Earth is constantly bombarded with debris from outer space. This debris is left over from when our solar system formed. Most of this orbits between Mars and Jupiter as the asteroid belt or as a belt of comets at the edge of the solar system. Occasionally an asteroid or comet is pulled out of its orbit, and crosses the Earth’s path. After the object enters the atmosphere, most are burnt up or explode. The travel time is very short because of the very high speeds of meteors, typically from 10 km a second up to an amazing 70 km a second. The biggest objects survive the journey. They have very high energy and can impact the earth and leave a crater. The largest objects melt on impact and are destroyed in a shower of molten rock. The smaller ones survive the impact and can be recovered as meteorites.
There are only 7 fresh sets of impact craters in Australia. The largest, Wolfe Creek Crater, is situated on the edge of the Great Sandy Desert in northern Western Australia. It is the second largest meteor crater on Earth from which meteorite fragments have been recovered. It was likely formed by a meteor only about 15 m in diameter, but weighing about 14,000 tonnes. The other large crater is Meteor Crater in Arizona. The traditional owners of the crater are the Djaru people who call the site Kandimalal, literally meaning “no potatoes” because they noted the bush potato did not grow there. An unpublished report on an isotopic study of a meteorite found 4 km away from the crater suggested the impact could have occurred around 300,000 years ago.
To determine the age of Wolfe Creek Crater, we used the technique of exposure dating1. We collected four samples from around the crater rim. The impact of the meteorite has tilted and overturned the rock, exposing rock that was previously shielded from cosmic radiation. We also tried to limit the age by dating sand at the base of a sand dune which was formed by diversion of wind around the crater. This technique is based on the last exposure of sand to sunlight. The most likely age for the impact based on the dating is 120,000 years ago. To compare the age to Meteor Crater, we recalculated its age using previously published exposure dating results. It is most likely to be 61,000 years old.
To better study the geomorphology of the crater, we used the technique of photogrammetry. Aerial photos were kindly supplied by Ted Brattstrom, a school teacher from Hawaii. He flew over the crater in a light aircraft in 2007 and snapped pictures of the crater from all directions using his SLR. The resulting 3D model (above) was used to create a digital elevation model of the crater. We calculate that the maximum width of the crater is 946 m in a NE-SW direction, reflecting the direction of the impact. The average diameter is 892 m. We predict a depth of 178 m and that it is filled by about 120 m of sediment, mostly sand blown in from the desert.
With 7 sets of impact craters dating to within the last 120,000 years, we can do some calculations as to how often these crater-producing events occur. Although the rate is only 1 large meteor hitting Australia every 17,000 years, it isn’t that simple. The craters are only found in the arid parts of Australia. Elsewhere, the craters are destroyed by geomorphic activity like river migration or slope processes in the mountains. Since Australia has an excellent preservation record with dated craters within the arid zone, we can extrapolate a rate for the whole Earth. Taking into account that most of the Earth’s surface is water or ice, and that arid Australia is only about 1% of the surface, the rate increases to 1 every 180 years or so. This is a minimum estimate because some smaller impacts were probably covered by sand during the ice age. The number of large objects is probably 20 times this number because stony meteorites are far more common but not as many survive the firey journey through the atmosphere or effectively make craters. Our results give us a better idea of how unlikely one of these destructive events is.
1. Barrows T. T., Magee J., Miller G., and Fifield L. K. (2019) The age of Wolfe Creek meteorite crater (Kandimalal), Western Australia. Meteoritics & Planetary Science, 54(11), 2686–2697. doi: 10.1111/maps.13378