Last issue we discussed the Henbury Meteorite Craters, just off the Stuart Highway south of Alice Springs, but these aren’t the only examples of meteorites striking the Earth within the bounds of Australia. In fact, it seems like news reports of an impending passage of a huge chunk of space rock and/or ice zooming close to Earth could be an almost weekly occurrence; close enough for us to need to be concerned.
This awareness of such close passage of these visitors from space is the result of the work of one US space scientist, Eugene Shoemaker. In 1977 geologist Walter Alvarez, working in northern Italy, discovered an anomalously high level of the element iridium in a narrow band of clay in rocks deposited at the end of the Cretaceous period, 65 million years ago. In fact, they coincided exactly with that time, and with the extinction of the dinosaurs and 70 percent of other species of life. As iridium is a scarce element on the surface of the Earth, Alvarez and his physicist father, Louis proposed that this was the result of a collision of a giant meteorite with the Earth.
Further work by other scientists identified this layer at every other site on Earth where there was a continuous deposit of rocks of this age, implying a truly global event. Since then, further work has uncovered a huge crater, largely underwater off the coast of the Yucatan Peninsula in Mexico, which exactly fits with this timeline.
It is estimated that this meteorite was about 10km in diameter and produced catastrophic global effects, with an energy release roughly equivalent to 100 million megatons. It is believed a tsunami of up to 300 metres in height travelled as much as 100km into continental North America. There was likely to have been such a huge ‘sloshing’ of oceanic waters that there would have been a series of tsunamis as waters rebounded back and forth across the oceans. All trees would have been flattened for as much as 1500km around the impact site, it produced a crater that was 20km in depth and 180km wide, molten chunks of rock were thrown close to 2000km to the north and a huge hole in the atmosphere — punched by the vast meteorite — would have brought the vacuum of space down to the surface, sucking up millions of tons of fine dust and debris which would have been trapped by Earth’s gravity and deposited over the entire Earth. It would have rained molten glowing material for days afterwards, starting huge forest fires across the world while the finer material would have blocked out the sun and stopped photosynthesis for several years. The fires and the shock waves within the atmosphere would have induced both sulphuric and nitric acid rain across the globe, acidifying the oceans and causing a collapse of marine ecosystems. It’s little wonder the dinosaurs quit at that time.
Shoemaker began to wonder if such impacts could happen again and began searching space for signs of potential cosmic visitors, and the more he looked the more he found. It has now become a part of many nations’ astronomical undertakings to plot such material.
There are some 182 impacts craters known on Earth, and new ones are being added all the time. It might seem obvious that they should be easily recognised, but a few are so large that they have only really been identified as the signs of extraterrestrial impacts through global surveys by satellites. The Vredefort Crater in South Africa has a diameter of 160km and is dated from two billion years ago. The Chicxulub Crater in Mexico is 150km in diameter and dates from that dinosaur killing impact of 65 million years ago. And there are three other known craters of more than 100km in width.
Australia is rich in the signs of meteoritic impacts. Our ancient landscape, little impacted by erosion, has preserved the signs of such events, and you can visit most of them if you know where to look.
The most famous of the impact craters in Australia is that at Wolfe Creek (WA, 19º10'S, 127º48'E), about 130km south of Halls Creek. It is regarded as the second most perfect impact crater on Earth and, like the Henbury Craters, was used by US lunar astronauts as part of their training to be able to identify craters. It’s about 900m across and the rim stands about 25m above the surrounding landscape. The crater floor was originally about 150m deep, but it has been partially filled by wind-blown dirt.
Boxhole Crater (NT, 22º 37'S, 135º 12'E) is an almost perfectly circular impact site that is 175m wide and with a rim that’s 10–17m high. Like the Henbury Craters, it is of relatively recent origin – 30,000 years – during Indigenous occupation of the area.
Another easily visited crater is Gosse's Bluff (NT, 23º 50'S, 132º 18'E), which is just off the popular and much travelled Mereenie Loop Road. This was formed about 130 million years ago and was originally as much as 600 metres deep. Its creation would have produced a cloud of debris that went as high as 20km into the atmosphere. The original crater was 22km across but has since been largely obliterated by erosion. What you see today was the smaller inner ring of rebound material in the centre that forms in most large craters when the Earth’s ‘equal and opposite’ response to such an impact causes a central uplift of material. Even so, the structure that you see is still an impressive 5km in diameter and up to 180m in height.
Australia has seven other craters above 10km in width, with three uncertain impact sites of greater than 200km in diameter (East Warburton Basin, South Australia; Bedout, off the Western Australian coast; and the ‘Australian impact structure’, in the Northern Territory at over 600km wide). These are all so indistinct that the determination of their actual origins remains uncertain, but there are plenty of others that are all more obvious and that give you a real taste for the violence of what had once happened right where you are standing. If you see a sign pointing down a dirt track indicating a meteorite crater, take the trip, spend a night and look up at the stars that once came down to touch the Earth at that very spot where you’re about to lay your head.