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Ready for a vacation? This lake may seem like paradise, but it's got an explosive origin story. On the North Island of New Zealand, you'll find Lake Tao, and you can see why it's a popular tourist destination. It's got amazing views, decadent hot springs, and freaking waterfalls, man. But around 25,500 years ago, you definitely wouldn't want to be in its vicinity because this placid lake formed from a massive volcanic super eruption. The volcano shot out tons of lava and volcanic ash, buckling the earth into a depression about the size of Singapore. Eventually, it filled with water and became Lake Tao. Amazing, right? And kind of terrifying. Hi, I'm Sage and this is Crash Course Geology. From the huge Mount Davabond in Iran to the tiny Tal in the Philippines to the
infamous Mount Doom in Mordor, volcanoes capture our imagination. Yeah, I know Lord of the Rings isn't real, but it's based on real volcanoes. Okay. Whether we're talking the real deal or paperier-mâché, volcanoes can be equal parts enthralling and scary because, well, they do this. Stand back, Dwayne. You're right. Well, in real life, they erupt. But why? And what actually is a volcano anyway? Geologically speaking, a volcano is a crack in the ground, aka a vent, that lava comes out of. It can also refer to the land formations that build up around those vents, like the mountain looking things we might think of when we hear the word volcano. But volcanoes aren't just mountains, especially because they erupt. Let's
take a look at how that happens. Magma is what we call molten rock that's underground, and it's actually pretty chill. Metaphorically, literally, it's like 1,000° C down there. The pressure is way higher than it is on the surface. And that pressure combined with the high heat allows gases like water vapor, carbon dioxide, and sulfur to dissolve into the magma. Magma is less dense than the surrounding rock, so it tends to rise toward the surface and collect in underground pools called magma chambers, which do a pretty good job of keeping things contained usually. But put a little crack in the rock above and things get explody. Think of that magma with all its dissolved gases like a
shaken up bottle of soda. As long as the cap's still on, everything's fine. But unscrew it even a tiny bit, and you'd better have your goggles on. As soon as the gases have a path to the surface, they come speeding up like the bubbles in your soda, carrying that hot molten rock with them. Magma then breaks the surface and becomes a lava, the name we give to molten rock when it's above ground. And voila, an eruption. Not all eruptions are equal, though. Some are huge and explosive, while others look more like a cool feature at a water park. That difference partly comes from the amount of gas that's dissolved in the magma. More gas equals more explosive power. But the other key is
how much silica the lava has. Silica is one of the most plentiful chemical compounds on Earth's surface and helps determine how gooey the lava is. Low silica lava, also called mafic lava, creates super runny flows that race across the surface in rivers or spurts up as fountains. High silica lava, called felic lava, is thick and sticky, preventing the gases from easily escaping. Pressure builds up until blammo, you get an explosion. And those eruptions from different types of lava create different shapes of volcanoes, too. Like the reiest lavas often create shield volcanoes, which are big, low, and flat. Kilawa is an example. Its lava flows harden into broad flanks of basaltt that end up looking kind of like
a shield. But if the lava is extra gassy or has slightly more silica, eruptions can get a little more explosive. Lava can sputter up along fractures on the ground called a fissure eruption. Or it can fountain from a central vent falling back down as terra chunks of rock that can be as small as ash or as big as houses. This can create small steep-sided volcanoes called cinder cones like Pari Coutine in Mexico. Cinder cones look tiny in comparison to composite volcanoes aka strat volcanoes like Mount Fuji in Japan. They're towering conicle peaks that loom over the landscape often with a distinct crater at their summit, you know. So,
the hobbits have somewhere to chuck that ring. Composite volcanoes erupt with remarkable variety, sometimes unleashing torrent of lava, other times sending up volcanic plumes of terra that fall back to Earth. These combinations of eruptions build up the composite volcano's iconic steep flanks. When lava is extra sticky, it can't flow away from the vent, so it may pile up around it, creating a lava dome. Sometimes lava domes stand freely on their own, but they can also form inside structures like calderas, which are created when a ton of magma erupts quickly and drains the chamber below, causing the surface to collapse. The result is a huge bowl-shaped caldera like Lake Tao. So,
volcanoes come in all kinds of shapes and sizes, and you can find them all over the world, but they don't pop up just anywhere. For magma to burst from the surface, conditions have to be just right. Many volcanic eruptions happen at plate boundaries, places where the tectonic plates that make up Earth's outermost layers squish together or spread apart. Like lots of volcanoes form at subduction zones where two tectonic plates converge. One gets forced beneath the other and back into the mantle. The mantle's heat causes the downgoing plate to release water which reduces the melting point of the rocks above. That's how you get magma collecting in reservoirs that can blast to the surface. Volcano can form at
divergent boundaries, too. Sometimes that happens on land in places called continental rifts, where two continental plates pull apart from each other, causing hot mantle rock to well upward. And this process is way more common underwater. Geologists estimate that more than 70% of the world's volcanoes are hidden under the sea at mid ocean ridges. Divergent plate boundaries where two oceanic plates are pulling apart. Geologists finally witnessed an eruption at a mid ocean ridge for the first time in 2025. Just so cool. Sometimes volcanoes can form in places called hot spots, where a big plume of super hot mantle rock rises up underneath the
crust, causing a rock to melt and erupt at the surface. We have hot spots to thank for Yellowstone, whose hot spot now sits right below the northwest corner of the United States. Now, you might have heard Yellowstone described as a super volcano, which could cause colossal destruction if it erupted again. But super volcano isn't a scientific term. Most geologists kind of roll their eyes at it. It's like their personal Nickelback. Look at this super volcano. Technically, a super volcano is a volcano that's had at least one super eruption, like the one that formed Lake Tao. What counts as a super eruption comes down to the volcanic explosivity index or VEI, which ranks the intensity
of volcanic eruptions on a scale of 0 to 8. For context, the strongest volcanic eruptions of the past century or so were all around a six on the VEI. But a super eruption is a VEI of eight, meaning it ejects at least 1,000 cubic kilm of ash, pummus, and debris. That's enough volcano stuff to fill 400 million Olympic swimming pools. The 100 meter butterfly would be just so different. But could this really happen at Yellowstone? Whole towns and cities covered in ash. Emergency services stymied global agriculture collapse. In theory, yes. But the last super eruption at Yellowstone was around 640,000 years ago. And geologists say there's an exceedingly low chance that a super eruption would happen in the next few
thousand years. Some aren't convinced another will ever happen again. And just because a volcano has had one or even two super eruptions in the past doesn't mean its future blasts will be as hefty. Even if a super eruption were going to happen at Yellowstone, we'd have some notice. Volcanologists, who are not a species from Star Trek, but rather scientists who study volcanoes, keep tabs on the amount of magma held within the volcano's underground reservoir using tools like seismometers. Recent studies suggest there is more magma down there than we initially thought. But more importantly, it's spread across many smaller underground pockets. It's
not linked up enough to erupt in one big boom. So you have geologist blessing. You can take a Yellowstone super eruption off your things to worry about list. But just because super eruptions aren't much of a threat to us doesn't mean regular old volcanic eruptions aren't. Across the world about 800 million people live within 100 km of an active volcano. Now the meaning of active can vary. Like some geologists use active to refer to volcanoes that are erupting right now. And others use it to mean any volcano that's erupted in the last 11,000 years or so. Dormant volcanoes aren't erupting right now, but might in the future. And extinct volcanoes were active once, but aren't expected to erupt ever again, even
though they can surprise us once in a while. But it's not all Mount Doom and gloom living near an active volcano. In small doses, volcanic ash is really good for crops. Volcanoes are where we find a lot of our most prized minerals like gold, silver, copper, lead, and zinc. And the heat of volcanic systems can be tapped for geothermal energy, which we'll get more into in a later episode. But some eruptions can cause serious destruction. Thick volcanic ash can blanket crops, collapse structures, and suffocate people and animals. Plus, eruptions can cause avalanches of hot ash called pyrolastic flows, and landslides and mudslides called laahars. So, it makes sense that some folks have
tried to control erupting volcanoes. Like in 1935, the US Army tried to change the course of lava flows from Monaloa by bombing it, which didn't really work. Truth be told, there's no way to stop a volcano from erupting. There's no big cork you can stick in there. But there are ways to forecast and prepare for eruptions. Geologists and volcanologists monitor warning signs of volcanic activity like earthquakes, swelling in the ground, and steam arising ominously from places it shouldn't. They also base forecasts on volcanoes recurrence intervals or the average time between past volcanic eruptions. This can give us clues to when a peak might erupt again. And when it comes to disaster preparedness, there
are a few things you can do. One is to learn about volcano risks in your area and sign up for alerts from your local government. Another is to form an evacuation plan and gather emergency supplies like food, water, masks, and clothing. There's more on volcano preparedness in the links below. So, yeah, volcanic eruptions can be scary. They can also be enthralling, fascinating, and even pretty chill. Some create huge calderas that turn into popular tourist destinations, and some just blow off a little steam. Earth's geology is chaotic like that. Sweet. Next time we're talking about another geological hazard, earthquakes. See you then. Thanks for watching this episode of Crash Course Geology, which is filmed in our studio in Indianapolis,
Indiana, was made with the help of all these nice people. If you want to help keep Crash Course free for everyone forever, you can join our community on Patreon.
