Introduced in 1912, Albert Wegener’s theory of continental drift is one of the most well-known Earth theories out there. In it, Wegener suggests the earth’s land started as one mass, a supercontinent called “Pangaea”, which ploughed through the oceans of the planet, sometimes into itself, before splintering off into the continents as we know them today. What it failed to ask, however, was “How did land get there in the first place?”
Australian researchers may have come closer to finding an answer.
Firstly, there are two major theories that exist around the creation of land on planet earth. The earliest, suggested by J. Tuzo Wilson in 1963, said that land was created when buoyant material in the earth’s mantle melted the crust and rose to the surface – a theory reliant on the internal processes of the planet. The second, which dates back to around the same time, said that giant asteroids pierced the once water-covered planet’s crust, and made way for the content of the mantle to rise, creating land.
While the first theory is often cited as the most widely accepted, a new study has found evidence that the asteroid theory reigns true.
Recently, Tim Johnson, an associate professor and metamorphic geologist at Curtin University, led a team to Western Australia’s Pilbara Craton, 1,300 kilometres from Perth. By studying one of Earth’s oldest minerals, zircon, the researchers were able to corroborate their age with spherical, sand-sized droplets, known as spherules that were “splashed out” in beds across the site. What it seemed to point to was the impact of meteorites on Earth’s surface billions of years ago.
What’s interesting about the asteroid theory is that many believe these early bodies brought with them the ingredients to create life on the planet, including water and carbon-based molecules.
As the first study of its kind to find physical evidence of the asteroid theory, it means that it could also lead others to question how life on Earth came to be, and why any of us are here at all.
We caught up with Johnson to find out more about asteroids, the formation of land and his new study that could unlock the answers to our long-held questions of life’s evolution.
VICE: So what initially led you to the Pilbara Craton?
Tim Johnson: Because it’s right in our backyard. But mainly because there’s lots of these fragments of really, really ancient crud. So bits of continents that are older than 3 billion years old. Presently, you can find them in continents everywhere – South America, North America, Europe.
But because those rocks are so old, in most places they’ve been just bashed about so much, almost all of the evidence for how they formed originally has been destroyed. The Pilbara, while it’s certainly been nudged at its edges by other bits of land, the southern rocks have been almost unaffected for the last 3 billion years. So you’re looking at, as close as we can get on the planet, to the most pristine piece of the Earth, how it would have looked 3 billion years ago.
So I understand there’s been previous research on the role of asteroids shaping the earth. But why is this, in recent years, the only study that has physical evidence of the asteroid theory?
The idea has been around at least since the 1960s, as a serious scientific idea that was published in 1966. And even before that. But the concrete evidence behind it means it really just remained as an idea. It’s a vague idea.
So we use grains of this mineral called zircon, which is very common in the granites, these pale coloured rocks that make up most of the continents.
And zircon is very useful because it is extremely robust. So it can survive billions and billions of years. So the oldest material we know about on Earth is the Jack Hills zircon in WA. They are 4.4 billion years old, so almost as old as the Earth itself. So that shows how robust they are. But they also contain lots of uranium, and that uranium is radioactive, It breaks down to lead over time. So we can use various sophisticated bits of kit that will be fired as laser beams and we can work out exactly how old they are.
So then you have a look at that as a timeline against which you can interrogate other things. And they also contain oxygen, like most mammals do. So what we did is measure the ratio of two of the isotopes of oxygen, it doesn’t really matter which, but that ratio can tell you something about the environment in which those circle grains formed. And the data from the Pilbara suggests that the very earliest rocks were formed at very shallow levels in the earth. So the uppermost few kilometres of rock that had interacted with a global ocean.
So we know that 4 billion years ago, the Earth was almost entirely covered in a global ocean. So that interacted with these top layers of the earth. And then those melted to produce the rocks that we see today. Now, when you’re talking about such shallow levels in the crust. The only way of producing that heat of putting enough energy into actually melting those rocks is with giant asteroids.
I would argue it’s the most plausible way of the earth forming. We just need to look up at the moon to see what would have been happening to the Earth. You can see it is heavily cratered. So it was hit by huge meteorites and asteroids in its early history. And at about the same time as Earth. So the many meteorite impacts that might have happened, the evidence for them has been lost on Earth.
So the pattern we see is the opposite to what you would expect from the alternative model for producing continental crust. And that’s for an internally generated process called mantle blooms.
How does this play into Alfred Wagener’s theory of continental drift?
So he proposed this idea of continental drift, that the continents are moving around floating around on the surface of the earth. But as far as I know he had nothing to say about why there’s continents there in the first place, just that they moved. So he’s talking about processes that happened much later on. Whereas we’re talking about things that happened in the first billion years, when many people have argued, including me, about plate tectonics. So the mechanism by which everything on earth happens today, we would argue that it wasn’t possibly that first, but other people would disagree with us. So we would argue that plate tectonics didn’t operate so early.
So what was the process of land formation when the asteroids hit the Earth?
I’m going to use an analogy here. What happens when a meteorite hits is a bit like what happens if you’re unlucky enough to get hit on the head with something hard. You’ll develop a big bump. It’s exactly the same when a meteorite hits the earth or any planet.
The Earth is like an onion, it’s layered. And we’ve got this rigid, rocky outer layer called the lithosphere. The rocky layer doesn’t really matter. But underneath that is a mass of very hot rock called the mantle. So that’s the layer underneath the crust. And basically, it’s very close to its melting point and wants to melt. But it can’t because it’s got the lithosphere acting like a cork. So when the meteorite impacts you get rid of some of that lithosphere. So it just evaporates or otherwise gets excavated and makes a hole.
Now you’ve removed the pressure from the top of it can erupt to form a big blob of dark basaltic rock, so this really thick, dark, massive molten rock that solidifies. And then if that gets big enough, then it sits there. And at its base, it gets hot enough that it can then melt itself. And then it’s that process that produces the pale colour granitic rocks, which are a relatively low density. And then once they start forming, you’ve got a buoyant crustal nucleus. It’s going to stay there because it can’t get back into the mantle because it’s too buoyant. So it’s kind of a two stage process. The first stage, the giant impact to make this big basaltic crust of nucleus which could be many hundreds of kilometres in diameter, and many tens of kilometres thick. And then you need the second stage where it stews in its own juices and melts at the base to produce these granites that stabilise the continents.
What can this tell us about the evolution of life on the planet?
So I don’t recall the precise age of the oldest evidence for life that we have, but it’s certainly something around 3.3 or 3.4 billion years ago. So exactly the same age as some of the oldest rocks in the Pilbara Craton. And I would argue very strongly that that is not a coincidence. So we know that one essential ingredient for life is water. And that’s definitely true. But we would argue there’s a good possibility that to kick start life, you might also need giant meteorites, so that they could be absolutely essential for life.
There hasn’t been huge amounts of research, because people are reluctant to admit – that might be the wrong word – that giant meteorite impacts would have played a significant role.
Because, as I said earlier, we don’t really see the evidence for those impacts. And yet the evidence is so tangential that people, even though they can see them, still argue, the moon got in the way of earth being hit by big meteorites. Geologists in general try to explain everything in terms of internal processes. So if anything is coming from outside, for them, it makes things a bit more complicated, so they prefer to pretend.
So in the end, this study is the first to provide concrete evidence of the asteroid theory?
Well, people have been proposing it as a vague idea. And it’s always been very attractive to me as an idea, but it hasn’t really picked up in the scientific community, because people say, “Well, yeah, maybe, but you can’t prove it.”
But now we’ve got some tantalising evidence that I think does put some meat on those bones.
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