Monday, February 02, 2015

Wolf Creek Crater W.A.

Unknown wonders: Wolfe Creek Crater

By Kenneth McNamara, University of Cambridge

Australia is famous for its natural beauty: the Great Barrier Reef, Uluru, Kakadu, the Kimberley. But what about the places almost no one goes? We asked ecologists, biologists and wildlife researchers to nominate five of Australia’s unknown wonders.

It is a testament to the size and isolation of many parts of Australia that it wasn’t until 1947 that the second largest meteorite crater in the world was discovered. Known as Wolfe Creek Crater, this imposing feature is located about 145km from Halls Creek in the Kimberley region of Western Australia. It can be reached after a two to three-hour drive down the Tanami Road, only accessible to conventional vehicles during the dry season.
Its discovery came during an aerial survey of this part of the Kimberley region, when geologists Frank Reeves and NB Sauve, along with pilot Dudley Hart, spotted an unusual circular structure almost a kilometre in diameter. Naturally intrigued by what they saw, they were keen to inspect it a little closer.

While known by Indigenous Australians for thousands of years, Wolfe Creek Crater wasn’t officially ‘discovered’ until 1947. Dainis Dravins, Lund Observatory, Sweden.
Two months later, Reeves and Hart reached the site on foot and made the first detailed investigation. Their suspicion that it was a deep crater was confirmed after they climbed up the outer sloping flanks of the structure and looked down to the floor, some 30 metres below. As they made their way up the slope of the crater rim they would have seen rusty balls of rock scattered on the ground or fused to the laterite.

Known as “shale balls”, these rusty rocks provide the evidence that the structure was a huge meteorite crater. These rust balls represent the deeply weathered remains of an iron meteorite that exploded when it collided with Earth about 300,000 years ago – clear and stark evidence of what made the crater.

The hole that it gouged out of the Devonian age quartzite rocks varies in diameter from 950 to 870 metres. The only bigger crater undoubtedly made by a meteorite impact is the Meteor Crater in Arizona.

Travelling at cosmic velocity, about 15km per second (that’s 40 times faster than a bullet from a high-powered rifle, or like crossing Australia in less than five minutes), the massive chunk of iron would have exploded on impact with the earth. Most of the meteorite, which was probably getting on for 100,000 tonnes in weight, would have been vapourised, along with huge quantities of the quartzite rock into which it ploughed.

Very little of the meteorite remains today, but sufficient has been collected for us to be sure that this was what made the crater. Like all iron meteorites, this one contained small quantities of nickel. In the weathered shale balls this nickel has been incorporated into what turned out to be a nickel-iron carbonate mineral not found anywhere else and which was new to science. This was named “reevesite” after Frank Reeves.

The ‘eye’ of Wolfe Creek Crater. Dainis Dravins, Lund Observatory, Sweden.

Although only “discovered” in 1947, the structure had long been known to the local Indigenous people, probably for thousands of years. The local Djaru people call the crater Kandimalal. In their dreamtime stories two rainbow snakes crossed the desert and in doing so formed the nearby Sturt Creek and Wolfe Creek. The crater is the place where one of the snakes emerged from the ground.
Huge quantities of sand have blown into the crater since it was formed, so it would have originally been far deeper. Its base is essentially flat, except for a slight rise in the centre. This is a feature of many meteorite craters and represents where the earth rebounded following the explosion.

There are a number of sink holes in the centre and unusually large trees grow here. These are mainly species of Acacia and Eucalyptus, some growing up to 8m high. It is likely that the trees draw on the summer water that is trapped in these sink holes. The relatively large number of dead trees interspersed with healthy ones attests to periods of lower rainfall. The sink holes are arranged on two intersecting lines and probably reflect the location of stress fractures formed by the explosion at the base of the crater.

As well as higher moisture levels, this central vegetation patch has higher soil salinity and nitrate content. One of the consequences of the higher soil moisture content has been the production of a circular, darker patch within which more vegetation grows. So when viewed from above, the crater looks remarkably like a huge eye peering up at the sky.

The vast expanse of Wolfe Creek Crater. Flickr/Neeravbhatt
Wolfe Creek Crater’s future looks pretty secure. It has legislative protection in the form of Class A Reserve status in a National Park. Its isolation also affords it an added protection.
Fortunately the chances of the crater sitting on a huge resource of iron ore is remote. Virtually all would have been pulverised when this traveller from the asteroid belt made its violent contact with the Earth at a time when the only terrestrial inhabitants to have viewed the spectacle would have been a few bemused giant kangaroos and diprotodontids.

Next: Christmas Island. Read all the unknown wonders here.

Map of Wolfe Creek Crater Google Maps
This article was originally published on The Conversation. Read the original article.

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