Heavenly bodies stir up routine catastrophes
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.
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).
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. Although a great deal is now known about crater formation, and the Vredefort site is increasingly being studied and modelled, much remains speculative.
The impact took place a very long time ago when the Earth was just over half its present age, and yet it has primary modern significance for mining. South Africa's vast semicircle of goldfields follows the outline of the Vredefort ring. Coincidentally, the impact area largely covered the earlier Witwatersrand basin where gold-bearing strata had been laid down by archaic rivers.
Experts differ about exactly how the gold came to be concentrated so highly in this area, but the energy released by the impact certainly had something to do with it. The strata appear to have been thrown on edge, and erosive processes finally exposed them as the well-known Reef at the highest point in Johannesburg. The world's deepest mines are to be found near Carletonville, where Anglogold is probing to a vertical depth of 3,5km in search of the deep veins of ore that were buried so long ago.
Few South Africans are aware of this geological wonder in the very heart of their country. Yet it has always fascinated those in the know - the geologists, minerologists and astrophysicists who pay attention to large rocks falling from the sky, and who warn that it could happen again. Here they share something with local mystics who believe the Dome is a sign from heaven portending the worst.
According to prophecy, fire will once again rain from the sky (or erupt from the bowels of the Earth), and we are all doomed to burn to extinction. This might sound alarming or merely funny depending on your view of fate. But it could indeed happen again. Scans of our immediate environs show that dangerous Near Earth Objects (NEOs) quite frequently stray into Earth's neighbourhood. Some have narrowly missed in recent years, and more are predicted.
Calculations based on the observed number of asteroids suggest that we should expect about three craters of at least 10km in diameter be formed on the Earth every million years. Very big impactors are rare, but if one the size of Vredefort should hit us, it would probably spell the end of life as we know it.
Colossal fires and tidal waves would sweep away landmarks, killing millions if not billions immediately. Ejecta and dust thrown from the impact zone would do the long-term damage, darkening the skies and chilling the seas for centuries, putting an end to agriculture and possibly disrupting the atmospheric processes from which we draw our air.
This is what seems to have caused the mass extinction of the dinosaurs. About 65-million years ago, an asteroid ploughed into what is now the Yucatan peninsula of Mexico, setting in motion the chain reactions that killed off Earth's dominant species in a few short years. Known as the Chicxulub crater, this is regarded as the world's third-largest.
The second largest is at Sudbury in Ontario, Canada - thought to have been caused by a large comet. According to one study, Sudbury produced about 31 000 cubic kilometres of impact melt, approximately six times the volume of lakes Huron and Ontario combined, and nearly 70 percent more than the melt at Chicxulub.
Defending Earth against these roving destructors is becoming a political issue in the leading Western nations. Public funds are committed to searching for NEOs and designing possible weapons to deflect them - although it is doubtful at this stage whether the largest and fastest could be stopped in time.
At any rate, worldwide interest in impactors (otherwise known as bolides) has focused attention on the Vredefort Dome and is starting to bring international tourists to the town of Parys, on the banks of the Vaal about 120km from Johannesburg. This type of feature is known as an astrobleme - a wonderfully evocative word for an eroded impact crater.
As with many old craters, appearances are misleading. The centre of the Vredefort Dome (also known as the Ring) looks merely like a small- to medium-sized crater.
Many people who visit the area think that a horseshoe of low mountains called the Bergland, lying to the north west of the Vaal River between Parys and Potchefstroom, is the crater rim. It isn't: the mountains are merely what remain of a central upheaval dome that formed at the core of the crater.
The truth is far more impressive, though harder to see from the ground. What is most likely the outermost concentric ring of the crater itself runs from the Mondeor heights south of Johannesburg along the ridges of the West Rand to Welkom in the Free State - a diameter in excess of 300km. An inner ring is also visible: it's called the Gatsrand, and you cross it when driving on the N1 through the Grasmere tollgate.
In mistaking the Bergland for the crater rim, early estimates put the size of the Vredefort impact zone at less than 100km. This merited inclusion in the top league of big craters, but as time passed and geologists explored the features of this unique area, the truth began to dawn: this was a crater of earth-shattering proportions, and possibly the largest on record.
Recent Landsat pictures put the minimum diameter at 250km, although there are still scientists who say the crater is no more than 107km wide.
There has never been any shortage of hot debating topics in the realm of meteorites and comets. In the past, the Vredefort structure was thought to have been an ancient volcano. There was even an oddball theory that the Moon had pulled away from Mother Earth at this point, sucking up molten rock from below the crust, but the age of the Moon (4,5-billion years) and its size quickly ruled out this possibility.
Vredefort is newer and smaller than any feature that might have been left by the departing Moon. In fact, evidence in the rocks points to an impact by a space invader. With no meteoritic fragments present after such a long passage of time, other evidence is invoked to prove impacting. Shock metamorphism - or changes in the rock due to high pressures not found in volcanic eruptions - is a sure sign.
Imagine a stone hitting a window: the glass shatters in myriads of splinters. In the case of impacted rocks, these shatter cones appear along with melt rocks and other signatures in zircons, quartz and feldspar grains. In the Vredefort area, so-called pseudotachylites - melted black seams in the rock having the false appearance of volcanism - are a sure giveaway.
The crater was blasted out of the Earth by a wandering asteroid that detonated in the atmosphere with the explosive force of millions of nuclear bombs and melted the crust instantly to a depth of up to 30km.
The low dome of granite in the middle was once viewed as the plug of cooled molten matter that had welled up from the magma surrounding Earth's core, and the rings were described as successive volcano rims.
Until about three decades ago, almost every large impact site on Earth was thought to be volcanic rather than extraterrestrial in origin. Then along came American geologist Eugene Shoemaker and his wife Carolyn, who pains-takingly documented crater after crater, from America to Australia and even on the Moon. They were convinced that many terrestrial and lunar craters were due to asteroidal impacts rather than volcanoes, but were sidelined by research authorities and scientist peers.
The big breakthrough came when Shoemaker and a colleague, David Levy, correctly predicted that a newly discovered comet, Shoemaker-Levy 9, would impact Jupiter in June 1994 - and impact it did, in 21 spectacular fragments. Clearly, major impacts could and did occur throughout the solar system.
A meteorite is any rock that remains after an impact. The impactor may vaporise, however, and leave no meteorite. Large asteroids from the belt of Asteroids lying between Mars and Jupiter are likely to vaporise when colliding with Earth, though the smaller ones would leave a meteorite. Meteoroid is the word to describe any solid body migrating through space on a collision course with other bodies. Some of these take the form of smaller swarms of bits of rock crossing the solar system.
The Vredefort impact was certainly not the biggest in Earth's history of punishment by heavenly bodies, either. Between three and four- billion years ago, Earth weathered a period of heavy bombardment by passing rock fragments, large and small. Then or perhaps later, comet storms also struck the earth; comets - which are lumps of ice and dust - have been credited with delivering liquid water in such abundance that the seas were formed.
Comets, too, have generated their share of scientific dispute. British astronomer Sir Fred Hoyle suggested that life on Earth may have been seeded by comets that transported the essential molecular materials from outer space.
Dubbed panspermism, this idea - supported by the discoverer of DNA, Francis Crick - is still hotly debated.
The status of Vredefort as the oldest and biggest known impact site on Earth has recently been challenged by Dr Gary Byerly of Louisiana State University in the US. He and his team found evidence of an even bigger blast in the Barberton area - crossing over to the Pilbara Block in Western Australia (the two land masses were once joined).
Byerly says five separate rock layers containing debris ejected from an ancient impact have been discovered and dated at 3,47-billion years old. They didn't manage to find a crater - preserving, for now at least, Vredefort's claim to fame.
However, none of the asteroidal or cometary impacts compares with the biggest ever, which occurred right near the beginning of Earth's existence, some 4,5 billion years ago. It is now thought that the Moon was formed when a huge lump of coalescing matter - large enough to be another planet in the process of formation - collided with the proto-Earth. From the loose agglomeration of the two bodies, some lighter crustal matter spun off to create the Moon.
The story of Vredefort is just one chapter in the epic of planetary formation in our solar system, and many others.
As gravity pulls drifting dust together to create stars and planets, the leftovers circulate as clouds of comets, belts of asteroids and random rock fragments. As these bodies continue to fall inwards towards us, they result in grand and terrifying catastrophes. It will happen again - but we hope not soon.
This article originally appears in the March issue of the South African edition of Popular Mechanics