Ep. 142: Plate Tectonics

Plate Tectonics

Plate Tectonics

The surface of the Earth feels solid under your feet, but you’re actually standing on a plate of the Earth’s crust. And that plate is slowly shifting across the surface of the Earth. Over geologic timescales, plate tectonics has totally resurfaced our planet, bringing continents together, and tearing them apart. We know we have plate tectonics here on Earth, but what about other worlds?

  • Ep. 142: Plate Tectonics
  • Jump to Shownotes
  • Jump to Transcript or Download (coming soon!)
  • Shownotes

      Transcript: Plate Tectonics

      Download the transcript

      Fraser Cain: Hey Pamela, are you enjoying your summer so far?

      Dr. Pamela Gay: I’m traveling my little luggage back off right now. [Laughter]

      Fraser: Right so whatever you see from the inside of an airplane that is your summer.

      Pamela: You know the inside of an airplane can let you see some pretty cool things.

      Fraser: That’s true. I know you’re going to be in Seattle, the UK, in China.

      Pamela: [Laughter] Then I am going to South America – 3 continents, one of them twice, in addition to our own so that makes 4 continents in one summer.

      Fraser: Good. Alright the surface of the Earth feels solid under your feet but you’re actually standing on a plate of the Earth’s crust. The plate is slowly shifting around across the mantle of the Earth.

      Over geologic time scales plate tectonics has totally resurfaced our planet bringing continents together and tearing them apart. We know we have plate tectonics here on Earth but what about other worlds in the solar system?

      Plate tectonics is one of the coolest stories. It is the lone scientist railing against the establishment proving the theory, bringing the evidence and turning everyone’s minds to the new theory.

      Pamela: And it is a theory that is younger than you and I.

      Fraser: So tell it! Tell the story! [Laughter]

      Pamela: There is a lone scientist as you quite elegantly put it whose name was Alfred Lothar Wegener. He like a lot of other people looked at a globe and went hmm you really can just sort of take South America and fit it very nicely into Africa.

      In fact if you have a globe you really hate and you need to do this with a 3-dimensional globe not with a map that has a Mercator projection. Find yourself a really old globe – one that has all the nations of Eastern Europe and Africa completely wrong so you don’t feel bad about destroying it.

      Fraser: Right, something with Yugoslavia on it. [Laughter]

      Pamela: Right, Czechoslovakia. Take a razor blade to the globe and cut out the land. You can actually rearrange the land masses to more or less form one large mega-continent. This had been noticed by Magellan.

      Pretty much as long as we’ve been able to get all the way around the planet in one piece we’ve known that the continent is fit together in a kind of weird way. People looked at fossils in South America and Africa and discovered that on the eastern coast of South America and on the western coast of Africa, two places separated by a whole lot of ocean, we have the exact same fossils.

      It wasn’t like it is easy for a hedgehog to hop in the ocean and swim across the Atlantic Ocean. Somehow we were able to get fossils on the same side of both continents or at least on facing sides of the two continents even though the continents were far apart. Coincidentally the continents when cut out of the globe fit together like puzzle pieces.

      Fraser: There are mineral deposits that start on one continent and then continue on the other continent. Yeah, it is amazing.

      Pamela: Alfred put forward the idea of continental drift that once upon a time, long, long, long ago, like order of 225-250 million years ago back in the Permian Period maybe all the continents formed one giant continent called Pangaea.

      Maybe for reasons that he wasn’t completely sure how to explain he said continental drift but there weren’t any forces driving this drift at that period. Something caused the land to tear itself apart and start pulling itself into the continents we know today.

      During WW II we as a society set up seismic monitors all over the planet. We increased the seismic monitors as the cold war progressed because you can use seismology to detect something under the ground being blown up.

      We would be able to go investigate this and if you scatter seismometers all over the planet you can do 3-dimensional mapping to figure out exactly where underground either an earthquake originated or a nuclear bomb was tested which was the real reason they paid for the technology.

      Fraser: How were Wegener’s theories received? He says, knowing the answer.

      Pamela: No one believed Alfred. They mocked him openly.

      Fraser: Yeah.

      Pamela: The planet was the way it was and this is after we’d already accepted things like evolution. This was after we had already accepted things like Earth goes around the sun, sun goes around the galaxy. We understood that the big bang existed.

      Fraser: The Milky Way was one galaxy and that those were other galaxies that were moving apart from each other.

      Pamela: Exactly, right.

      Fraser: We knew about how old the universe and the galaxy were.

      Pamela: Yeah, yet poor Alfred Wegener was mocked. People made fun of him. They said he was wrong. He plugged away at it looking for evidence. He looked at well look at all the seismology; there are clear lines of anger on our planet where we have volcanoes and earthquakes originating.

      The whole ring of fire – he was able to mark out all these different plate boundaries. If you look at mountain ranges, you can see the different levels of the planet upwelling in these great amazing rifts. You see these when you fly over the continent.

      He was able to just look at all these different lines of evidence including things like the flipping of the magnetic field of the Earth in some of these levels. Over time slowly he swayed people to believe him.

      Nowadays with GPS we can look out and go aha, England is moving away at a rate of roughly 3 centimeters a year. This is similar to the rate at which your fingernails grow. Now we believe him but like many new ideas.

      Fraser: How long did it take?

      Pamela: It took about a decade. We’re kind of slow to come around to new things occasionally.

      Fraser: It took about a decade for scientists to go from openly mocking him [laughter] to most scientists being onboard with the theory. Why was it so successful?

      Pamela: It explained it.

      Fraser: It explained it and amazing wonderful evidence.

      Pamela: What’s cool is now we can look at maps that are based in part on where we find fossils. We’re able to say we know these continents touched up until the Jurassic Period. We know these continents touched up until the Cretaceous Period.

      We’re able to follow – thanks to the fossil record – the divergence of the continents. We’re able to basically figure out well go back to the Permian it’s all one continent.

      Over time things migrated out to being what they are today. Even today we can see Saudi Arabia is working on tearing itself apart right now toward becoming its own land mass separate from Asia.

      Fraser: Now we have a much better scientific understanding of exactly what’s going on with plate tectonics. What is the story? What’s causing the drift?

      Pamela: It’s basically we have a hot planet. Things circulate. As the planet is trying to give off its heat we have this layer of liquid rock basically. We have very heavy lithosphere down at the bottoms of the ocean and much lighter materials making up the continents.

      The continents are able to kind of float around atop of a partial melt called the asthenosphere. We have plates made up as lithosphere with the heavy ones down at the bottom of the ocean. All of this is floating on top of this partially melted layer that is moving very slowly at geologic time scales.

      We have circulating hot liquid rock underneath. As the heat is released we have the plates moving relative to one another and heat bubbling out in the form of volcanoes at different places along the boundaries of the plates. It’s sort of like you can imagine you have a crusty material on top of a lava lamp.

      Fraser: You should really just refer to the boiling custard that we all make, right? [Laughter]

      Pamela: Right we found and example the other day online that this is clearly like everyday boiling custard of which I still haven’t seen what boiling custard looks like. I’m going to imagine a big giant lava lamp with crusty nastiness on top of it.

      As chunks of the thick part of the lava lamp come up to the surface and well back down, the stuff floating on top of that lava lamp is getting moved around. It’s getting broken apart so that heat can escape.

      Fraser: Right and this is just a result of the fact that there’s still a lot of heat escaping from the Earth. Even though the Earth is cooling there’s still a ton of heat down there.

      Pamela: Over time the heat that’s being generated from radioactive decays is going to slowly go away as the radioactive material finally uses itself up. Over time the heat that’s still left over from when the planet was formed is going to radiate away.

      Eventually our continents are going to lock themselves down in a given place. For now, everything is moving around.

      Fraser: We’ve got sort of two situations, right? We’ve got the places where the plates are coming together and we’ve got the places where the plates are coming apart. Why don’t we talk a bit about that?

      Pamela: We also have the places where the plates are just rubbing up side-by-side.

      Fraser: Let’s look at those 3 examples then. Where do we have them coming apart?

      Pamela: Saudi Arabia is one example of places coming apart. We also have the mid-ocean ridge out in the Atlantic Ocean where everything is completely coming apart.

      There are all sorts of neat basically black smokers and colonies of really weird critters living down purely off of the heat that’s coming out of this divide down at the bottom of the Atlantic Ocean.

      Fraser: It kind of looks like a crease where Earth is coming out and going in both directions. [Laughter] Right, Pamela?

      Pamela: Exactly.

      Fraser: It’s almost like fresh materials. That’s what it is, fresh materials just spreading out from those creases. Then you’ve got the situations where they’re coming together so what’s going on there?

      Pamela: Where the plates are coming together is where we start getting mountain ranges formed. Currently the Himalayas are still in the process of forming. India is still plowing its way northward.

      What’s cool is we can actually age where different parts of the planet careened into each other by the height of mountains. The Himalayan Mountains are the tallest mountains in the world and they are the youngest mountains in the world.

      India is still moving, still creating these mountains very, very slowly. This is where you have the convergence, you have mountains.

      Fraser: Right. Everest is still getting bigger.

      Pamela: Everest is still getting bigger.

      Fraser: You have situations where you get them sliding side-by-side, just rubbing past each other.

      Pamela: This is one of the problems that we have here in America. The whole western coast of California is trying to move. As California attempts to become more equatorial, you end up with all sorts of earthquakes.

      Basically the continents build up pressure, build up pressure and we have all kind of experienced this trying to move heavy objects across the floor.

      You have a heavy cardboard box filled with stuff on the asphalt of your driveway where there’s a lot of friction. You push and push and eventually it just gives and it slides.

      Occasionally you fall on your knees as it suddenly slides way more than you expected. That’s what happens when you have some of these earthquakes.

      You have along the fault line pressure building, pressure building and all of a sudden the pressure overcomes the frictional forces between the two plates and the plates slide. You get destruction.

      Fraser: I actually live on that same fault. I’m a little more north but pretty much right along that ring of fire right along the coast of the Pacific plate.

      We have the exact same risk up here in Canada where we can have really awful huge earthquakes every few hundred years.

      Pamela: If you look at a map of the planet that tracks where all of the different motions are going on you have much of America is actively attempting to go west and then California itself is trying to move in new and interesting directions.

      We have Australia working on trying to move itself north. We have India still working on trying to move itself. Europe is moving over toward Asia. This is where you have the Euro Mountains and the Alps all along in there.

      The Andes Mountains is where you have the edge of a crust trying to move inland essentially.

      All these different mountain ranges, all of this is caused by the motions of the plates. It leads to some rather traumatic things both under water and above land that we can see.

      Fraser: When we did our show quite awhile ago we talked about the Drake equation. If we were talking about how to expand the Drake equation one of the things that you proposed is whether or not a planet had plate tectonics. That could be necessary for life. I was just wondering why that is.

      Pamela: Plate techtonics aren’t so much a requirement for life but a diagnostic of things that are required for life. We have plate tectonics because our planet still has a molten moving circulating core.

      It’s these motions inside of our planet that create our magnetosphere, that create the magnetic field around the planet Earth that is able to protect us from incoming cosmic rays. It protects us from the sun and a whole lot of things that could kill us.

      Fraser: It also runs the carbon cycle.

      Pamela: Right, we have the circulation of it rains, things die, and things end up in the ground. Over time we’re actually flipping over layers of our planet just like you might till a garden.

      Fraser: So if we didn’t have the plate tectonics going on the carbon would just all remain on the surface and build up and build up. That sounds kind of familiar. [Laughter] Sounds like some other planet in the solar system.

      Pamela: Without being able to sequester carbon away underground first of all we wouldn’t have oil. Second of all we kind of would have a runaway greenhouse effect.

      One of the problems that we’re running into right now is we’re circumventing our planet’s carbon cycle. In a perfect solar system things would die on the surface of the planet and the planet would eventually bury them very deep.

      It would eventually turn them into things like coal and oil and petrified wood and all sorts of cool things that are far, far away from the surface of the planet. More importantly, they aren’t part of the atmosphere.

      Fraser: We’re just pulling them under as one plate goes under another plate. Just take it back down under the crust.

      Pamela: Without this we end up with things like carbon monoxide and carbon dioxide and methane and other complex organic badness building up in our atmosphere. Methane conveniently breaks down in sunlight but carbon monoxide and carbon dioxide don’t.

      If you get too much of them in the atmosphere the planet will slowly heat up. Right now we’re taking all of that nice friendly sequestered carbon-based material and burning it in power plants. That includes the engine of your car. That’s just a really small power plant.

      We’re releasing things that should be buried underground back into our atmosphere and we’re causing our planet – at least in part – to slowly heat up. This is bad.

      Fraser: Right. We know we have plate tectonics here on Earth but what about other places in the solar system?

      Pamela: When we look at other planets, like Mars and Venus, we don’t see plate tectonics that we recognize. We don’t see the big mid-ocean rifts because they don’t exactly have oceans.

      We don’t see the giant mountain ranges that are created from plates colliding so we don’t see basically the diverging and converging plates that we see here on the planet Earth.

      We do see evidence for motion. We do see they don’t have perfectly smooth surfaces and not every mountain is caused by a crater. Not every mountain is caused by a volcano.

      We do look at these two worlds and see evidence that in the past they did have some level of geologic activity. They may not have had the giant plates that we have but their surfaces certainly moved themselves around and their surfaces certainly were different in the past.

      It’s just that the majority of the restructuring happened due to volcanism which we talked about in our last episode.

      Fraser: Right but we talked about the carbon cycle here on Earth and we know that Venus has no plate tectonics so is that part of the problem for Venus?

      Pamela: Venus is just a whole variety of badness.

      Fraser: It’s closer to the sun.

      Pamela: It’s close to the sun. Once upon a time it had water on it but that water got too hot and ended up becoming water vapor. Water vapor is a greenhouse gas.

      The carbon on the planet ended up in the atmosphere. Now it has an atmosphere that is rich in organics that trap the heat on the planet. It rains sulphuric acid which is just a whole new form of badness.

      It has hydrochloric acid. It has basically acid in liquid form falling out of the sky. The thick impermeable layer of clouds that we can’t look through in the optical, those clouds also trap infrared heat.

      Sunlight goes through the clouds and heats up the planet. The rock of the surface of the planet tries to radiate away that heat in the form of infrared.

      The infrared just basically reflects back off of the clouds building up so that you end up with Venus hotter than the planet Mercury.

      Fraser: But that heat contributes to the fact that it has no continental drift, right?

      Pamela: Actually that is part of what’s going on with Venus. You don’t have the same huge temperature gradient that we have here on Earth. On Earth we have circulating hot liquid rock deep in our planet. As it rises, gives off its heat and settles back down it’s this convective process that is driving plate tectonics.

      On Venus you don’t have the huge temperature gradient as you come up through the rock and then hit really hot atmosphere; really hot surface of planet.

      Without the huge temperature gradients that you have here on the planet Earth you don’t have any circulation. You don’t have the driving forces that cause the plates to move.

      Instead you just end up with occasionally the entire planet resurfaces itself all at once as it gives off its heat.

      Fraser: You have no more carbon cycle so no way trap the heat back under. Things just stay the way they are.

      Pamela: You end up with a planet that we have trouble landing robots on because they’d melt quickly.

      Fraser: [Laughter] Yeah, then what about Mars?

      Pamela: Mars is tiny. People really don’t I think in general understand how small Mars is. Because it is tiny it never really held onto its heat.

      It’s sort of the difference between taking a giant loaf of bread out of the oven and taking a completely flat pan of cake out of the oven. That really thin flat pan of cake is going to cool off much faster than the big loaf of bread just because it has more surface area compared to the volume. It can radiate its heat through that surface area much faster.

      Mars just cooled off. It did have some heat, some liquid rock in its center. It just wasn’t enough to get the whole surface of the planet moving around. We can see evidence that it once had liquid rock in the form of the Olympus Mons volcanoes.

      We have giant volcanoes all in one place all on the surface of Mars, not that they’d be anywhere other than the surface of Mars. They’re all in one clustered area. They were able to grow to be some of the biggest mountains in the solar system.

      Fraser: Right that’s the Tharsis Bulge.

      Pamela: Yeah, one giant hot spot on the planet just fed and fed the growth of this mountain set.

      Fraser: Okay like here on Earth we’ve got the Hawaiian hot spot. You’ve got this long mountain chain or island chain of volcanic islands and that’s one hot spot. You’ve got the continental plate floating over the top of this hot spot and moving.

      You get cracks in the surface makes an island and then the plate moves and you get another island and so on. I guess on Mars it just never moved. It just kept going and going and going for billions of years.

      Pamela: What’s really cool is like what we said in the last episode the giant valley set that’s beside Olympus Mons – this is Valles Marineris – was formed with well if you relieve the pressure underneath the crust, if you remove all of the liquid rock from underneath the crust, it’s going to slump. It’s going to crack.

      That was enough to start this valley. The valley was also cut by water and it’s a complex geologic feature that people are still working to fully understand but I just love the idea that you grow the planet in one place and you have to shrink it in another.

      Fraser: Last week we talked about cryo-volcanism. Is there sort of similar things happening with like Europa?

      Pamela: One of the really cool things about Europa is you can actually see how the different parts of its crust move around. You can see the cracking, the striations; you can see the places where the different sections of the ice move apart from one another.

      You end up with upwelling of fluid. This is very much like the diverging areas that we have at the bottom of our ocean except it is occurring at the surface of a planet that we can conveniently image much more conveniently than we can image the bottom of our own ocean.

      Fraser: Right and there are big long cracks where you can see the new materials come up from underneath and it is spreading.

      Pamela: You can see the effects of where different places move relative to one another where you can actually see where a crack used to be a straight line.

      Two separate sections of ice moved parallel to one another and that crack got split and now one section of the crack is further north than the other section of the crack.

      Fraser: I think about when you break through or maybe you’re walking through a really thin ice puddle with boots on and the pieces of ice crack open. Chunks of ice floats slide across the top of other ones. It’s sort of a very similar situation going on but it’s with water and ice not rock.

      I guess the underlying mechanisms are all just the same. You’ve got a difference of energy, you’ve got liquid, and you’ve got solids.

      Pamela: You have circulation. You have convection. The physics is the same and what’s cool is by changing how much gravity there is and how viscous the material is, you’re going from lava to water.

      You’re also going from high gravity Earth to low gravity Europa. You can end up with very similar physics. It’s not identical, but it’s close and it’s cool.

      Fraser: It’s really cool. I think that covers our journey into plate tectonics, way out of your field Pamela. [Laughter] But thanks a lot for.

      Pamela: I’ve been hanging out with some really cool geophysicists periodically. If you ever want to meet really cool scientists who aren’t astronomers, go hang out with a geophysicist.

      Fraser: I guess the funny thing sort of is that – a friend of mine was talking to me about this – he says you guys go everywhere because it’s all connected. You have to.

      We’ve got a bunch of shows planned on biology because it is connected to astronomy, to the search for life and exploration of the solar system and plate tectonics, geology, and all that stuff and physics, quantum mechanics. It’s all connected.

      Pamela: This is where the word universe comes in because everything is included.

      Fraser: Right, you talk about everything in the universe. Exactly, I hope the listeners appreciate the wide range of topics we get to choose from.

      This transcript is not an exact match to the audio file. It has been edited for clarity. Transcription and editing by Cindy Leonard.