Short Wave - What Did Earth Look Like 3.2 Billion Years Ago?

Episode Date: May 15, 2020

The surface of the Earth is constantly recycled through the motion of plate tectonics. So how do researchers study what it used to look like? Planetary scientist Roger Fu talks to host Maddie Sofia about hunting for rocks that paint a picture of the Earth a few billion years ago, in the early days of the evolution of life.Follow host Maddie Sofia on Twitter @maddie_sofia. Email the show at [e-mail]. See pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences.NPR Privacy Policy

Hey, everybody, quick heads up. Next week, the Shortwave Science Movie Club is back. We'll be talking about the science in the 1997 sci-fi drama, Contact. So, if you can, try and see the movie before the episode drops next week. All right, on with today's show. You're listening to Shortwave from NPR. So imagine you're floating over the earth, say, a couple billion years ago. What would you recognize?

Well, you would see large bodies of water. We don't know how much land there was back then, but there was definitely some. If you ask Roger Fu, it might look surprisingly familiar. So you'll probably see the same kinds of mountain belts, valleys, and rift basins that you might see today. Roger's a professor in the Department of Earth and Planetary Science at Harvard University. He says you'll also notice Earth isn't covered in craters, like the Earth is covered in craters, like, other planets are. That difference stems from the fact the Earth's surface is constantly recycling itself through the action of plate tectonics.

Plate tectonics. We all remember learning about it. Kind of. Roger, I'm a microbiologist and I have to be honest with you. I read your paper and I was like, oh, buddy, you are not in biology anymore. The only reason I know about plate tectonics was because our high school did this nerdy ocean science competition, like this kind of buzzer, like, jeopardy style, and we all, like, kind of read textbooks in our spare time. That's what spare time is for. Eventually, all that extra book reading paid off

because Rogers Lab recently published a study showing that the Earth's tectonic plates started shifting hundreds of millions of years earlier than we thought, which is important, because by knowing when, those shifts happened. We can say something more about the environment in which life evolved. So today on the show, Roger Fu tells us how we know what was happening with Earth's tectonic plates billions of years ago and how the action of these plates set the stage

for evolution of life as we know it. I'm Maddie Safaya, and this is Shortwave, the Daily Science podcast from NPR. Before we get into it, quick, very general refresher on plates. tectonics. The outer layer of our planet, the stuff we're sitting on, is made up of a system of hard plates. Rigid blocks of rock that move relative to each other. And they glide around on top of a layer of softer rock that makes up part of the Earth's mantle. These raft-like plates drift around, colliding, causing each other to crumple or slide over top of one another. It's why we have most of our mountains and earthquakes. And Roger wants to know in Earth's long history, when those plates started moving.

Answering that question was a bit of an adventure. Okay, so Roger, to figure out when these plates started shaking and bacon, moving around, you had to go on a hunt for some very specific rocks. where did that take you? Yeah, so we follow the old rocks. We go to the parts of the world where rocks from 3 billion years ago are actually preserved.

And this is hard because actually of plate tectonics. So plate tectonics like recycles the surface of the earth over and over again. Right. Only about 5% of the Earth's surface represents the first half of Earth history. Oh, that's really interesting.

Yeah, in other words, if you're a piece of continent three and a half billion years ago, there's very little chance you survived at the present day. So specifically, we went to an area in northwest Australia called the Pilbara. Right. So this is an area where there isn't a ton of turnover due to plate tectonics, so you can find some really old rocks there. That's right.

Yeah. So just by the luck of the draw, these rocks have been knocked around on the surface of the earth. It probably wandered all the way from the pole to the equator many times. but over the course of these three billion years, it was never pushed down into the interior of the earth, in which case it would have been heated and melted. What does it look like, Roger? Yeah, it's a really beautiful place.

And most of the terrain is kind of these green rolling hills with these kind of spiky, kind of drought-resistant grasses. It's better to look at than walk through in that sense. Yeah, it's prettier than it feels is what you're telling me. Exactly. The same field season, we took these samples. I made the grave mistake of taking light-duty hiking shoes that also had some holes in it. And I ended up duct-taping my feet every day just to kind of armor it a little bit more against the spiky grass. Wow, that's funny. So, okay, so you're hiking along. You find your rock that you're looking for. You collect your samples. And then you take them back to your lab and try to, to determine their magnetic history? What does that mean?

Yeah, that's exactly right. So we take the rocks from the field. We keep track of how the rocks are oriented. So in other words, which side's up? And then we take it back to our lab and we measure the direction of the magnetic field in these rocks. So it turns out all naturally form rocks contain magnetic components. Oh, I mean, I knew that. I knew that. Keep going. I knew that. Totally knew that.

Yeah. Yeah. So all natural rocks contain these minerals, so these little grains of material that actually are magnetic. And they actually behave like little compass needles. And if you take a rock, any old rock, and then you measure it in the instruments that we have, you can detect the direction that these little magnetic grains are. pointing in. Wow. So you can literally take a rock and say, okay, we know this rock was pointing in this direction. That's exactly right. Yeah. And the reason this is useful in our case is that the magnetic field of the earth exists at different angles. I exist in different directions, depending on where you are on Earth. And specifically, if you change in latitude, if you go from one latitude to a different latitude on the earth, the angle to magnetic field changes. So if you can measure the angle to magnetic field in these rocks, you can figure out

what latitude they formed at. Wow. Okay. Okay. So you figure that out and then let me know if I have this right. Then you compare them to other nearby rocks that you know the magnetic history of, you know which way they were pointing. And that helps you understand like when they started moving? By looking at our data of where this rock was relative to the equator and comparing two other studies. We showed that this rock actually moved from a position close to the equator, so in the

tropics of the earth, to a position that's farther from the equator, so in the kind of mid latitudes. Got it. And we can quantify how quick this drift was, how quick this motion was, and from that, we know that this continent

was moving at the same rate, at the same kinds of velocities that modern continents move. That's cool. And so you know when that happened because you know the age of the rocks as well? That's right. So other people, other workers that have visited these rocks before us, have looked at particular parts of these rocks or particular mineral grains in these rocks that actually preserve information about how old they are. So for each of these measurements of how close a rock was to the equator, we can also put an age on that position.

So Roger and his team, by collecting and analyzing these very, very old rocks in Australia, came up with an estimate. Their research suggests Earth's tectonic plates were in motion at least 3.2 billion years ago, several hundred million years earlier than we thought. And another cool thing about Roger's research, it weighs in on a peculiar geoscience mystery. So there's this very long-standing question in earth science of how the earth seems to have had water on surface

for at least the last four billion years. Roger says at that time, the sun was probably about 30% fainter compared to today. So the earth should have been completely frozen. But he says there's geological evidence, liquid water was on the Earth's surface then. You know, one of the key ingredients that allowed life to evolve on this planet? So what could have made the planet warm enough for liquid water? You know where I'm going with this. So one of the leading hypotheses for why the Earth managed to

maintain an equilibrium and temperature, managed to have this thermostat, is that plate tectonics causes the recycling of carbon into the interior of the earth and then also puts out carbon into the atmosphere. And it does so in such a way that the surface temperature is kept within a certain range. So are you telling me that the movement of these plates that we're living on is partially responsible for the development of our atmosphere and the temperature of our planet? Yeah, that's exactly right. So this is a question that, geologists have been pondering for like a really long time. How cool was it to add this piece to it, to find this out?

Yeah. Yeah, it felt very gratifying to know that we have contributed to resolving this very old question. It feels like the effort was well worth it. Yeah, it was worth the duct tape boots is what you're telling me. That's right, yeah. It was worth the pokes in my foot every day, every step. really. Yeah.

All right, Roger. Well, I really appreciate you. This was super fun. Yeah. Yeah, this is really fun. Roger Foo is an assistant professor of Earth and Planetary Sciences at Harvard University. Today's episode was produced by Emily Vaughn with help from Rebecca Ramirez. It was edited by Viet Le and fact-checked by Emily Vaughn.

I'm Maddie Safia, and you've been listening to Shortwave from NPR.