not volcanic but it had been formed by impact. Shoemaker theorised that craters on the Moon were formed the same way. Only when the Apollo spacecraft returned from the Moon with rock samples was this question finally answered in the affirmative.

1994 Comet

The Shoemakers went on to discover scores of craters and in so doing became the founders of the subfield of planetary science  that studies impacts. In 1994 he finally convinced geologists that impacts could happen anytime, anywhere. Together with Carolyn and David H. Levy he predicted that Comet Shoemaker-Levy would hit Jupiter – and it did so right on time during July of that year.

The comet was torn apart by the gravity of the massive gas planet but even so one of its fragments punched a hole in Jupiter larger than the Earth. The world’s public sat up in alarm at the thought that such a catastrophe could happen here – and indeed had happened several times, causing mass extinctions. We could be next.

Eugene was killed in a vehicle accident in Australia in 1997 but Caroline visited South Africa and lent her support to geologists who believed Vredefort was a true impact crater – and the largest, at that. By then anyway the consensus was that Earth had been extensively cratered. Only about 137 have been counted compared with more than 800 on the Moon, but this is because craters here are hidden by desert sands, oceans, forests and ice. Many disappear as land masses are subducted during plate movement.

Extinctions

Vredefort has not disappeared because the Southern African subcontinent has remained very stable over time. The impact structure was covered by the Karoo supergroup from about 280 million years ago but the Vaal River, born on the Karoo surface, has eroded much of the covering.

The Vredefort crater is the largest and oldest on Earth – at least as an intact though very eroded structure. It is so old (2 023 thousand million years) that it could not have caused a major extinction. The only life on Earth at that time was blue-green algae, cyanobacteria, and they could ride out the blast.