Explore Early Earth | Part 2: Meet a Geologist

Welcome back, young scientists. I’m Dr. Universe.  If you’re anything like me, you’ve got lots of big questions about our world.
 
What was early Earth like? How did life start on Earth? And what’s up with plate tectonics?

In part two of this two-part episode, we meet geologist Johannes Haemmerli of Washington State University. We learn about early Earth, some brand-new research on plate tectonics and the idea that this process helped make Earth habitable.

As always, submit burning questions at askdruniverse.wsu.edu.  Who knows where your questions will take us next.

Ask Dr. Universe is a service of Washington State University geared toward an 8- to 13-year-old audience. Listeners working on the Middle School History of Earth Next Generation Science Standards may particularly enjoy this episode:

| MS-ESS1-4. | Construct a scientific explanation based on evidence from rock strata for how the geologic time scale is used to organize Earth's 4.6-billion-year-old history.
 
| MS-ESS2-2. | Construct an explanation based on evidence for how geoscience processes have changed Earth's surface at varying time and spatial scales.
 
| MS-ESS2-3. | Analyze and interpret data on the distribution of fossils and rocks, continental shapes, and seafloor structures to provide evidence of the past plate motions. 

As always, submit burning questions at askdruniverse.wsu.edu. Who knows where your questions will take us next.

Transcript

Dr. Universe
Hi friends. I'm Dr. Universe, and if you're anything like me, you've got lots of big questions about our world. 

Welcome back to Part 2 of Early Earth. Last time we talked with an astrobiologist about what life was like after Earth formed about 4.5 billion years ago. Today we're going to keep talking about what the rocks tell us about early Earth.  

Let's get started. 

I met Johannes Haemmerli when I was answering a question about gems. Dr. Haemmerli is a geologist at Washington State University. He studies rocks and minerals. And recently he was part of a project that looked at plate tectonics. 

The part of Earth that we stand on is called its crust—but it's not just one continuous piece of crust. It's big chunks of crust called plates, and those plates are subducting. That means one plate can slide underneath another plate. That causes all kinds of things like earthquakes, massive ocean waves called tsunamis, and volcano eruptions. And we'll learn today it might have helped make Earth a planet that supports life. 

I was hoping that we could start out talking about what it was like on early Earth. 

Dr. Haemmerli
That's a really good question—and we don't really know. That's the answer, I think. There's not that much evidence we can really investigate.

 The oldest terrestrial material we have are tiny minerals called zircons. They're on average maybe 100 micrometers big. They’re about twice the size of one of your hairs. The good thing about them is they're quite robust, so they survive a lot. 

 So, once they form in a rock, even if this rock decays and forms tiny little fragments, these little tiny minerals are quite robust. They can survive several cycles of deposition, and we can use them and age date them, and that's how we know that we had something solid 4.4 billion years ago—something in which these zircons were able to crystallize. 

 Dr. Universe
How do you figure that date out? Like, if you had a sample of that zircon, how would you figure it out? 

 Dr. Haemmerli
That's a good question because until about 20 years ago, we didn't know that we have such old material. These are really sophisticated machines and techniques that’s how we can do that. 

 So, we basically drill into these zircons. We release some of this material from that zircon, and this material then goes into a mass spectrometer. 

 The mineral zircon is very special because it incorporates a bit of uranium, and uranium decays to lead. And we know what the decay constant is. So, we know how long it takes to produce a certain amount of this radioactive lead when it decays from uranium. 

 So, by simply counting how much uranium we have in our rock and how much lead we have in our rock, or in this case, in our zircon, we can actually calculate how old our zircons are.

 Dr. Universe
So, basically the uranium is breaking down into lead, and you can look at that and figure out how much is already broken down and how much is still there and work backwards to see how old the zircon is. 

 Dr. Haemmerli
Exactly. 

 Dr. Universe
And a mass spectrometer is an instrument that's measuring the different bits of uranium or lead.  

 Dr. Haemmerli
And the important thing is, these mass spectrometers, they're very, very sensitive. So, we can actually distinguish between different isotopes. We cannot just measure lead. We can also measure lead 206 and 207, and we basically use that ratio then to calculate the age of a zircon. 

 Dr. Universe
Recently you’ve been studying something to do with plate tectonics. Can we talk about what plate tectonics are? 

 Dr. Haemmerli
Yes. So, plate tectonics is essentially a connected framework of plates on the Earth’s surface, which move around, and they're all connected.

 Now what happens with what we call modern-style plate tectonics is that we have so-called subduction zones. And how that works is that we have relatively dense crust, which typically forms on the oceans. And this crust is much denser than continental crust. What happens is when these plates collide, the dense oceanic crust gets subducted under our continental crust, which is less dense.

 And, in this process, when the oceanic crust goes beneath the continental crust, the subducting oceanic cross gets denser and denser and denser, which is basically almost like a self-sufficient engine, which then the material of the subducting plate gets denser and denser and denser, and it pulls it further and further and further down.

 And that's what we call modern-style plate tectonics, that we have these oceanic plates being subducted under the continental crust, and it's basically an engine and going on on many, many plates on Earth, and all these plates are connected. If one plate moves, the other plate moves pretty much.

 Dr. Universe
So, when you say the oceanic plates are denser, does that mean they're essentially heavier—so  that's why they're slipping under the continental plate? 

 Dr. Haemmerli
Exactly. They're heavier. They're made out of other material than our continental crust.

 Dr. Universe
And the place where those two plates come together, and the heavier oceanic one slides under, that's a subduction zone. 

 Dr. Haemmerli
Exactly. That's a subduction zone. And often when you have subduction zones, we create, for example, a lot of earthquakes because these plates are grinding on each other, and they can release a lot of energy.

 Dr. Universe
So, what did you recently figure out about plates and early Earth? 

 Dr. Haemmerli
So, the one thing we don't really know is when was the modern-style plate tectonics regime established. What time did these modern subduction zones form? That's actually very important to know because we think one of the very important components on Earth that we have a so-called habitable planet that we can actually have life are subduction zones.

 What we have in subduction zones is we have an engine that recycles a lot of components, which are very important for life. If you think about CO2, if we didn't have subduction zones, we would probably have much, much higher CO2 levels in the atmosphere. Because what subduction zones do is take some of the CO2, which is stored in minerals, down into the subducting system. Some of it goes back in the mantle, and some of it is being recycled throughout volcanoes. 

 And this is basically a cycle going on and on and on, which keeps the CO2 level relatively constant on Earth. So, relatively speaking, right? Of course, now we have a lot of emissions, which change the CO2 level in the atmosphere, but on a bigger scale, thanks to we think plate tectonics, it keeps our atmosphere relatively constant, which makes it essentially a habitable planet.

 So, one of the big questions in geology is when did modern stye subducting systems get established? And one of the problems is that when we don't have that much material left. Because that material would have to sit around for about 4 billion years, right? And there's many, many processes on the Earth, and that material can be destroyed, for example, in subduction zones being recycled, being weathered. It's just not there anymore. It's not pristine anymore. It's very hard to find. 

 Now in order to find out when did this first sort of modern-style plate tectonics get established, we need to find direct evidence for that.

 And that's very hard, and that's been a big debate. Now, what we found—and that paper was led by a scientist from the Geological Survey of Western Australia, Dr. Ivan Zibra—and they found rocks that are about 2.8 billion years old, and they have all the characteristics that we also find in modern-style plate tectonics settings.

 So, the important thing is the specific minerals in these rocks are very, very similar to the minerals that we find today in subduction zones. That was very exciting because often these rocks, even though we maybe think they formed in subduction zones 3 billion years ago, they are just decayed.

 They're not fresh anymore, and we have a really hard time to tell whether they really formed deep down. When we look at these minerals, we can basically calculate when we see certain minerals under what exact pressure and temperature conditions—in other words, at what depth—did these exact minerals form.

 So that was done in this paper, and it turned out that these rocks went down about 30 kilometers. And we were able to basically calculate how long did it take when they were on the Earth’s surface or almost at the Earth’s surface, and how long did it take to go all the way down? And then how long did it take to come back up to the surface? 

 We found out that this very rock, about 2.8 billion years ago, was under a sea—so was basically at the sea floor for a certain time—and then, within about 20 million years, it was transferred all the way down to maybe 30 to 40 kilometers depth.

 And we were able to basically reconstruct the entire cycle from the surface down to about 40 kilometers and back up to the surface again, which is very exciting because we can do that in modern environments, but all the way back to 2.8 billion years ago, that hasn't really been shown in such a nice coherent way. 

 Dr. Universe
If that rock was on an ancient sea floor, and then that piece of crust was going under another piece of crust, that means that probably there were plates almost 3 billion years ago. 

 Dr. Haemmerli
Exactly. This might have been the transition or the beginning of modern-style plate tectonics—when we really see some materials being transported quite deep down and then being brought up again. And the important thing is that's pretty much what we see in modern-style plate tectonics today. So, we can really take these old rocks, look at them, look at young rocks, and we see pretty much exactly the same signatures. 

 And the interesting thing is that's pretty much exactly the same material that we have all over the Palouse, the Columbia River basalt. These were also basalt, so if we actually brought down the Columbia River basalt, we would essentially form exactly the same rocks as were formed 2.7 billion years ago.

 Dr. Universe
So, is it accurate or a stretch to say that that kind of plate activity happening so long ago probably played a role in life being able to happen? 

 Dr. Haemmerli
That's a good question. I think what happens, probably as soon as we had plate tectonics, it started to really stabilize Earth in a way, right?

 So maybe the atmosphere would've changed and so forth. So, I think maybe it was really the beginning that things stabilized and allowed for life to be developed—maybe much more rapidly than if there wasn't any plate tectonics. Maybe there would never be life on Earth if there weren't any plate tectonics and especially subduction zones.

 Dr. Universe
The last time that we talked, I asked you if you had a favorite rock or a mineral, and you couldn't answer—and I want to know if you have one yet. 

 Dr. Haemmerli
Tell you what, it's not diamonds, that's for sure. That's boring in my eyes because it's only carbon involved. You know what, I probably have to say my favorite mineral, and now many people would laugh at me if I say that, but it’s a mineral called scapolite.

 And it's just maybe my favorite mineral because I've done a lot of research on it. It doesn't look very spectacular at all, but it's chemically very interesting. So that's probably it. 

 Next time I'll have a rock for you. 

 Dr. Universe
Okay, that sounds great. I look forward to that. Thank you so much for talking to me. I learned so much. 

 Dr. Haemmerli
Awesome. Well, thank you for taking the time. Thank you for being interested in it. 

 Dr. Universe
That's all for this episode, friends. Big thanks to Johannes Haemmerli for giving us a window into geology.

 As always, if you've got a science question tickling your brain, you can submit it at askdruniverse.wsu.edu. That's A S K D R U N I V E R S E dot W S U dot E D U. 

 Who knows where your questions will take us next.