Australasian Science: Australia's authority on science since 1938

The Truth About Impact Craters

Wolf Creek crater

Wolf Creek crater

By Fred Jourdan

The Earth is scarred from meteor impacts, but how old are they and do these ages match the dates of mass extinction events?

Asteroid and comet impacts have been central to the formation and evolution of our solar system, and represent one of the most important elements shaping planetary surfaces. Throughout their history, the solid planets of the inner part of the solar system have been intensely bombarded by meteors. The number of impacts has drastically decreased since the solar system formed 4.6 billion years ago, but impacts have nevertheless played a crucial role throughout its history.

On Earth, things are a bit more complicated as geological processes have erased all traces of many impact events, and only a relatively small number remain observable today. At present, 176 confirmed impact craters are recognised worldwide, with 26 of them found in Australia.

Perhaps more than anywhere else in the solar system, meteor impacts have played a critical role during the Earth’s history. Impacts have been responsible for delivering the constituent elements of our planet, the formation of major ore and petroleum deposits, and have affected the evolution of life on Earth.

The study of impacts and their effects is currently a very active field at the crossroads of many scientific disciplines, including mineralogy, environmental sciences, geophysics and planetary science. At Curtin University I am working in collaboration with a team of colleagues from Europe and the USA on terrestrial impact craters. Each of us specialise in different disciplines, and I am particularly interested in the age of impact craters and their significance to the history of our planet.

What Is An Impact Crater?
Compared with most geological events that take place over millions of years, impacts are sudden and catastrophic events. In most cases, the incoming object is relatively small, consisting of an asteroid or a comet, while the larger object will be something like a very large asteroid, a satellite such as the Moon, or a planet.

The round scar left after an impact event is what we call an impact crater, and this is what contains all the clues needed for geologists to determine what happened.

During an impact event, the surface impacted by the meteoroid is called the target rock, and this must withstand extreme conditions for a few seconds. The energy carried by the projectile is converted to heat upon impact, and the target rock can reach temperatures of a few thousand degrees, causing melting of parts of the target rock.

Once it has resolidified, this rock is called melt rock, and it is important for measuring the age of the impact. Deeper sections of the target rocks are pulverised while other parts are hurled out of the crater in every direction, forming an ejecta blanket of angular rocks and droplets of melt rocks surrounding the crater.

In most cases the meteoroid is entirely vapourised and mixed with the molten target rock. All this spectacular action is over in a few seconds, and the crater cools quickly.

Impact craters on Earth vary in size from a few metres to several hundred kilometres in diameter, depending on the initial size of the projectile. Many craters are visible from the air and you can see them by using web applications such as Google Earth.

What Is The Age Of Your Crater?
Knowing the age of an impact is scientifically important, as this will help scientists to relate these events with other specific events. For example, having precise and accurate ages for impact craters is crucial when we try to understand whether a particular impact event has contributed to a mass extinction.

When the ages for a large number of craters are known precisely it allows us to estimate the flux of asteroids that have struck Earth during its history. Such information permits us to estimate the probability that an asteroid or comet of a given size will collide with Earth in the future. Of course, this does not replace direct observations of the sky, but it indicates to us what we may expect in the future.

Knowing the age of a crater is always interesting for the public as it helps people to put things into perspective. For example, the Meteor Crater in Arizona was emplaced approximately 50,000 years ago and is now a world-famous tourist destination.

Impact craters on Earth range in age from the very recent (witnessed by humans) to more than two billion years old for the Vredefort and Dhala craters located in South Africa and India, respectively.

Not So Fast
In a recent study we showed that only a surprisingly small proportion of craters have their age precisely known. Based on rigorous statistical analysis we demonstrated that the accuracy of most of the ages needs improvement, with only the ages of only 13 craters in the world precisely and accurately known. Sadly, none of these craters are located within Australia, despite several attempts to date them.

This does not mean that we don’t have know the approximate age of some of the remaining 163 craters, but the precision on their ages is not sufficient to be relevant for in-depth scientific study. For example, the best estimation of the age of the Strangways Crater in the Northern Territory is currently 646 ± 42 million years old. This means that the age for the event can be anywhere between 604 and 688 million years. This is not a very practical result when high precision is required!

We have also clearly demonstrated that many of the ages currently attributed to terrestrial craters require revision, such as the Shoemaker Crater in Western Australia, which is currently given at 1630 million years old based on old data.

Such a lack of good quality age data prevents geologists from exploring whether individual or several impact events occurred at the same time as relatively well-dated events such as mass extinctions. In fact we have discovered that only on one occasion did a large impact happen at the same time as a mass extinction event. This is the infamous extinction of the dinosaurs associated with the 170-km wide Chixculub Crater and the 24-km wide Ukrainian Boltysh Crater, both of which occurred about 66 million years ago. However, this case is further complicated by the simultaneous occurrence of large volcanic eruptions, which played a significant role as well (AS, April 2010, p.14–17).

While no other cases of impact-related extinction events have been demonstrated, absence of evidence is not evidence of absence. Further work may reveal other cases of exact synchronicity between mass extinctions and large impacts.

Working On Improving Things
At the John de Laeter Centre at Curtin University we are actively dating impact craters from around the world. How do we determine the age of a crater?

First, we need to recover samples of the melt rock created during the impact. This melt rock is the crucial key, as it is the only part of the target rock that has been heated enough during the impact to reset the natural radioactive clock of its constituents. Measuring the age given by this radioactive clock is called isotopic geochronology, and this is how we know the age of the solar system and when our first ancestors were starting to roam Africa.

Isotopic geochronology works on the principle that radioactive elements decay to other elements at a known rate. For example, half of the potassium-40 decays into argon-40 in 1.3 billion years. As the clock in the melt rock is reset during the impact, it will start a new sequence of decay and, if not perturbed by geological processes, will give the age of the impact when measured in the laboratory today.

At the Western Australian Argon Isotope Facility, we use the argon–argon dating technique as this is particularly well-suited to the study of impact events. We use an infrared laser to melt impact rocks and a noble gas mass spectrometer to measure the argon gas extracted from the samples. As impact melt rocks are more difficult to date than conventional rocks, we generally carry out measurements on a number of samples from the same crater to be sure of the final age.

We are actively measuring the age of terrestrial impact craters that have important scientific value. For example, we have dated the Keurusselkä impact crater in Finland at 1152 ± 4 million years old, making it one of the oldest and most precisely dated impact structures known in Europe.

We also recently worked on the 50-km diameter impact crater at Siljian in Sweden, which was considered responsible for the third largest mass extinction on Earth. The results we obtained showed that the crater was 382 millions years old and demonstrably unrelated to the Frasnian–Famennien mass extinction dated at 376 million years.

Some of our latest work focuses on the Lonar Crater in India. This little crater is important for geologists because it is one of the rare impact events on Earth that has a target rock type similar to those found on Mars and the Moon. It therefore offers a natural planetary laboratory. This impact was thought to be about 50,000 years old, but precise argon–argon dating now shows that the impact occurred half a million years ago!

The list of our projects goes on, but sometimes argon–argon dating simply does not work and we are left searching for better samples or trying other dating techniques.

For example, we tentatively dated the Dhala Crater in India. Field geology makes this impact a solid candidate as the oldest impact scar preserved on Earth, so we were eager to test this hypothesis with colleagues from India and Canberra. We used argon–argon and uranium–lead dating techniques, but unfortunately both approaches failed and we still cannot say whether this impact is actually the oldest on Earth.

Much More Work To Be Done On Australian Craters
In Australia there have so far been 26 confirmed impact structures identified. Australia has one of the best-preserved records of impact events in the world, a consequence of the relatively undisturbed nature of Australian terrains.

However, these craters remain poorly studied, and none of them have been precisely dated. Apparent “ages” are generally attributed to all of these craters but the reported information might be misleading depending on the level of precision required.

An exception perhaps is the Australasian tektite fields. Tektites consists of glass droplets of impact melt rock, and tektite fields cover at least 10% of the Earth’s surface.

Australasian tektites have been dated using the argon–argon technique at about 800,000 years old. However, many researchers think that this impact occurred somewhere in Indochina rather than Australia. In any case, in my laboratory we are trying to address the flagrant lack of good quality ages available for Australian craters.

We are currently investigating the huge 90-km diameter Acraman Crater, which is located in South Australia. Through the use of field relationships its age is currently given at about 580 million years old, but it has never been successfully determined using isotopic techniques.

One of our future studies will focus on the Wolfe Creek Crater in the northern part of Western Australia. This crater is almost as impressive as the Meteor Crater in Arizona, and is one of Australia’s favourite tourist destinations, particularly since the crater became famous thanks to the movie of the same name. Uncertain age estimates suggest that the Wolfe Creek impact occurred roughly 300,000 years ago, but this will need to be further investigated using argon–argon dating.

There are other very interesting structures in Australia where fresh, analysable melt rocks have been recovered. These include the 20 km long elliptical Amelia Creek Crater, the 24 km diameter Gosses Bluff Crater (which is certainly older than the previous age estimate of 142 million years), and the large 60-km diameter Tookoonooka impact in Queensland.

Impact craters as craters are a fascinating part of Australia’s heritage. At Curtin University we are working on understanding their history better.

Fred Jourdan is a Senior Research Fellow at Curtin University.