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Apocalypse followed impact of giant asteroid -

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Robyn Williams: Now, what was the biggest rock to hit the Earth? Well, there's the one that could have helped make the Moon long ago, one that killed all those dinosaurs 65 million years ago, and now one lying buried in South Australia that could be on the same scale. Vast. This is Dr Andrew Glikson at the ANU.

Andrew Glikson: I would estimate it at about a total of 400 kilometres, but it's a twin structure, it consists of two separate segments. It's possible that it represents a single asteroid which has split into two, and we have evidence of the shock metamorphism and the crustal effects. But the time is our big problem. Asteroid impacts are particularly hard to date. You have to date both the impact of the rocks and the overlying formations and the thermal signature of the particular event.

To date we have two ages, we have 300 million years old thermal event in the granites which have been shocked. We also have relics of 420 million years old material which is probably the material from which the granites have formed. Then overlying this 1,600 million years old granite are Permian glacials which are 296. So we really don't know the exact age of the events, which is a problem because if we wanted to study the consequences of this large event, possibly the largest one recorded so far, we need to know the age. And then once we know the age we can go to the sedimentary sequence and look for the consequences. We will zero in on the particular horizons which can be dated both from fossils and from isotopes, and see whether we have evidence of the ejector, the tsunami which invariably follows large impacts, and possibly evidence of mass extinctions.

Robyn Williams: Where did it hit?

Andrew Glikson: Well, of course Australia was a part of Gondwanaland, but it hit in the present north-eastern part of South Australia. It was too large to be true, so I had to study more than 100 drill cores in the eastern structure, and they all contained these shock metamorphic effects. When this was completed and published, I turned my attention to the western structure which is far less well explored in terms of drilling and seismic work, but lo and behold we found evidence of shock metamorphism there as well.

So now we have a whole story but not a digital story, for the reason that these structures are buried 2 to 3 kilometres underground, which means we cannot look at the detail of the craters and of the domes which formed by elastic rebound nearly in as much detail as you can look at structures when they are exposed.

Robyn Williams: Something that size hitting the Earth must have had a gigantic effect.

Andrew Glikson: That's exactly right. We have a classic type case now, the K-T boundary impact, the Chicxulub crater, and the global ejector from it which has been discovered by Alvarez, has by now been studied by so many people, there are hundreds and hundreds of peer reviewed papers, maybe more than 1,000, so this gives us a benchmark for what other large impacts could have been. Of course conditions on the Earth were different, different fauna, different flora. But we now know that essential mechanics of such impacts, first you get the flash in the atmosphere, the consequent incineration of forests, so there would be vast regional fires which would of course have had a devastating effect, as well as release carbon dioxide.

The crater would form and the seismic effect would be in orders of magnitude larger than any which are caused by normal terrestrial volcanic and earthquake effects, perhaps size magnitude 10, 11 or 12. So whole plates on the Earth will actually be disrupted. The dusting will follow, and this will result in what is called an asteroid winter, which is a period possibly not much longer than weeks or months when virtually parts of the Earth will freeze. But as we know, carbon dioxide has longer atmospheric residence time, which means that there will be then an asteroid summer, and some large regions of the Earth will cook. So it's really the apocalypse. The fire, the freeze, and then the heating, not to mention the dusting.

Robyn Williams: If someone goes to the area in South Australia, will they see anything?

Andrew Glikson: Yes, they will see a lot of drilling rigs, they will see roads and vehicles and maybe some geothermal experimental stations, otherwise it will be a plain, the dunes, and the whole story is underground, deep, two or three kilometres underground.

Robyn Williams: Is there a way you can look underground using various visual technologies?

Andrew Glikson: Well, they take cores in the case of geothermal research, and we look at the cores, we look at the rock. Whether they can look visually, yes, they can locally, they have sophisticated logging techniques that take every kind of measurement; electrical density, magnetic and so on. But the drill holes are normally coated with mud, so looking visually is not necessarily the best way.

Robyn Williams: You talked about Alvarez, and of course he found there was throughout the Earth this thin layer of material which gave evidence of the fact that there had been some sort of disruption, and then the material fell, and it showed this sort of layer. He was dealing with a gigantic asteroid that is supposed to have killed the dinosaurs and so on. Is yours on the same sort of scale?

Andrew Glikson: His was estimated to be approximately eight kilometres or so from the size of the crater.

Robyn Williams: What about extinctions?

Andrew Glikson: This one is a problem because since we don't know the age yet, in so far as it could be 300 million years old, which is the age of the granite or one of the thermal effects on the granite, then there is no major extinction at the boundary. This is one of the reasons that we suspect the age of impact could be older. How much older, well, one possibility is it is late Devonian, 356 million years ago. Late Devonian is a period of both asteroid impact and mass extinction, so it's a possibility, but we still need some isotopic signature in the rocks we are looking at. I was even hoping at one stage it could be as old as the late Ordovician, because in the late Ordovician we have a major mass extinction and we don't know the exact factor underlying it.

Robyn Williams: How are you going to follow this up now?

Andrew Glikson: We are trying to date the zircons which already have been looked at but we are trying to use modern up-to-date techniques. Whether we get the extra age of the impact from this measurement, it's not clear, because, as you know, each core granite could have been imprinted by more than one thermal event. This one we know has been imprinted by two, but we can't say that any of them actually has to do with the impacts. So hopefully there will be other signatures, but at this stage it's a problem.

Robyn Williams: Dr Andrew Glikson at the Australian National University, trying to date that immense rock, or two rocks, lying underground in South Australia, 400 kilometres across, can you believe. That really would make your fillings shake on impact, wouldn't it!


Guests
Andrew GliksonVisiting Scientist
Australian National University
Canberra ACT

Further Information
Andrew Glikson at Australian National University

Credits
PresenterRobyn Williams ProducerDavid Fisher