Magnets preserve proof of plate tectonics
We all have Earth’s magnetic field to thank for the fact that we’re here at all - without it, we’d have been zapped by space radiation. But our magnetic field can also help us to piece back together the history of how the planet, and life as we know it, evolved here. That's because it can help us to reconstruct what the planet was like in the past since it formed 4 and a half billion years ago. A century ago, Alfred Wegener proposed the theory of continental drift. Many were unconvinced but, as time went on, more and more evidence started to emerge which proved that he was right: that the Earth’s surface was formed of tectonic plates which were constantly moving and jockeying for position, taking the continents with them. The clues to validate Wegner’s theory were found in the form of magnetic signatures imprinted into these rocks from the Earth’s field as they formed, billions of years ago. And from these signatures, geologists like Harvard’s Alec Brenner and Roger Fu can work out the location of the rock when it was forged, allowing them to assess its movement over time, and what, as Alec explains, the Earth was like all that time ago...
Alec - Early earth would've been a very different place than the earth we live on today. And it's been changing ever since it formed four and a half billion years ago. And of course, plate tectonics is part of that picture. Earth's surface was probably a magma ocean. It would've been constantly bombarded by giant asteroids left over from the birth of the solar system. But within about a hundred million years, that surface would've cooled down. Oceans probably would've formed. Its interior was hotter, volcanic eruptions were probably also quite a bit hotter. There were still occasionally gigantic asteroid impacts. The only living things would've been bacteria. Our research adds to this picture by looking at what plate tectonics and earth's magnetic field were doing in the background while all of these changes were happening,
James - How did we transition out of that vision of the earth that Alec just described?
Roger - So this is the hard part. We know that as, Alec said, very soon after initial formation of the earth, let's say 4.4 billion years ago, we probably had a stable crust. The next part is where it really gets tricky and we don't have anything close to a consensus, and that is how earth's crust became partitioned? And this is something that we only see on earth, and that forms the core of plate tectonics. That we have these two different types of crust that collide with each other and some are denser and sink into the mantle and some are lighter and stay on the surface.
Alec - This is where we come in. We measure the magnetic signals that are preserved by ancient rocks that tell us how these rocks may have drifted horizontally over the surface. The reason that this works is that, to scientists like ourselves, rocks are time capsules that can record and then hang on to information about where, how and when they formed. And the kind of information we are looking for in rocks is magnetic information. And the idea here is that rocks contain tiny grains of magnetic minerals. These are hematite and magnetite, and those grains act like little compasses that can be frozen in place where they form. So just like you can use a compass to find your way around on earth when you're navigating outdoors, you can also use these little mineral compasses preserved inside rocks to look back in time at how those rocks moved around on earth's surface. And the place we went to look for these rocks is in the desert of Western Australia. It's called the Pilbara Craton. And in our research we went there and measured the magnetic signals of these little minerals preserved in rocks that are about 3.3, 3.2 billion years old. And what we found was that the Pilbara was drifting pretty quickly over Earth's surface back then. In fact, actually at a similar rate to how fast the new world and old world are drifting apart due to plate tectonics right now. And not only that, but we actually also stumbled on the signature of what we call an ancient geomagnetic reversal, or basically a flip versus magnetic magnetic field. And that's where the North and South poles of earth's magnetic field exchange places. And this happens pretty often on the modern earth. And what that means is that the ancient Earth's magnetic field acted a lot like it does today with a North and a South pole that originate from convection in Earth's core.
James - That is amazing to think, over billions of years, which even in geological terms I think I'm right in saying is a long time, that the movement of tectonic plates has remained so consistent. I wonder if either of you could speak to at what point the surface of the earth may have become an environment on which life could be sustained. What were the main criteria that needed to be solved from earth as it was in its state before that?
Roger - So you'll get a lot of opinions on this. At the most basic level, having a surface that's not, for example, thousands of degrees hot, that probably occurred quite early on. Whether life that evolved then could have sustained itself, whether there was all the other conditions on earth, like a stable temperature, the right elements, that is a much tougher question, a much tougher criterion to achieve. We don't know exactly when that happened, but the evidence shows that, probably by about 4 billion years ago, certainly about 3.5 billion years ago, there was life established on earth.
Alec - It doesn't necessarily require, per se, that plate tectonics specifically was going on, or for that matter the magnetic field. It doesn't necessarily require that those factors are present, but the fact that we can now see that they were present certainly adds to the story. We can see today how plate tectonics and earth's magnetic field lead to conditions that life can take advantage of. For instance, the magnetic field being present, stabilising the atmosphere against being bombarded from space by cosmic ray. Or, plate tectonics not only creates the environments that living things can make a living off of, but it also plays a whole bunch of different roles all over the earth system in stabilising the climate over billions of years, for instance.
Roger - If there is a way that plate tectonics is really vitally important for life, that will probably be through this mechanism of temperature regulation that's famously called the Walker feedback. This is where extra CO2 in the atmosphere is stored in rocks on the sea floor, and then gets subducted, or gets tucked away into the mantle over long time scales. Without this ability to ship away the surface CO2 in the form of rock and bury it in in the deep earth, it's believed that CO2 would build up in the atmosphere. And of course, CO2 is a greenhouse gas and makes the surface hotter and hotter.