Australia once joined to North America


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02-02-2019 12:37 PM


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02-02-2019 12:37 PM



02-02-2019 01:39 PM

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2019-02-02 12:37:57

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Australia once joined to North America -

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Robyn Williams: And one observation you could make from up there in the sky is that the coastline of northern Tasmania is nowhere near Arizona. You could also just look at the map of course. So how come it seems like they were once joined? Here's Science Show producer David Fisher.

David Fisher: To geologists, the rocks along Tasmania's northern coast don't fit. They stick out as different from adjacent formations. So where did they come from? How did they get there? And are they related to formations elsewhere in the world? After much searching, the rocks of the US south-west were thought to be a possible match, and a good place to look. And American geologists had a similar mystery to solve. Ancient formations in the South West seemed to be sheared off. Do they extend onto some other continent? So Jack Mulder went deep down into the Grand Canyon where ancient rocks are exposed, and where stories of our planet's history come to light.

Jack Mulder: The first striking thing to me is when we got down to the Grand Canyon you could sit on these rocks and they were just a dead ringer for what we were looking at in Tasmania. So, physically the rocks look the same. And then when we started doing some chemical analysis and age dating, that all lined up as well and it just looks very similar on a variety of scales.

David Fisher: The age of these rocks is revealed by studying the mineral zircon.

Jack Mulder: The rocks we are talking about here are ancient sedimentary rocks, so they were once layers of sand and mud deposited in a shallow ocean. And some of those grains are this mineral called zircon. Zircon is a really interesting and very useful mineral because we can date it. We can tell how old those grains are and that's because zircon contains tiny little amounts of uranium in there which decays to lead. The first thing we do when we get these samples is we essentially crush them up and separate all these tiny little zircon grains out of them. And then once we have those zircons we analyse them on a mass spectrometer that's linked up to a laser. So essentially we blast these tiny little zircon grains and we can measure the different ratios of uranium and lead coming out of these grains to tell how old those zircons are.

David Fisher: And how old are they?

Jack Mulder: The youngest zircons we were finding were about 1.1 billion years old, and there's a spread of ages all the way out to about 3 billion years old. So there's a lot of history preserved in those tiny little zircons.

David Fisher: The dating process pinpoints a specific time when a crystalline mineral was trapped in its surrounding rock. Each sample contains a specific quantity of crystals from specific dates. Together they form a unique signature. If the signatures of two samples match, chances are they were produced from the same source.

Jack Mulder: These zircons are growing in things like granite that were originally magmatic rocks. When these zircons start to crystallise out of the magma, they will cool down enough that the lead that is decaying out of the uranium will get trapped in the mineral. And from that point onwards that lead will continue to accumulate within our zircon crystal, and then when we measure the amount of lead in there we can figure out how long that grain must've been sitting there accumulating lead because we know how fast uranium decays to lead.

David Fisher: Why could your sample in Tasmania not have come from somewhere else? How can you be sure that the two are linked?

Jack Mulder: That's a good question, and that's something we spent quite a bit of time thinking about, and essentially it comes back again in a large part to these zircons. So these zircons in sedimentary rocks are coming from older rocks. So old mountain belts are eroding, eroding all this sand and mud off which eventually gets transported down in rivers and deposited in these sedimentary basins. When you look at the range of ages of zircons within sedimentary rock you can use that as a fingerprint of the older source rocks that these sediments came from.

So when we did this with the Tasmanian rocks we compared those ages to nearby rocks in Australia, so older rocks in Australia that could have potentially been the source for the Tasmanian sediments. What you see when you do that is that those two signatures don't overlap. So the zircons in the Tasmanian rocks are not coming from anywhere in Australia that is at least exposed at the present day.

David Fisher: Could it not be linked to some buried deposited in Europe? How can you be sure that the Grand Canyon deposit was related to your Tasmanian deposit?

Jack Mulder: It's a great question, and looking at very ancient rocks like the ones we've been looking at there's always uncertainty. But I guess one of the strengths of this study is that not only do we have this zircon fingerprint that we've been working on, the rocks themselves were deposited in very similar environments and were very similar ages. So when you start to add up all these different lines of evidence you can be more confident that indeed these Tasmanian and Arizonian rocks are very similar.

David Fisher: How useful is this information?

Jack Mulder: It's a very useful discovery for a lot of people because it helps us understand the continents about a billion years ago. During this time in Earth's history most of Earth's continents we think were assembled together in a supercontinent called Rodinia. And people have debated about how the different continents were arranged within that supercontinent, and the new data from Tasmania helps link Tasmania and wider parts of Australia and Antarctica onto that western margin of the US. So that's quite an exciting discovery.

David Fisher: Does this information and this idea fit with what was previously thought, or is it a completely new idea?

Jack Mulder: Well it builds on an extensive body of work that probably really started to get going around the early '90s on this supercontinent Rodinia. So one of the most controversial aspects are what continents were assembled along the margin of the western US. There's a number of different models that have been put forward, these include sticking Australia and Antarctica against that margin. Other people stick some blocks that are now in China and throughout Asia along that margin. And the new data from Tasmania provides more support, I think, to the Australian, East Antarctica connection with western US and narrows down the relative configurations of those blocks. So it's not a completely new model but it is building and provides some important geological support to previously proposed models.

David Fisher: Have you got an idea why there was a supercontinent? Why should the planet have, way back, as you say, in deep time, why should there has been just one landmass?

Jack Mulder: Look, that is one of the big questions in geoscience, is what drives this supercontinent cycle because we haven't just had one of those supercontinents, we seem to get them every 400-500 million years the continents will join together into these large land masses, and the fundamental driving mechanisms for that process are still quite poorly known.

David Fisher: So we may all assemble together again in the future?

Jack Mulder: Absolutely, and we've already named the next supercontinent and that's called Amasia. It's happening again, absolutely.

David Fisher: Beginning now.

Jack Mulder: At the present day we are in the middle of a supercontinent cycle, so we call these supercontinent cycles the time from one supercontinent fully assembled, it then breaks up, the different continents drift apart and then they coalesce into another supercontinent. So that's a supercontinent cycle. The present-day distribution of continents on our planet largely reflects the breakup of the supercontinent Pangaea. So we are heading for a new one.

David Fisher: How many times has it happened in the past?

Jack Mulder: So that's, again, something that is debated. But we think the first true supercontinent was probably formed between about 1.5 and maybe 2 billion years ago. That one was called Nuna. Then Nuna broke up, the different continents drifted apart and they came together to form Rodinia, so that is supercontinent number two, it's about a billion years ago. Then Rodinia broke up and began to assemble into the landmass called Gondwana, which is debated as a supercontinent. But essentially Gondwana eventually collided with bits of North America to form Pangaea. So there's three, maybe four supercontinents that most of the geological community would agree on, and there is speculation that there might have been some even older ones. But as you get back in time you get fewer rocks to work with, so these things become even more speculative.

David Fisher: It's a fascinating story. Getting back to the site in Tasmania, can you just place it for us, where this outcrop is? It's not a secret is it?

Jack Mulder: No, absolutely not. One of the great things about these rocks in Tassie is they are very easy to get to and they are quite literally in people's backyards. So probably the best place to see these rocks is along the north-west coast between Wynyard and, say, Stanley, all along the coastline there in places like Boat Harbour and Sisters Beach.

David Fisher: How long is that bit of coastline roughly?

Jack Mulder: 100 kilometres maybe. These rocks are quite extensively exposed in north-west Tassie, and then we also find them down in south-west Tasmania in a number of places as well. A quite extensive package of rocks. We're not talking about one outcrop that we stumbled across, this is quite a regionally extensive group of rocks.

David Fisher: Could be presented as a tourist attraction. Yet another reason to visit Tasmania.

Jack Mulder: Look, absolutely, it just adds another layer to the iconic scenery that Tasmania is so famous for.

Robyn Williams: Jack Mulder is a research fellow at the School of Earth Sciences at Monash University in Melbourne. And yes, the earth is moving. Arizona in Tasmania. Who'd have thought?