This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. Water is strongly implicated in many of the most powerful volcanic eruptions. In this episode, we'll talk about the two main ways in which this comes about. The first is when magma comes into contact with external water, whether from oceans, groundwater or glaciers. The second is when water is already present within the magma.

Michael Manga studies how geological processes affect and are affected by water. He also studies similar processes on other planets and on the icy moons of the outer solar system. He is a professor in the Department of Earth and Planetary Science at the University of California, Berkeley. Michael Manga, welcome to Geology Bites.

Michael Manga: Oliver, it's a pleasure to join you today.

Oliver Strimpel: Let's talk about the first of the two ways that water affects eruptions, the so-called phreatomagmatic eruptions, when external water and hot magma come into contact. How does this amplify the explosive power of an eruption?

Michael Manga: You highlighted something in that question, Oliver, that's really important. Maybe we should define the word phreatomagmatic. "Phreato" means water, and "magmatic" refers to the melted rock. The key is that magma is hot and water is cold, and heat is transferred from hot objects to cold bodies. As the magma cools down, it becomes brittle and it breaks. Maybe some of your listeners have had the unfortunate experience of putting some water into hot glass, and it shatters as it cools too quickly. This will make the small pieces of magma that become volcanic ash that can erupt explosively. The water, in turn, is of course cooling the magma, but it's heating up because it's taking that heat, and liquid water becomes steam, and steam takes up much more space than the liquid water did. That huge volume expansion carries the fragmented magma, the volcanic ash, with it as it expands.

In most physical systems that we think about, we have motion, which you can think of as kinetic energy. When you rub your hands together, your hands are moving; they have kinetic energy. And that energy is dissipated to make heat when your hands rub against each other; that's friction. Most processes in the Earth take kinetic energy and convert it into heat. What's special about phreatomagmatic eruptions is that the energy transfer goes the other way. We can take the heat in the magma and convert it into kinetic energy to make these eruptions so explosive.

Oliver Strimpel: So because, as you said, when water turns to steam there's a huge increase in volume, can steam therefore be generated even when the magma contacts water at great depths, and therefore under great pressure, under the ocean?

Michael Manga: As we go deeper inside the Earth, or deeper under the surface of the ocean, the pressure goes up, and as pressure goes up, gases contract. This matters for two reasons. First, liquid water at high pressures doesn't convert into steam; it doesn't boil. It becomes a supercritical fluid. And the second effect of higher pressure is that the boiling temperature goes up as well. So at the bottom of the ocean, the temperature at which water boils can be as high as 400°C, and it can be very difficult for magma to heat water up to that temperature. So the high pressure inside the Earth has two effects: it increases the boiling temperature, and in fact it can keep boiling from happening. And if you don't boil water, you don't have the big volume expansion that drives explosive eruptions.

Oliver Strimpel: And so what happens instead is that you get pillow lavas forming?

Michael Manga: Yeah, pillow lavas. The name is very descriptive: you see these little blobs of lava that look like pillows, hence their name. Those are erupted in a style we call effusive. Effusive means lava flowing over the surface in a sort of coherent, intact manner, as opposed to breaking up into little pieces, or fragmenting, which is what happens in explosive eruptions. Pillow lavas form when low-viscosity magmas are extruded onto the bottom of the ocean, and they cool pretty quickly, which is why they get the pillow shape, as opposed to making the very thick lava flows we see on land. So the interaction with water in submarine settings that makes pillow lavas is still very important, cooling the outside of the lava, but it does suppress explosive eruption.

Oliver Strimpel: In 2022, we experienced the most powerful eruption in recent memory, and that was the Hunga Tonga eruption. Can you describe that eruption and compare it to other eruptions, just to put it in context, such as the Mount St. Helens eruption in 1980 or Mount Pinatubo in 1991? And I guess water must have been implicated in that one.

Michael Manga: Yeah. The Hunga eruption of 2022 was the most powerful volcanic eruption in our lifetimes. It was so powerful that people in Alaska, 8,000 kilometers away, could hear it. The plume reached a height of 57 kilometers, which is higher than any eruption we have documented. It set a record for the number of lightning strikes. Just as thunderstorms create lightning, volcanic eruptions will also create lightning, by very rapidly carrying ice and magma to great heights. This eruption also created global tsunamis, through coupling between the atmosphere, the ocean and the solid part of the Earth. And it was, of course, devastating for the Kingdom of Tonga. So it was an extremely powerful eruption, and much of the power was because it was a shallow submarine eruption. The magma fragmented underwater, and those fragments could interact with the surrounding liquid water to make steam. And because the pressure was pretty low, since it was a shallow eruption, a lot of steam could be generated, and that steam helped power the great energy of the eruption.

You asked about how to put this in context. You and I were alive during the 1980 Mount St. Helens eruption. This eruption was 10 times bigger than Mount St. Helens, and was similar to the Pinatubo eruption in the Philippines. It is small, though, in comparison with some of the biggest eruptions we know about, what we call super-eruptions, the ones that make things like the big calderas we see in Yellowstone National Park.

Oliver Strimpel: About 10 years before the Hunga Tonga eruption, there was another eruption, called the Havre eruption, not too far away, but that had a very different outcome. Can you tell us about that?

Michael Manga: Absolutely. The Havre submarine eruption occurred south of Hunga, along a volcanic arc. Volcanic arcs are produced where plates sink into the Earth's mantle. The northern part of the subduction zone is the Tonga arc; the Kermadec arc is south of that, just north of New Zealand. The Havre volcano is in the Kermadec arc, and it erupted in 2012. It was a deeper submarine eruption. The vent was 900 meters below the ocean surface, and that's deep enough that the way the volcano erupted was really quite fundamentally different. It was also smaller, although once again bigger than Mount St. Helens was in 1980. If we call this a deep submarine eruption, the Havre eruption was the largest deep submarine eruption since 1650, and it created a raft of floating pumice. Pumice is a volcanic rock full of bubbles, and there are enough bubbles in this rock that the fragments can float on the ocean surface. It created a huge raft of floating pumice, and that pumice, because it floats, circumnavigates the planet with the ocean currents, eventually washing up on beaches.

The eruption style, though, is quite different. The magma, as it entered the ocean from the volcanic vent, was full of bubbles, but it hadn't yet fragmented, and so it was extruded, or squeezed, into the ocean. There were enough bubbles in it that it was lighter than the ocean, so the magma floated up to the ocean surface, and as it interacted with the surrounding water, the magma quenched. It cooled very quickly and broke up into the pieces of pumice that float on the ocean. Since we're in my office today, I can show you a piece of the pumice, if that would be of interest.

Oliver Strimpel: Yeah, please.

Michael Manga: Your listeners probably can't see, but these are two pieces I collected in New Zealand. They were transported a long distance for a long time. And unlike most things that break, they're not angular; they're rounded on the sides. They're rounded because these pieces of pumice, as they float on the ocean surface, are rubbing against each other, kind of like pebbles on a beach, and so they get progressively rounder and smaller as well. Of course, you can't feel them to see how light they are, but I do have a container of water, and if I put the piece of pumice on the water, you can see that it floats high and dry. And of course it has to float, right? It got all the way to New Zealand from the volcanic vent. And this pumice will float for years and years.

The thing we're learning about submarine eruptions, which I guess we hadn't properly appreciated, not having had the chance to witness submarine eruptions, and especially deep vents, until 2012, is that ocean depth is fundamentally important. At great depths, you suppress fragmentation, but the fragments are still lighter than water, so they can float on the ocean surface. At shallow depths, you have an explosive eruption surrounded by liquid water, and you can get very explosive eruptions, more explosive than we had previously anticipated.

Oliver Strimpel: Roughly at what depth do we switch over from these highly explosive steam eruptions to the more contained ones that form either pillows or pumice?

Michael Manga: Yeah, that's an interesting question that I have actually tried to think about. How do we map depth onto eruption style? Let's go from the deepest to the shallowest. The critical pressure for water to form steam is reached at about 2,200 meters. So deeper than 2,200 meters, you should expect mostly effusive eruptions. And the average depth of mid-ocean ridges, where oceanic crust is formed, is 2,500 meters. The details of what that critical depth is depend on the eruption rate, how fast the magma is erupting, and whether it can fragment. But for the Havre eruption in 2012 that formed the raft of floating pumice, if the vent had been shallower than two or three hundred meters, it probably would have been an explosive eruption, much like Hunga. And that's probably useful for hazard assessment. You can look at the volcanoes around the Earth and identify those in submarine settings that are below sea level, or could become below sea level if they were to erupt.

Oliver Strimpel: Are there some cases where eruptions start below that 2,200-meter threshold, but because they produce so much lava, they build up a kind of seamount, get to that critical height and then start to become steam-driven?

Michael Manga: The answer is yes. Any volcano we see poking out of the ocean had to start its life below sea level. The islands of Hawaii are good examples, where the tops of the mountains are way above sea level, high enough that there are even little glaciers on the tallest peaks on Hawaii. So those volcanoes grew from being in a deep submarine setting to a shallow submarine setting to being above land. And the converse is true too: as volcanoes erupt and their flanks collapse, what was once above sea level can then migrate below sea level.

Oliver Strimpel: Yeah, but you're not likely to go from a great depth to less than 2,200 meters of depth in a single eruption. You're talking about a sequence of eruptions over many years.

Michael Manga: Yeah. To grow a volcano a kilometer in height will take a long time, probably many eruptions. And I don't know of any good examples where we have documented the detailed growth of a volcanic edifice under the ocean. That question highlights one of the challenges in understanding submarine eruptions: to look at them and observe them, we have to go under the ocean, because our cameras can't see below the ocean surface. That requires sending either autonomous vehicles or remotely operated vehicles, or using sonar or some kind of remote mapping technology, to repeatedly survey the sea floor. It's not something we can monitor easily and continuously.

Oliver Strimpel: Is this effect of magma contacting external water also what drives geysers, hot springs and mud volcanoes?

Michael Manga: Geysers are a special type of hot spring. They erupt boiling water, and they erupt episodically. When they do erupt, they're erupting a mixture of liquid water and water vapor, or steam. Actually, geyser eruptions can be energetic enough to carry little fragments of rock with them. There's almost a continuum between geyser eruptions and hydrothermal explosions, where hydrothermal explosion means "hydro," water, "explosion," things break apart and rock fragments get ejected. For all the geyser fields that we know of, their energy source is in fact magma underground. There's no magma directly erupting in the geyser, but the heat source is in fact magmas that are solidifying and cooling underground, transferring their heat to the groundwater.

Oliver Strimpel: So when you get an intermittent eruption of a geyser, is it the periodic passage of some water over this magma?

Michael Manga: When we look at a geyser erupting, we're seeing the very surficial manifestation of that heat transport deeper down. The groundwater flow that's carrying that heat is probably much more steady. The water that's taking the heat out of the magmas is probably circulating to depths of many hundreds of meters, or even a few kilometers, and carrying that heat up to shallower reservoirs. And actually, from studies of geysers, direct imaging with cameras or geophysical measurements beneath geysers, it looks like there are cavities in which heat and steam accumulate. In fact, these may be analogous to the reservoirs in which magma accumulates leading up to an eruption. So there are some similarities between geyser eruptions and volcanic eruptions, in that they're episodic; they're not erupting continuously. And there are reservoirs that are accumulating the gases and the materials that erupt, and the heat. When the pressure gets big enough, the eruption begins, the fluids are evacuated, the eruption ends, and then the process repeats. But many geysers are quite irregular. There's only a very special subset that are truly periodic, where periodic means the interval between eruptions is always the same. And even for those that are periodic, that periodicity varies over decadal time scales, or can be influenced by earthquakes or other processes.

Oliver Strimpel: What about mud volcanoes?

Michael Manga: Mud volcanoes also erupt because the pressure where the mud is coming from gets too high, and so materials are ejected to the surface. With too much pressure underground, mud volcanoes erupt sediment, meaning mud and rock fragments, along with a mixture of water and gas. Often that gas is methane, natural gas. That methane is produced from the organic material that's buried with the sediment, and as it's heated to a high enough temperature, that organic material is converted into methane. We often find mud volcanoes in places where we also extract hydrocarbons, oil and gas, from the Earth. And mud volcanoes can erupt this mixture of fluids, water, gas and sediment, from as deep as 10 kilometers. They create landforms that resemble magmatic volcanoes in many ways. They can erupt sort of explosively. They can create mudflows. Mud volcanoes can create mountains. They can create things that look like lava flows.

In fact, I had the great pleasure of going to Azerbaijan, which is famous for many things, but for many people the mud volcanoes are the big tourist attraction in Azerbaijan. There are fields of mud volcanoes on land as well as under the sea, under the Caspian Sea. And the source of the gas and the mud and the fluids is the same sedimentary formations from which Azerbaijan is extracting oil and gas. If I were just looking at a satellite image of the morphology of the flows, I would not easily be able to tell, maybe not be able to tell at all, that those were made of mud and not lava. They flow downhill. They make long flows. They look a lot like rhyolite flows, very silica-rich flows. And they can also create spatter, fragmented mud that falls on the ground. And on Mars, there is debate about whether some of the landforms people see, little cones, are magmatic cones or were produced by mud volcanoes.

Oliver Strimpel: Presumably mud volcanoes don't last as long as magmatic ones. They must be very soft and easily washed away.

Michael Manga: Yes. On a planet where there's precipitation, that precipitation will erode the sediment faster than it would erode lava. And I guess they're best preserved on land in very dry places, but also under the ocean. A lot of the mud volcanoes on Earth form in submarine settings, and they're better preserved there.

Oliver Strimpel: What happens when there are eruptions under ice? Do you get the same amplifying effect? And can we see this kind of thing happening in Iceland?

Michael Manga: Ice is just the solidified form of water. So in the same way that magmas, when they interact with water, can make steam and help drive explosive eruptions, the same process can happen when magma erupts under ice. But first you have to melt the ice, and all that meltwater that you're producing does help cool the magma and help it solidify. So lava flows that erupt under ice often cool fast enough that they don't erupt explosively. But there is a distinctive signature of lavas as they cool. When magmas cool, they contract, and when they contract, they make cracks to accommodate that change in volume. Maybe some of your listeners have been to the Giant's Causeway in Northern Ireland. Here in California, there's a place called Devils Postpile. These are sort of hexagonal vertical columns of lava. The columns are bounded by what we call cooling joints, the cracks made as the magma cools, and those cracks propagate perpendicular to the cooling front. So if you cool a flat lava flow underwater, you get vertical cooling joints. If you're completely surrounded by ice, those cooling joints will be perpendicular to the ice-lava contact, and so they'll often radiate, a distinctive signature of lavas that cool under ice.

In 2010, you may remember, the Eyjafjallajökull eruption happened in Iceland and closed down air traffic in northern Europe. That was probably the greatest disruption to air traffic in Europe since World War II. There, the volcano erupted under ice, and the meltwater flowed into the volcanic vent, helping to drive the explosivity of that eruption. That water had two effects. The water helped quench the magma to make the volcanic ash, and the steam generated from that cooling helped power the eruption.

Oliver Strimpel: Let's talk about the second important way in which water affects an eruption, namely when water is already present in some form within the erupting magma. How does that amplify eruptions, and where exactly does that water come from?

Michael Manga: There are many parts to this question. Let's start, maybe, with where the water comes from. Magma is melted rock, and when you melt a rock, different elements in the periodic table either like to stay in the solid that's left over or like to go into the melt that's produced. Water and lots of the other gases prefer to go into the melt. So if you take a rock that has water in it and melt it, most of that water ends up going into the melt that's produced. And where's that water coming from? There can be little bits of water trapped between crystals inside a rock, in what we call the pore space. But lots of minerals also incorporate water directly in their mineral structure. Clay minerals are an example. There's a mineral called amphibole that incorporates water in its structure. So when you melt a rock that contains amphibole, or rocks that contain clay, the water that was present ends up going into the melt.

And that water in the melt is fundamentally important for why we have volcanoes at all. Volcanoes erupt because the melt that's produced is less dense than the surrounding crust. So for magma to erupt, we have to make it less dense. One way to make something less dense is to fill it full of gas, and at low pressure, the water that was dissolved in the magma can no longer stay dissolved. It comes out of solution and forms bubbles, and it's the presence of those bubbles that helps drive magma to the surface.

So how do we get all this water into the interior of the Earth in the first place? The process of plate tectonics, where we create plates at the Earth's surface that move horizontally and then sink into the Earth through a process called subduction, is a great way to bring water from the Earth's surface, from the oceans, into the Earth. That water is brought into the Earth through subduction in two ways. The sediments on top of the subducting plates, as they sink into the Earth, carry with them the water between the sediment grains, and the minerals that contain water get subducted as well. Then at high pressure, those minerals can't hold that water, and it's released into the overlying part of the Earth that we call the mantle. That helps drive melting of the mantle. And when you melt wet rock, the water goes into the melt, it's carried upwards with that melt, and it then powers the volcanic eruptions we see above subduction zones.

I guess the other concept we should talk about is why it is that we make bubbles inside magma from that dissolved water. The solubility of gases inside liquids increases with increasing pressure, and your listeners will know this from their everyday experience. When you open a carbonated beverage, when you take off the top or you open the can, you decrease the pressure, and that carbonated beverage starts to fizz, because you've lowered the pressure. That's because the solubility of those gases is lower at low pressure, so the water can't dissolve that carbon dioxide, and it starts forming bubbles. Magma does the same thing. At high pressure, it can dissolve a lot of water, but as the magma rises to shallower depths, the pressure decreases, the water that was dissolved in the magma forms bubbles, and those bubbles grow. And if the magma keeps rising, the solubility of those gases keeps decreasing, and the gases expand. It's that rapid expansion of the gas and the formation of bubbles that drives magma to the surface and makes volcanoes erupt, and erupt explosively.

Oliver Strimpel: So are we more likely to see wet magma, or the effects of water and the bubbles that you just described, in the tectonic settings where water has just been subducted, i.e. in or above subduction zones?

Michael Manga: Yeah, exactly. Most of Earth's most explosive volcanic eruptions happen in places where you can get a lot of water into magmas, and the best places to do that are subduction zones, where you're bringing water down with the subducting plate. That water can enter the source region for the magma and then gets carried back up to the surface. At mid-ocean ridges, where plates are being formed, the magmas tend to be relatively dry. At subduction zones, they tend to be quite wet.

Oliver Strimpel: I'm not sure if this is related or a separate factor, but I know that the silica content of the melt is a critical determinant of viscosity, with the viscosity increasing quite dramatically as silica content increases. And since high-viscosity magma tends to block the conduits as it cools, it can lead to explosive eruptions when the pressure builds up and it kind of blows its top. Is this silica content variability independent of the volatile or water content of the magma?

Michael Manga: Maybe we should put the viscosity of magmas in perspective. The lowest-silica magmas we see, the types of rocks we call basalts, have a viscosity, or consistency, very similar to something like syrup or honey. The more silica-rich magmas, like those we see in volcanic arcs, have a viscosity similar to asphalt, so more than a million times greater. The ability to flow is inversely proportional to viscosity, and so that means the magmas in volcanic arcs will flow a million times more slowly, given the same forces driving their motion. So the amount of silica in magmas has a huge effect on how they can erupt.

Your question was whether there's a relationship between the amount of water and the amount of silica. Observations say that's the case, and there's also a reason why. But I should point out that understanding how we make silica-rich magmas is still the subject of quite active research: understanding which processes dominate, and why. There are two main ways in which we change the silica content of magmas. One is that you can heat the surrounding rock, melt some of it and incorporate that into the magma sitting in the crust. We call that assimilation, where you incorporate the surrounding rock. The other process is that as magmas sit in the crust, they cool, and they start to crystallize as they cool. Those crystals can settle out of the magma and be separated from the remaining liquid. We call that fractional crystallization: we crystallize, and we remove some of those crystals. And it turns out the first crystals that form are relatively low in silica. That means the magma that's left over has more silica. So through this combination of crystallizing and removing crystals, and assimilating surrounding crust, over time the magma gets more silica-rich. And earlier we talked about water liking to stay in the melt. So in the process of melting the surrounding rock, or of crystallizing, whatever water was present gets concentrated in the melt that's left over. So in the process of enriching magma in silica, you're also enriching it in water, and the two tend to increase together over time.

Oliver Strimpel: Let's talk about how water is implicated in eruptions elsewhere in the solar system. Mars, for example, used to have lots of surface water, and it has some giant volcanoes. Can we see evidence of water affecting the volcanic activity there?

Michael Manga: Mars used to have abundant liquid water at its surface, and there's still plenty of water on the surface today. We can see ground ice. The Phoenix lander exposed some of it when it dug into the ground. We can map some of the water with gamma rays from spacecraft, and there are big ice sheets at the north and south poles. I mention ice because there's a distinctive landform that's produced when lava flows flow over ice. We said that magma transfers heat to the surrounding environment, and if that's water, it can vaporize that water. When you flow over ice, you can melt some of that ice, and then you vaporize it. So now imagine a lava flow flowing on top of ice. It's heating the underlying ice, which could be in the ground. The steam that's generated is buoyant and expanding, and it blasts its way through the lava flow, making a tiny little volcano. We can see fields of these little volcanoes on top of lava flows in Iceland, and people call them rootless cones, rootless because there's no actual magma source deep underground; it's just the lava flow on which they're sitting. And people have mapped out fields of rootless cones on Mars, so that's evidence that lavas flowed over frozen ground.

The youngest volcanism on Mars happened in a place called Cerberus Fossae, just south of the Elysium volcanoes. There are some long fissures we see on the surface of Mars there. This is also where many of the recent marsquakes detected by the InSight lander came from. And it looks like there was recent explosive volcanism in that area that was generated by magma interacting with external water, either ice that it melted, incorporating water like we see in Iceland, or groundwater.

Oliver Strimpel: What about some of the icy moons in the outer solar system, such as Saturn's moon Enceladus?

Michael Manga: In my lifetime, this was probably one of the most exciting observations from space exploration: detecting active eruptions on Enceladus. Enceladus is a small moon of Saturn. It's got a radius of 250 kilometers, it's covered with ice, and it's very cold out there. So it's hard to imagine that four and a half billion years after the formation of our solar system, something as small as a little ice ball like Enceladus could have both liquid water and be erupting that liquid water. But it is. The Cassini spacecraft imaged that plume, and it also flew through the plume, collected ice particles and measured their composition. So now we understand that those eruptions are sourced from an ocean underneath the ice-covered surface, probably at a depth of, say, 10 kilometers. That ocean is a global ocean in contact with the rocky interior. We know that because the eruption is ejecting silica nanoparticles that come from that interaction with the rocky interior. And those are very powerful eruptions. They're sending ice particles into one of the rings of Saturn, called the E ring. Not everything reaches escape velocity and leaves Enceladus, so a lot of the erupted material falls back on the surface as snow. That actually makes Enceladus the most reflective object in our solar system, from all that snow.

And the eruptions on Enceladus are interesting. They're probably most like geysers. Geysers, we said, erupt boiling water, and as liquid water rises through these cracks at the south polar region of Enceladus and gets to low pressure, that water also boils. The volume expansion from boiling helps drive the eruption. And once everything enters space, whatever liquid was there cools very quickly and forms ice particles.

Oliver Strimpel: And the source of the heating to make it boil is tidal distortion?

Michael Manga: Yeah. The source of heating for the oceans in the outer solar system's ocean worlds is energy from tidal deformation. The orbits of these satellites around their host planet are not perfectly circular. So when the satellite gets closer to the planet, the gravitational attraction is bigger and the satellite deforms more, and then when it moves further away, it deforms less. That squishing and unsquishing of the satellite creates heat through friction, and that heat from friction warms the interior. For some of these ocean worlds, that warming is enough to melt the ice and make a liquid ocean.

Oliver Strimpel: Europa, which is one of the Galilean moons of Jupiter, is also covered in ice, and it's crisscrossed with fractures. Do we see water erupting out of the ice there as well?

Michael Manga: Yeah. Whether there are eruptions analogous to those we see on Enceladus happening on Europa is uncertain; you might almost say controversial. The Hubble Space Telescope, for example, saw a plume, or an anomaly, of water vapor above the surface, but it's not detected all the time. So the question is, was it a false detection, or contamination of some signal somehow? Or it could be that the eruptions are episodic, so they're on sometimes and not on other times. From the Galileo mission, it's not obvious that anything it saw could be attributed to active eruptions. But certainly when we look at the surface of Europa, we see features that are most easily explained, I think, by liquid water being either on the surface or very close to the surface. There's a type of terrain on Europa called chaos terrain, where you see something that looks like a bunch of icebergs that have been displaced and rotated a little bit. That requires mobility of those blocks of ice, and to enable mobility, it helps to have a liquid, like liquid water. And we do know that Europa has a liquid ocean underneath its ice-covered surface. It's a deep ocean, probably about 100 kilometers deep.

Oliver Strimpel: And again, like Enceladus, is it tidal deformation heating it up and causing the water to seep up or blow through the cracks?

Michael Manga: Yeah. Jupiter has four big moons, the Galilean moons, Io, Europa, Ganymede and Callisto, and Callisto, Ganymede and Europa are probably all ocean worlds.

Oliver Strimpel: What about Io, then? We did an episode on it earlier. It's the most volcanic object in the solar system, but is there water involved there?

Michael Manga: Io is the most volcanically active body in our solar system. Unlike its three neighbors, the ocean worlds covered in ice, it's closer to Jupiter. Its tidal deformation heating is much greater, and so it's much hotter. It's so warm that all the water that would have been there has long since boiled away. But to make magmas erupt, you need something to lower the density. So it could be that sulfur on Io plays the role that water plays in magmas on Earth. It's the volatile component that starts forming gases and bubbles that helps drive the volcanic activity on Io.

Oliver Strimpel: If you could decide where to send a new exploratory space mission to learn more about volcanism in the solar system, especially with regard to the effect of water, where would you choose?

Michael Manga: Enceladus, I think, is an easy choice, in part because we can directly sample the ocean. The eruptions on Enceladus are delivering that ocean to us above the surface, making it easy to sample and easy to collect. And it's a very fresh sample as well. But I am always reminded that every time we visit a body we have not gone to before, there are great surprises. I never would have guessed, until New Horizons flew by Pluto, that Pluto was such an interesting ocean world with such a rich geological history. I expected it would be a big ball of ice, and it's anything but. And I think the same is true for our exploration of Enceladus and the other moons in the Saturn system. They turn out to be quite a bit more interesting than we originally expected, in part maybe because our imagination is not good enough.

Regardless, every decade or so NASA commissions the National Academy of Sciences to undertake a decadal survey to identify the highest-priority targets for space exploration. The most recent Planetary Science Decadal Survey recommended a Uranus orbiter to visit Uranus, and Uranus has a number of interesting moons. That would be its highest-priority flagship mission, followed by an Enceladus mission. So Enceladus and its eruptions are clearly a high community priority for exploration. Of course, we have missions on the way. Let's see. The Dragonfly mission is heading to Titan. Whether Titan is an ocean world or not is the subject of some debate. I think until a year ago we all would have said Titan has an ocean. There's a paper published in Nature on December 17th, 2025, arguing that in fact Titan may not be an ocean world.

Oliver Strimpel: But it would be an ocean of methane, though?

Michael Manga: Titan has lakes and rivers and an atmospheric cycle where there is a liquid, and that liquid is hydrocarbons, methane. But that's in the very shallow part of Titan. As we go deeper below the surface, just like Enceladus or Europa, the interior is probably mostly water ice, and if you get deep enough, the water ice can melt to make an ocean of water. We used to think Titan had an ocean; that may not be the case. But we do have other missions besides Dragonfly, which has still to launch. We have Clipper on its way to study Europa. The Jupiter Icy Moons Explorer, with the acronym JUICE, is on its way to study three ocean worlds of Jupiter: Ganymede, Europa and Callisto. So whether or not new missions come to fruition, we have a couple of spacecraft on the way.

Oliver Strimpel: What are you working on at the moment?

Michael Manga: In the last couple of years I've had the great pleasure of going on a couple of oceanographic expeditions to study submarine volcanism. The first was an International Ocean Discovery Program expedition to the Aegean, part of the Mediterranean, to study the volcanic history of Santorini volcano and the nearby Christiana and Kolumbo volcanoes, that whole volcanic field. Its goal was to understand the interaction between volcanism; tectonics, that is, the faulting and rifting that create the basins in which the volcanoes sit; and the evolution of climate. As sea level goes up and down, that change in pressure influences the volcanic system. In that setting, we can go all the way back 5 million years and look at the evolution of the system, with sea level going up and down, the faults being activated and the volcanoes erupting. And we can do that from the birth of the volcanic centers to their end, over multiple volcanic cycles. We drilled over 7 kilometers of holes and recovered thousands of meters of sediment to reconstruct that volcanic history, and also to understand the processes that drive submarine volcanism.

And then in September of last year, I was on the Australian research vessel Investigator, studying the Hunga eruption, recovering samples and mapping, to understand what drove that eruption. Although Hunga powered that 57-kilometer-high plume, most of what erupted collapsed and went underwater and was not transported in the atmosphere. And those underwater currents traveled very far and very fast underwater, snapping the communication cables that connected Tonga to the outside world. So the expedition was aimed at trying to understand how the volcanic eruption collapsed, what happened to that material, and what governed where it went and how it was transported.

Oliver Strimpel: And did you see a caldera there?

Michael Manga: It was a big eruption, and so it left behind a hole in the ground. One of the questions that motivated the expedition is understanding what that hole is. Was it a crater? A crater is produced by just blowing stuff up. You can think about an impact: a meteorite hits the surface of the Earth and excavates a crater. That's as opposed to what we call a caldera, where the ground collapses because the magma underneath is evacuated and the overlying crust sinks into what used to be an area filled with magma. We're still trying to figure out whether the images we see support the caldera or the crater interpretation. The diagnostic signature would be identifying the faults on which the ground would have collapsed. So hopefully, as we continue to process all our data, we'll be able to resolve all these interesting open questions.

Oliver Strimpel: Michael Manga, thank you very much.

Michael Manga: Thank you for the time today, and thanks to your listeners for their interest. Volcanoes are really fascinating. It's a great life to be a volcanologist, and these are great times to be a volcanologist. And of course, there's so much still left to learn.

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