Professor Alan Collins, the Douglas Mawson Professor of Earth Sciences at The University of Adelaide, is building a dynamic map of the Earth’s plate tectonics spanning nearly a third of our planet’s existence, from 1800 to 500 million years ago. The map might reveal critical mineral locations, but Collins also believes tectonics are part of the reason life formed on Earth.
When most people think about plate tectonics, they visualise Gondwana breaking up and becoming the Earth that we know now. This is understandable, as the logic behind plate tectonics comes from the evidence we have of continental drift. But for a long time, people thought there wasn’t anything before that – that this had been the primordial continent.
Our work is showing that Gondwana was just one of numerous amalgamations of continents that have happened periodically in the history of our planet.
We know Gondwana broke up about 200 million years ago, but the Earth formed around 4.6 billion years ago. Our work is looking at the movement of continents between 2 billion years ago to half a billion years ago – the time period we call the Proterozoic eon. I’m building a map of the movement of the continents for the 1.5 billion years before Gondwana broke up.
There’s certainly lots of inferences that have to be made to get to the cartoon you can see in the above animation. What we’ve done is definitely not the final word on what the world looked like over this time – it’s only the first attempt. But it’s a step change in the way we can test hypotheses for how our planet came to be habitable, with the diversity in ecosystems and life and chemistry that it has.
There are a lot of rocks around the planet that formed in this time period. And a lot of rocks form on the margins of tectonic plates. What I try to do is look at what we can say from the evidence preserved in these rocks about where the nearest plate margin was when they were formed. And then I start to put that jigsaw puzzle together.
This really is the product of an army of thousands of geologists all over the world publishing papers. We then try to integrate this regional geology into a global picture.
It helps that we can date rocks quite accurately. It is particularly helpful for my work that many rocks preserve the magnetic field of the Earth at the time they formed – like a fossil. This is because many rocks have iron in them, and iron often forms a magnetic mineral called magnetite, which is iron oxide. If the rock forms from lava coming out of the volcano, as it cools down below about 550 degrees, it literally freezes into it the magnetic field of the Earth. As long as you keep it oriented, you can take that rock and put it into a very sensitive magnetometer in a lab, and you can measure the magnetic field preserved in that rock. Of course, it will point to where North was at the time that rock formed.
Picture the Earth’s magnetic field. It flows vertically out of the poles and wraps around the Earth in a big semi-circle. This means that at the poles, the magnetic field is actually pointing straight down. At the equator, the magnetic field is parallel to the surface. At everywhere in between, there’s a specific angle. And this information is recorded in the rock when it was formed.
We can look at these data and ask questions. Has it rotated since it formed? Has it moved latitudes? This is what we call the paleomagnetic record. We have thousands of these little snapshots, and we reconstruct the rocks’ movements from there.
The big end of town in the minerals industry is interested in this work. They’re looking for the next big, critical metal deposit. These areas don’t necessarily stick out of the ground; they can be buried by younger rocks. But we’re most likely to find them at these ancient plate margins.
Things happen at plate margins. That’s where we have volcanoes. That’s where mountains are formed. And that’s where a lot of our biggest metal deposits in the world are found. Critical minerals and rare earths are of vital importance for a low-carbon economy.
Copper is one of the most important because we need an awful lot of it. In the modern Earth, the biggest copper mines in the world are found in South America, in Chile. And they’re still in their tectonic environment where they formed. But if we can look where these past plate margins were, what we call subduction zones, it would be helpful for looking for these deposits.
Of course, it’s not quite as simple as that: you don’t get a copper deposit everywhere there’s a subduction zone. You get them where the subduction zones have something interesting about their chemistry. Maybe there’s slightly different rocks being subducted that are quite rich in copper, and as they melt they get concentrated into the roots of a volcano above it. With our reconstructions, we can look at where those past plate margins are, then look for these sweet spots.
It might help us locate another Olympic Dam, in South Australia. We’ve been able to demonstrate how the ancient subduction underneath this area has fertilised the crust above it.
It’s not going to be as straightforward as an X marking the spot, but we will identify places with the right ancient plate tectonic setting to preserve these minerals. It will then be left to companies to go and test for them, one way or the other.
Read more: is plate tectonics the hidden key to life?
No other planet has plate tectonics. Many of us are absolutely convinced that it’s plate tectonics that has provided this strange Earth system that we have, with life being able to exist and to evolve into the complexity it has – the influence of mountains on the formation of the atmosphere, and the mixing of chemical nutrients in the ocean that kickstarted life.
This is the bit I find absolutely fascinating. I’m just a geologist who likes walking around looking at rocks. But we just might be also glimpsing the origins of existence here.
As told to Graem Sims
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