Around twenty five impact craters are known in Australia, ranging in size from less than 20m in diameter to perhaps more than 100km (see the World Crater Inventory). Some date back hundreds of millions (even billions) of years. Others are as recent as just a few thousand years old. Most probably did little more than localised damage. At least two Australian impacts may have contributed to mass extinctions.
There are several features geologists look for when identifying a geological structure as that of an ancient impact. Most craters are circular in shape, with an inner ring of raised rock known as an uplift. The central uplift structure forms when the underlying rocks rebound after the impact, throwing up a central mound or ring of rock. Surrounding the uplift will be a depression that is usually several times larger than the raised area. In time the outer ring may weather away, leaving only the central uplifted structure.
Other evidence for a large impact comes in the form of shocked quartz or breccia, or even veins of melted glass. These altered minerals form when sediments are heated and compressed by the impact, often to a greater degree than volcanism or earthquake activity could have produced. Occasionally elements that are rare on earth may occur in abundance in an asteroid or comet. The Chicxulub impact in Mexico, thought to be one of the causes of the extinction event that killed off non-avian dinosaurs, left an elemental calling card that can be found in many places around the world. The element known as iridium is rare here on earth (although does exist in the earths molten core), but can occur in higher concentrations in asteroids or comets. An iridium 'spike' occurs as a thin layer just above Late Cretaceous sediments all over the world, suggesting that a large impact threw up enough dust (containing iridium from the vaporised asteroid or comet) into the atmosphere for it to eventually settle almost everywhere.
Of the two dozen or so impact structures known in Australia, I shall look at four of the largest; the Woodleigh crater, the Bedout High, Gosses Bluff and the Tookoonooka crater. For other craters around the world, see the Global Impact Studies Project Craterbase.
Woodleigh: 364 ± 8 MYA, <120 (60?)km diameter
The Woodleigh crater was found on Woodleigh Station, east of Shark Bay in Western Australia. The crater may be up to 120km in diameter, although some estimates are closer to 40km. The larger estimate suggests a bolide (asteroid or comet) 5km in diameter, which would make this the fourth largest impact structure in the world. However, a study presented at the Geological Society of America's 38th Annual Meeting in 2003 suggests a diameter closer to 60km.
The impact structure is entirely underground. The central uplift structure 20km in diameter was first detected by drilling activities in the late 1970s, however its significance as an impact structure was only realised in 1997 during a gravity survey. In 1999 a core sample was taken. The thin veins of melted glass, breccia, and shocked quartz found would have formed under pressures 100,000 times greater than atmospheric pressure at sea level, or between 10 and 100 times greater than those generated by volcanic or earthquake activity. Only a large impact could have generated such conditions.
The Woodleigh impact, originally thought to be Late Triassic or Late Permian, most likely dates to around 364 MYA, corresponding to a minor extinction event in the Late Devonian Period when around 40% of species disappeared. Evidence for another impact in the Moroccan desert at around the same time suggests that more than one impact was involved in the extinction event.
Glikson et al 2000 Woodleigh - a new 120 km-diameter buried multi-ring impact basin, Carnarvon Basin, Western Australia, of pre-Jurassic post-early Permian age: evidence of a meteoritic component injected into sub-crater basement. Catastrophic Events Conference (PDF file)
Whitehead et al 2003 Planar deformation features in the Woodleigh impact structure, Western Australia, and their bearing on the degree of structural uplift in the structure. Geological Society of America 38th Annual Meeting
| The Bedout structure 300km west of Broome, in the Canning Basin off the coast of Western Australia, has been sited as one of the possible impacts that contributed to one of the greatest extinction events known. At the end of the Permian Period, around 250 MYA, it has been estimated that more than 90% of marine species, and 70% of terrestrial species, may have become extinct. The extinction event seems to have been a sudden, global occurrance, lasting less than a million years (which in geological terms is very rapid). |
The Bedout structure (pronounced "Bedoo") has a central uplift of around 40km in diamater, with a transient crater size of around 100km diameter. The original crater probably measured around 200km in diameter in total, comparable to the Chicxulub crater in Mexico which may have contributed to the extinction of (non-avian) dinosaurs at the end of the Cretaceous Period.
A team lead by Dr Luann Becker (University of California, Santa Barbara) published a more detailed paper about the structure in 2004. Some of the co-authors suggest that the structure is an impact crater based on the presence of a type of glass called maskelynite, a mineral usually only formed under the intense temperatures and pressures of an extraterrestrial impact. However other scientists have suggested the mineral found may be a similar type of glass created by volcanic activity. The absence of shocked quartz in the core samples may indicate that the Bedout High structure is actually of volcanic origin, although the core samples taken were not deep enough to reach the layers beneath the crater where shocked quartz would be expected to be found. Even at least one of the co-authors (Robert Iasky, of the Geological Survey of Western Australia in Perth) is unsure whether the structure can be definitively catagorised as either a crater or of volcanic origin.
Several other Permian/Triassic (P/Tr) sites from this region of the world have yielded evidence of large impacts. At Graphite Peak in Antarctica, the large size of the shocked quartz grains (>100 micrometres) seem to indicate a large impact occurred close to that site. Shocked quartz grains ranging from 150 micrometers in size have also been found at Fraser Park, adjacent to the site at Wybung Head in the Sydney Basin.
Meteorite fragments from the P/Tr boundary at Graphite Peak range in size from 50-400mm, and the chemical composition of the fragments matches that of similar finds in Meishan, southern China, and perhaps Sasayama, Japan, these also dating to the Permian/Triassic boundary. This may indicate that several objects impacted the earth at around the same time. Indeed, the Araguainha Dome in Brazil is an impact structure 40km wide that also dates to the P/Tr boundary. Alternatively, all of these sites yeilding shocked quartz and meteorific fragments may be evidence of one large impact in Southern Gondwana, which managed to spread ejecta over a significant portion of the ancient world.
Also in 2004 a large crater-like structure almost 2km below the East Antarctic ice sheet was reported. The Wilkes Land crater may be more than five times the size of the Chicxulub crater, and has been tentatively dated to around the P/Tr boundary. As well as possibly being the main cause of the Permian/Triassic extinctions, it has also been suggested that such a large impact could have initiated the tectonic rift that eventually separated Australia from Antarctica, hastening the break-up of the giant super-continent of Gondwana.
Becker et al 2003 Bedout: an end-Permian impact crater offshore northwestern Australia (PDF file - conference abstract)
Becker, L., R.J.Poreda, A.R.Basu, K.O.Pope, T.M.Harrison, C.Nicholson, R.Iasky5 2004 Bedout: A Possible End-Permian Impact Crater Offshore of Northwestern Australia. Science 304(5676):1469-1476 (online Abstract)
Bedout: A Possible End-Permian Impact Crater Offshore Northwestern Australia (HTML - full paper)
Impact At Bedout 'Smoking Gun' Of Giant Collision That Nearly Ended Life On Earth Is Identified Science Daily website, 14th May 2004
Gigantic meteor crater found in Antarctica Science Daily website, 1st June 2006
|Gosses Bluff is located west of Alice Springs and south of the Macdonnell Ranges, in the arid Missionary Plain in the Northern Territory. The bluff itself is a circular ring of hills 5km in diameter and 200m high, in the centre of a crater. It was formed 142 million years ago by the impact of an asteroid or comet up to 2km in diameter. The hills are part of the crater’s central uplift, which formed when the earth's surface recoiled from the impact. A circular drainage system 24km in diameter marks the outer ring of the crater. The bluff is deeply significant to the Western Arrernte Aboriginal people, who own the Tnorala Conservation Reserve that contains the crater. |
This impact occurred at the very end of the Jurassic Period at a time when the largest dinosaurs, the sauropods, declined in number. This impact alone would not have been large enough to cause mass extinctions on a broad scale, but would certainly have caused a lot of local damage. The Morokweng crater in South Africa dates to roughly the same time period, although at 70km in diameter was a much larger impact than that which created Gosses Bluff.
|The Tookoonooka crater in south central Queensland is about 55km in diameter. It was discovered in the early 1980s when petroleum exploration revealed an anomalous circular structure. Tookoonooka has a central uplift dome that is 22km in diameter. |
Impact melt breccias have been found about 25km southeast of the central uplift structure, and have also been found filling the annular structure around the central uplift. The clasts consist of impact breccia fragments, as well as devitrified impact melt glass. Breccia samples have also been found during drilling into the central uplift dome; these breccias have a devitrified glassy matrix, with angular clasts.
Tookoonooka has been dated to around 128 MYA, based on studies of local pollen sequences. At this time a large inland sea covered much of Queensland, suggesting this impact landed somewhere off-shore. No dinosaur remains are known from Queensland from around this time, so the effect of the impact on local species is uncertain. Most Queensland species date to 110 MYA or later, with the sauropod Rhoetosaurus living 40-50 million years before the impact. It may have had more of an effect on marine species, however most Queensland marine reptiles are also only known from 110 MYA or later. Without a good sample of species from immediately before the impact, it is impossible to say whether the Tookoonooka impact caused an extinction event. Extinction event or not, an impact of this size would still have done a lot of damage to the local area.
Here is a brief summary of the possible causes of mass extinction events. Bolide impacts are but one theory to explain why large numbers of species die off in a relatively short time span.
Climate change could be a major factor in mass extinction, especially when the change is too rapid for some species to adapt to. Many species became extinct at the end of the Pleistocene, 10,000 years ago, when the world climate changed abruptly from the last ice age to our current warmer conditions. Ice core studies in Greenland suggest the last ice age may have ended within the span of a single human lifetime. It is possible that a long-lived stone age human may have been born in glacial conditions, and died in a much warmer world.
Over-hunting by humans is another possible cause of extinction, although not on a simultaneous global scale. However no-one has yet to produce widely accepted evidence that human hunting alone has been responsible for wiping out large numbers of species. Habitat loss, whether humanly or climatically induced, seems to be the more likely cause. Modern humans have only been around for about 200,000 years, so extinction events further in the past can not be blamed on them anyway. Over-hunting by other carnivorous species is also an unlikely cause of extinction.
Intense volcanic activity can pour vast amounts of material into the atmosphere, in the form of ash and sulphurous compounds, as well as devastate large areas with magma flows. When Mount Krakatoa exploded in the year 535 AD, the resulting ash cloud blanketed most of the world, blocking out the sun for up to eighteen months in some places. There are many documented accounts from the time: "The sun ... seems to have lost its wonted light, and appears of a bluish colour. We marvel to see no shadows of our bodies at noon, to feel the mighty vigour of the sun's heat wasted into feebleness, and the phenomena which accompany an eclipse prolonged through almost a whole year. We have had ... a summer without heat ... the crops have been chilled by north winds ... the rain is denied ..." (Cassiodorus)
"The sun became dark and its darkness lasted for eighteen months. Each day it shone for about four hours, and still this light was only a feeble shadow ... the fruits did not ripen and the wine tasted like sour grapes." (Michael the Syrian)
However this event was not sufficient to cause global-scale mass extinctions. Multiple eruptions over a longer period however may have blocked out the sun for much longer, and sulphurous compounds released into the atmosphere would have fallen back to earth in the form of acid rain. Greenhouse gases such as carbon dioxide could then have raised global temperatures, further stressing those species that managed to cope with the initially colder conditions.
"The sun ... seems to have lost its wonted light, and appears of a bluish colour. We marvel to see no shadows of our bodies at noon, to feel the mighty vigour of the sun's heat wasted into feebleness, and the phenomena which accompany an eclipse prolonged through almost a whole year. We have had ... a summer without heat ... the crops have been chilled by north winds ... the rain is denied ..." (Cassiodorus)
The Deccan Traps are layers of volcanic basalt up to 2,400 metres thick found in the Deccan region of India. The volcanic activity that caused this outpouring of lava seems to date to around 66 MYA, and continued for at least another million years to the end of the Cretaceous. One feature of Deccan basalt is that it contains high levels of a poisonous element known as selenium. Egg-laying animals are vulnerable to selenium poisoning, as it only requires very small amounts to kill the embryos inside. Unexpectedly high levels of selenium have been found in sauropod eggs from the Latest Cretaceous in France, with the amount of the poisonous element increasing towards the Cretaceous/Tertiary (K/T) boundary. It has been proposed that a high rate of hatching failure may have weakened dinosaurs towards the end of the Cretaceous Period, making them even more suceptible to the large Chicxulub impact 65 MYA.
In the last 600 million years, around 60 extraterrestrial objects of 5km or more in diameter have impacted with the earth. Large bolide impacts seem to be associated with the top five most destructive mass extinction events, and perhaps with many of the minor extinction events as well. It has been suggested that really large impacts may have triggered subsequent volcanic activity, compounding the effects on the world climate. However this theory has not been adequately proven. The impact of a large asteroid or comet could throw vast amounts of dust and pulverised rock into the atmosphere, much like a large volcanic eruption. An ocean impact would also send huge tsunami waves radiating outwards to devastate coastlines. The force of some large impacts has been estimated to have been between 10 and 100 times larger than any volcano or earthquake on earth could produce. Multiple impacts around the world would compound the effects; in fact many smaller impacts may have been more destructive on a global scale than a single large impact.
Perhaps any one of the above events by itself would not be sufficient to cause mass extinctions. Many of the known mass extinction events seem to be associated with several of them. They often occur at times of climatic change and increased volcanic activity, sometimes associated with a large bolide impact as well. A world already stressed by changes in climate or by intense volcanic activity may have been more susceptible to a large asteroid or comet impact.
Around 180 MYA at the end of the Early Jurassic, as many as 80% of deep-sea species disappeared. Massive volcanic eruptions may have warmed the earths climate, causing frozen methane hydrate trapped in the ocean floor to be released in the form of methane gas. Methane in the ocean would have reacted with dissolved oxygen, forming carbon dioxide. The lower oxygen levels may have killed off a lot of deep-sea creatures, with the increased carbon dioxide content of the atmosphere causing a further warming of the earths climate. Especially hard hit were the bivalves: an estimated 80% of the species disappeared. Ostracods, belemnites and some marine plants were also affected. A similar massive release of methane may have contributed to the Late Permian extinctions around 250 MYA. In this case, a large bolide impact may have triggered the release.