Back When the Planet Had Just One Plate
As magma oceans began to cool on a very early earth, a solid shell formed on the planet’s surface above the still-molten mantle. However, the exact mechanics of how that proto-Earth was transformed from its ancient state to the world of shifting tectonic plates we see today are still a mystery. In a report published in the journal Nature, geologist Michael Brown and colleagues describe efforts to model how different mineral combinations react at specific combinations of temperature and pressure. That model, Brown says, shows that the modern plate tectonic process of subduction may not have been needed to form some of the world’s oldest rocks.
Michael Brown is a professor in the Department of Geology at the University of Maryland in College Park, Maryland.
IRA FLATOW: This is “Science Friday.” I’m Ira Flatow. A bit later in the hour, a look at all that water in California. And we’ll talk about the role of farmers in conservation.
But first, a trip back in time, say maybe 4.5 billion years ago. The moon has just been split off from the Earth, and our planet, think magma, oceans. Eventually those oceans began to cool and a solid crust forms on the Earth’s surface, sort of like a cooling custard.
But what happened next? Geologists say, in the journal “Nature” that in the early days of earth there may not have been plate tectonics, moving pieces of crust. But perhaps just one big plate, sort of like an egg shell covering the whole world. Joining me to talk about that and how that would have worked is one of the authors of that report, Michael Brown. He’s Professor of Geology, University of Maryland in College Park. Welcome to “Science Friday.”
MICHAEL BROWN: Good afternoon, Ira.
IRA FLATOW: A single shell, like an egg shell– how is that idea different than what we think now, the generally accepted idea?
MICHAEL BROWN: Well, the difference is that at present, the Earth’s outer shell is broken up into about 10 large and six small plates. We call them plates because they’re largely internally rigid, so they can move relatively, one with respect to the other.
For example, in the middle of the Atlantic Ocean they move apart, creating an underwater mountain range called the Mid-Ocean Ridge. In California, along the San Andreas Fault, one plate moves past the other sideways. We call that a transform fault or a transcurrent fault. And under the Pacific Ring of Fire, the Pacific Ocean and the Nazca ocean off of South America are both being pushed underneath the continental edge, down back into the mantle. That’s a convergent margin where we destroy oceanic material.
So we have this ability to create new material at Mid-Ocean Ridges and destroy it at the subduction zones around the Pacific Ring of Fire. That’s the way the Earth works at present. But the Earth’s mantle underneath the crust was hotter in the past. And if it was hotter, it may not have behaved the same way.
IRA FLATOW: So you’re saying that back then that it was a solid crust, underwater. Because we had all those oceans. And one big plate, not the multiple plates that we have.
MICHAEL BROWN: That’s correct. I think that this is probably the global geodynamic regime throughout the Archean, which is the period of Earth’s history from four billion years ago to 2.5 billion years ago. It’s a little difficult to calculate exactly how hot the interior of the Earth would have been at the end of the magma ocean.
The upper mantle, the upper part of the material beneath us, may have been 100 or 150 degrees warmer than the present day. And that mantle would have continued to get hotter until about three billion years ago. And then since that time, since maybe three billion years ago, it’s been cooling down to the present day. And at that maximum, it could have been 250 or 300 degrees warmer.
If the material underneath us is warmer, as it rises up in response to convective motions, it produces a higher percentage, a higher volume of melt. And that melt has a more magnesium-rich, less salistic composition. It’s different than the ocean basins at the present Day.
IRA FLATOW: And so is that what starts to form all the different plates, that you have different kinds of minerals?
MICHAEL BROWN: No, I think I think what that does– you have to imagine a world where there are many more volcanoes distributed across the world, across the globe than we see at present. We have occasional volcanoes at present. You can think of Hawaii in the middle of the Pacific. You can think of Iceland. And we have volcanoes at plate boundaries, like around the Pacific Ring of Fire.
But this world would have had the possibility for volcanism anywhere across the surface. It would have had the possibility of volcanism being generated by up wellings in the upper mantle, as well as these larger, what we call plumes, that are generated at the core mantle boundary. Thus these big tubes of hot magma that come up through 2,900 kilometers of Earth’s mantle.
IRA FLATOW: So how do we get from this solid crust into all the plates? What is the progression there?
MICHAEL BROWN: Well, that’s the $64,000 question. I can give you a couple of possibilities that we’re investigating. One possibility is that these large plumes, these large tubes of hotter material that come generated from the core mantle boundary, that these are able to produce so much magmatism at the surface that they push this outer shell down sufficiently that it starts to sink under its own weight. This would generate a local, on the scale of something like the Pacific Ocean perhaps, but a local area of subduction. And it may be that with enough of these plumes in the late part of the archaean that we can generate a globally linked network of plate boundaries. What will turn– sorry.
IRA FLATOW: I’m sorry, go ahead.
MICHAEL BROWN: An alternative would be that simply through what we might call wear and tear that this shell begins to get weaker in certain linear zones. And as it begins to weaken in certain linear zones, those linear zones start to link up. So we have a network of weaknesses in this outer shell that eventually are able to move apart as magmatism forces them apart and some move together. And again, we eventually generate a globally linked network of plate boundaries.
IRA FLATOW: Do we have any rock formations today that date all the way back to that time?
MICHAEL BROWN: The oldest rocks on the planet, and there’s very little real estate left of that time, are in the Western parts of Canada, the Acasta Gneisses, in southwest Greenland, and then in the east Pilbara terrain that we studied in the Barberton Mountain land in South Africa. And these go back to 4 billion years. We have a very limited record of what happened before 4 billion years. In that first 500 million years, we have some remnant minerals called zircons that are found in sedimentary rocks in Western Australia.
IRA FLATOW: Do the rocks tell the story themselves? These old rocks have evidence of the solid crust, or is that very difficult to find?
MICHAEL BROWN: The oldest rocks have evidence of what we call the first continental crust. This is the evolved material that’s generated by multiple episodes of distillation through either crystallization of liquids or remelting of material that generates something that has a more silica-rich composition and that will float relative to the ocean basins as we have at present. The continents make up about a third of the surface of the globe. And that’s the land area.
And it’s the land area because that material that comprises the continents is most silistic, it’s less dense, and it therefore is able to sit above the mantle, rather than go back into the mantle under normal circumstances. Whereas the ocean basins are heavier and therefore they’re shallow. They form seas. They’re underwater.
IRA FLATOW: So that explains it. That’s why the land is up and the oceans are down. The stuff that’s at the bottom of the oceans are down.
MICHAEL BROWN: They’re different compositions, and therefore they sit at different levels on the surface of the Earth.
IRA FLATOW: Now you talked about a hot magma Earth. I mean how can water be covering the earth when the Earth is so hot?
MICHAEL BROWN: Well, water covers volcanoes in the Pacific. If you think of the Izu-Bonin-Mariana’s volcanic chain, it’s mostly under water. We have volcanic peaks peaking through the water’s surface. And we have lavas that are erupted there. Hawaii stands up above the surface of the water quite a way. But if we took the entire Pacific Ocean away, this would be the largest mountain on Earth.
IRA FLATOW: Hm. And how do you know that all this water was around there at the beginning of that shell time?
MICHAEL BROWN: Because we have information from geochemistry that tells us that the surface of the Earth could not have been more than perhaps 100 degrees Celsius warmer than the present day. And we have evidence recorded in these zircons that I mentioned earlier, this mineralogical record of the first 500 million years of Earth’s history that show that there must have been surface water. what So we have a cool environment at the surface.
IRA FLATOW: Yes, I see the picture. So what’s the controversy here? What’s the alternative theory that you’re pushing against?
MICHAEL BROWN: Well, the alternative would be that it’s business as usual. That the Earth has behaved in a plate tectonic manner all the way back to soon after 4.5 billion years ago. In other words, this outer shell that would have formed first of all, as the magma ocean crystallized after the moon-forming impact, would have very soon developed into a network of spreading, moving apart, converging, moving together, and moving sideways plate boundaries that would have enabled that shelf to, if you’d like, crack into a number of plates.
Those plates might have been smaller than the present day, in which case there would have been more of them. They may have moved faster or slower. There may have been differences.
But the argument would be that we’ve had a plate tectonic regime since very early on in Earth’s history. And one of the arguments in favor of that is a particular fingerprint in the chemistry of igneous rocks. And I think one of the things that our paper does is demonstrate that this chemical fingerprinting is not a unique signature of a particular tectonic environment. And we have to be more careful how we identify things when we go way back in time.
IRA FLATOW: So how do you go about providing new evidence or convincing people that your model is better than the old one?
MICHAEL BROWN: Well, I mean the community as a whole does this. This is how science works. We have so little rock available back that far in time that we have to use the limited information we have from those rocks and a variety of modeling techniques. We were able to model the stability of minerals at particular pressures and temperatures for particular compositions of rocks. And we’re able to predict what the melt compositions would be and what their chemistry could be.
And we can compare these with the rocks that we actually have from that past era. So it’s a community effort to try and understand this. I think at the moment there’s a consensus in favor of something like plate tectonics starting three billion years ago perhaps.
I don’t mean by that there’s 98% of us believe that, but probably more than half of the geologists interested in this topic would subscribe to that view at the moment. And that may change with time. This is the way we work. We try to prove ourselves wrong. We can’t prove ourselves right.
IRA FLATOW: So at least you get the conversation going is what you’re saying.
MICHAEL BROWN: Exactly, and if we didn’t have disagreements, we wouldn’t advance science. If we all agreed, there’d be no fun in the game.
IRA FLATOW: Well, everybody thinks that scientists know everything. One of the most surprising things to people who listen to “Science Friday” is hearing scientists disagree with one another. But it’s actually very informative.
MICHAEL BROWN: And it’s fun.
IRA FLATOW: It’s fun, OK. Mike Brown, Professor of Geology, University of Maryland, College Park. We’ll have you back for some more fun some time. Thanks for joining us.
MICHAEL BROWN: Thanks very much. I enjoyed it. Bye bye.
IRA FLATOW: We’re going to take a break. And when we come back, we’re going to talk about farmers on the front lines of conservation. Now farmers are basically deciding to farm differently and what that means for carbon footprint. And farmers don’t like to call themselves environmentalists, but what they actually do really does help the environment.
We’ll talk with some farmers, and hopefully you’re on the line. If you’re a farmer, give us a call. Our number 844-724-8255, 844-SideTalk.
We’ve had farmers out on their combines and tractors calling us in the past. We hope to get some more. You can also tweet us out there @SciFri. 844-724-8255. Stay with us. We’ll be right back after the break.