Drilling for Gold Uncovers Unexpected Treasure

Scattered around the Western Australian outback is evidence of the bombardment of Earth by extra-terrestrial bodies, eleven confirmed impact craters in all from the infamous Wolfe Creek in the Kimberley to the ancient Yarrabubba in the Mid-West. Now evidence of another meteorite crater has been found near Ora Banda in the Goldfields, less than 100-km northwest of Kalgoorlie, on land owned by Australia’s third largest gold-mining company, Evolution Mining.

Meteorites often begin life in the asteroid belt between Mars and Jupiter, remnants of asteroids that have shattered in collisions with other similar objects. Complex gravitational interactions can send them hurtling towards the Earth at speeds of up 70 km/s, impacting the ground with tremendous force and carving out huge craters in the process.

The Ora Banda crater was discovered by experts who used electromagnetic surveys to map the rocks beneath the ground and it appears to be about 5-km in diameter, more than five times the diameter of the Wolfe Creek crater. Given the level of erosion and the soil filling the sides, Dr Jayson Meyers, a Perth-based geologist, says that the asteroid could be around 100-million years old.

“In geology, you tell someone you found a meteorite crater and they immediately roll their eyes and are very sceptical because they're so rare,” he said. “We were convinced and pretty excited from a technical standpoint – It doesn't have a lot to do with the gold, it's actually smashed the gold around a bit – but we can say hand over heart that an asteroid hit this spot.”

Dr Meyers was brought in as a geological consultant and saw the tell-tale signs of a meteorite strike when he inspected rock samples. Rocks subjected to significant shock pressures have fractures that are conical in shape and are known as shatter cones. The cones can be up to metres in size, and the apex of the cones generally points to the shock source. Under a microscope they can look like shattered glass.

The passage of high-pressure shockwaves through the rock modifies it in other ways too, and because it is buried the definitive classification as an impact crater can only come after verification by drilling and sampling the material directly. As you would expect, the process of verifying and dating new craters is strict, and there are less than 200 confirmed impact structures worldwide, 27 of which are in Australia.

And Western Australia, with its ancient crust, and tectonically stable and expansive desert landscape, is prime crater hunting territory for scientists. Jonti Horner, Professor of Astrophysics at the University of Southern Queensland, explains.

“Western Australia has some of the oldest crust on Earth, so has had a really long time to build up an impact record. But because of weathering, only the largest craters, or the youngest, are easy to find. Meteor Crater in Arizona is a great example – it is pristine because it is too young to have been weathered away yet (which is helped by it being in the middle of the desert).”

Professor Horner says that the meteorite that created the Ora Banda crater could have been up to 300-m across. “A rough rule of thumb is that an impactor will create a crater about 19 times bigger than its diameter.” Such large meteorites are thankfully rare, probably striking the Earth no more often than once every few hundred thousand years.

The reason that impact craters are much bigger than the size of impactors is because a lot of energy is released during the impact. Professor Graziella Caprarelli, an expert in Earth and Planetary Science, did a back-of-the-envelope calculation to demonstrate the destructive effect that large meteorites can have.

“Assuming a very conservative hypervelocity impact of 11 km/sec, the energy of an impactor such as the one described could be tens to a hundred megatons. This is equivalent to the energy of a large to major earthquake which can cause major damage and destruction of infrastructure and consequent loss of life, and can be felt at distances hundreds of kilometres from the region of impact.”

Other than the velocity of the meteorite, its composition, and the ground at the point of impact, can also have a significant effect on the size of the crater, as does the angle of its entry into the atmosphere. “The physics of impacts of terrestrial planetary bodies is very complex and involves a lot of modelling,” says Professor Caprarelli.

Larger meteorites are even more terrifying, although far less frequent. According to Professor Horner, 1-km diameter objects are only thought to hit us every half a million years or thereabouts, but would be bad news for a state, or even a country. “Some studies a number of years back suggested a 1-km asteroid hitting Earth would kill 1/4 of our population.”

So, should we be worried? Well, no. At least, not during our lifetimes.

NASA’s Jet Propulsion Laboratory maintains a database of near-Earth objects (NEOs), computing high-precision orbits and monitoring the risk of impacts over the next 100 years. And their NEO observations program, running since 1998, has been successful in finding more than 90% of near-Earth asteroids larger than 1-km, and a good fraction of the NEOs larger than 140-m.

And there is nothing in the database that would suggest that we are in imminent danger. We can all continue to enjoy the annual roster of meteor showers that light up our skies, without having a fear of impending calamity.

Of course, it is possible that an unseen hunk of rock could be enroute to Earth right now, too small to be noticed and yet big enough to cause widespread devastation. The Tunguska event, where a fragment of comet or asteroid around 100-m in size exploded above unpopulated parts of Siberia and levelled trees across an area the size of London, is a reminder that these events are unpredictable and do happen.

Given enough warning though, Professor Horner thinks that we have the technology to divert asteroids of this size. “That's why people are scouring the skies looking for things that might be threatening. The more warning that we get, the better the odds we'll avert disaster.”

No doubt many more impact events in our history have left a mark on the Earth and are awaiting discovery by scientists. Not far from the Ora Banda crater is another meteorite crater candidate, just outside the town of Coolgardie. Around 200-m below the surface, it appears to have a diameter of about 800-m and could be around 800,000 years old.

Modern technology is making the discovery of these sites easier, and modern science is using them to build a more complete picture of the history of our planet and its place in the solar system.