Volcanoes are incredibly powerful, but what causes one to form? Becca from our programs team talks about the origins of these explosive entities.

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ERIC: From the Museum of Science in Boston, this is Pulsar, a podcast where we dredge for answers to the most molten questions we've ever gotten from our visitors. I'm your host, Eric, and there are over 1,300 active volcanoes on the surface of our planet. Our visitors have lots of great questions about them, and one of the most common is: what causes a volcano to form at a given location? For the answer, I turned once again to Becca, one of my fellow educators on the Puseum Programs team who always seems to have the answer to any volcano questions I've ever had. Becca, thanks for joining me again on the podcast.

BECCA: Thanks, Eric. I am very excited to talk about one of my favorite subjects yet again.

ERIC: So where do volcanoes come from?

BECCA: This is a great question. And there are several answers. Our planet has plate tectonics, and our plate tectonics act in many different ways. And the way that these plates interact with each other, the outer layer of our crust causes for us to have plate boundaries. And these boundaries can be one of three types, two of which can produce volcanoes. So we have transform boundaries, which do not produce volcanoes, we have convergent boundaries where two plates are colliding together, they can produce volcanoes. And we have divergent boundaries where two plates are pulling apart from each other. And they can also produce volcanoes.

ERIC: So we've got those plate tectonics. And that's the outer layer of the surface moving around. What is the thickness of that layer is moving around? Is it like a couple miles or is like a couple hundred miles?

BECCA: It can be pretty deep, especially depending on where you are on the planet. Ocean crust is much thinner, only about maybe seven to ten kilometers thick. Continental crust can be quite a bit thicker, more like 40 kilometers thick, or even a little bit more, depending on where on the continent you are.

ERIC: So plate boundaries where two plates meet each other are where you usually find volcanoes and they're not likely to pop up in random spots. So we wouldn't get a volcano just appearing anywhere in Massachusetts without good warning.

BECCA: In Massachusetts at the moment, probably not. But unfortunately, just like many other branches of science, there are exceptions to the rules. And there are volcanoes that can pop up that are not specifically on plate boundaries. They are what we call hotspot volcanoes. And they occur when there is a plume of magma that comes up from the mantle and pokes a hole through the crust, usually under oceanic crust, but sometimes under continental crust.

ERIC: So is that what's going on in Hawaii, why we get those chain of volcanic islands, the volcanoes all the way in the middle of the ocean?

BECCA: That's exactly right. Our volcanoes in Hawaii are hotspot volcanoes. And because we have this nice long chain, it's actually the Pacific plate that has moved over this hotspot causing for different islands to be created. And in fact, we have a new Hawaiian island that will be popping up sometime in the next several thousand, maybe even longer, years from now.

ERIC: So prospect your real estate now when we know another island's gonna show up.

BECCA: Oh, yeah, Hawaii is growing.

ERIC: You mentioned a couple different ways those plates can interact with each other. Why don't we start with them smashing together - you called that convergent?

BECCA: Absolutely. Convergent boundaries are when two, or more I suppose depending on where you are, plates will collide with each other. Now oftentimes we end up with an oceanic plate colliding with a continental plate. And because the oceanic plate is thinner, but more dense, and the continental plate is thicker, but less dense, the oceanic plate often gets dragged underneath the continental plate, something we call subduction. And as it's getting dragged down, some of it will melt and turn into magma. And then that magma ends up rising towards the top and ends up poking a bunch of holes, basically, into that continental plate. They're creating a long chain of volcanoes down the entire edge of the continental plate where that oceanic plate is subducting.

ERIC: I feel like sometimes I see this process in a book or in an animation. And it's got, like, the two slabs of land kind of pushing against each other, and one goes down and one goes up, and you lose the scale of what we're talking about here. I mean, entire sections of the earth slamming into each other at incredibly slow speeds, but with such incredible force that chunks and layers get pushed under each other, and volcanoes form, because you have that magma just bubbling up with intense pressure. I mean, do you ever stop and think about how intense the scale is? How huge the stuff is? It is truly amazing.

BECCA: I mean, just take, for example, the Ring of Fire. This is an entire area around the Pacific Ocean that is just cluttered with volcanoes that are active, some of them dormant, some of them not active at the moment, some of them active very regularly. And this area is just caused by the subduction of the Pacific plate underneath the plates around it, which is pretty amazing to think about.

ERIC: So you also said that sometimes the plates can pull apart from each other. So what happens there? There's no, like, hole in the earth that's opened up all the way down to the next layer. So what what happens to fill in that space? Do you get volcanoes there?

BECCA: Absolutely. Now that is a great question. When two plates pull apart in a divergent boundary, this does often happen in the ocean with two oceanic plates. But it can also happen with continental plates as well. But say for example, on the two oceanic plates, we have the Mid Atlantic Ridge, this is an area where two oceanic plates there are pulling away from each other. And new crust is coming up in the form of magma that then cools and becomes new crust on our planet. And by doing so it's actually coming up through underwater vents and volcanoes. So we ended up with a long chain of underwater volcanoes down the middle of the Atlantic Ocean, which is a pretty interesting thing to think about. And it's something that we don't see a lot of evidence from, because it's so deep under the ocean that we have to send submersibles there in order to actually see what's going on.

ERIC: And I wish we had more great video of these underwater volcanoes. When we've showed them, it's usually like one or two short clips of lava coming out and then steaming up and then cooling off. But you don't really get a sense that it's a volcano, it just looks like one isolated little ten-foot-wide spot. I really want like a full-fledged volcano underwater, like, excursion to go and be able to map that. Do you think we have the technology soon?

BECCA: I would hope so. I mean, we're developing new technology all the time, it would be great to be able to map them. Although the underwater volcanoes may not be as volcano-y as you're thinking just because they don't have that massive ash explosion, because it's being trapped by the water. So it won't spread as far as it would in the air. Although it definitely does have some different gases come out and has that lava that comes out of it too. And it certainly looks somewhat impressive, but it would be great to get some video footage of it.

ERIC: So the other kind of boundary is when two plates aren't really going towards or away from each other, but kind of slip side to side. Do you get volcanoes there?

BECCA: So on a transform boundary, two plates are sliding past each other. And so they don't actually create any volcanoes because there's no magma chamber underneath this type of boundary. But this does create some massive earthquakes. Now earthquakes can happen at any of these plate boundaries, and certainly do but transform boundaries, their biggest hazard are earthquakes. So the San Andreas Fault in California, that is a transform boundary where those two plates are sliding by each other. We don't have any volcanoes in this part of California. But we do happen to have a lot of earthquakes that happen there because of these two plates moving by each other.

ERIC: So when we see pictures of volcanoes, there's cones and shields, different shapes of volcanoes. Does that have to do with the specific area? And what's going on under the ground? Does it have to do with what kind of boundary you get? Do you always get a cone from one of these kinds of ways to make a volcano?

BECCA: That's a great question. It definitely differs depending on what the type of boundary is. That definitely has a major impact on the type of volcano that will be produced. But it also does have a lot to do with the mineralogy, with the elements that are inside of the magma itself. The more silicon content, the more likely that it will be a more explosive volcano, whereas the less, the more likely it will be more of what we call effusive or just kind of bubbling up a little more and having a little less of an eruption. Generally, hotspot volcanoes like Hawaii are going to produce those massive shield volcanoes. So that's going to happen in hotspot situation and also can happen in a divergent boundary situation. Whereas in a convergent boundary situation, you're more likely to develop those really tall, really massive explosive volcanoes like the Mount St. Helens or any of the ones down in South America. You would find the more cone-shaped more explosive ones on that type of boundary. So it has a little bit to do with the type of boundary but also the type of minerals that are in the magma itself.

ERIC: And it also seems like every volcano has its own personality, where you tend to see in the news when the ones on Hawaii erupting, you see footage of the slow oozing lava, and you don't see too many like giant explosions. But with Mount St. Helens, it was before I was born, but I've read about how, like, instantaneous and catastrophic that was. I mean, there was warning, but it was also not lava flows. It was like, the whole side of the volcano, imploding and detonating and a huge amount of energy released all at once.

BECCA: Yeah, it can be incredibly hard to predict volcanoes because as you just said, every single volcano is different. No two volcanoes are going to behave in the exact same way. And that is why volcanoes are so fascinating, because every single one is unique. And that's also what makes predicting future eruptions incredibly difficult. We can look for warning signs like an increase in earthquakes or the magma chamber rising and falling a little bit. But there is no exact way that we have right now to predict exactly when an eruption will occur.

ERIC: Well, Becca, thanks for telling us all about where volcanoes come from.

BECCA: Absolutely. I'm always happy to talk about volcanoes. Thanks, Eric.

ERIC: Next time you're at the Museum of Science, get up close with rocks that were formed and volcanic eruptions in our Rock Garden on our front lawn. And while you're at home, visit the rock garden virtuallyand learn how we can put a date on volcanic activity from thouands or millions of years ago in our sparks of science video series. Until next time, keep asking questions

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