Sage: Ready for a vacation? 

This lake may seem like paradise. But it has an explosive origin story. 

On the North Island of New Zealand, you'll find Lake Taupō. And you can see why it's a popular tourist destination: it's got amazing views, decadent hot Springs, and frickin' waterfalls, man. 

But, around 25,500 years ago, you definitely wouldn't want to be in its vicinity. Because this placid lake formed a massive volcanic supereruption. 

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 Taupō.

Amazing, right? And... kinda terrifying. 

Hi, I'm Sage, and this is Crash Course Geology. 

[Theme music]

From the huge Mount Damavand in Iran to the tiny Taal in the Philippines to the infamous Mount Doom of 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 papier-mâché, volcanoes can be equal parts enthralling and scary. Because, well, they do this.

[Presses button on papier-mâché volcano. Waits.] Stand back, Dwayne. Oh. 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 land formations that build up around those vents, like the mountain-looking things we might think of when we hear the word "volcano".

Bit 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 a thousand degrees Celsius. 

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 vapour, 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 cracks in the rock above, and things get... explode-y.

Think of that magma with all its dissolved gases like a shaken-up bottle of soda. As long as the cap's 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 up with them. Magma then breaks the surface and becomes lava, the name we give to molten rock when it's above ground. 

Et voilà: 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 Lava is. 

Low-silica lava, also called mafic lava, creates super runny flows that race across the surface of rivers or spurt up as fountains. 

High-silica lava, called felsic lava, is thick and sticky, preventing the gases from easily escaping. Pressure builds until — blam-o! — you get an explosion. 

And those eruptions from different types of lava create different shapes of volcanoes, too. 

Like, the runniest lavas often create shield volcanoes, which are big, low, and flat. 

Kilauea is an example. Its lava flows harden into broad flanks of basalt 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 in the ground, called a fissure eruption. 

Or it can fountain from a central vent, falling back down as tephr: 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 Parícutin in Mexico. 

Cinder cones look tiny in comparison to composite volcanoes, aka stratovolcanoes, like Mount Fuji in Japan. 

They're towering, conical 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 torrents of lava, other times sending up volcanic plumes of tephr 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 results is a huge, bowl-shaped caldera, like Lake Taupō. 

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 layer 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. 

Volcanoes 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 tjat 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 and eruption at a mud-ocean ridge for the first time in 2025. 

Just, so cool!

Sometimes volcanoes can form in places called hotspots, where a big plume of super-hot mantle rock rises up underneath the crust, causing rock to melt and erupt at the surface. 

We have hotspots to thank for Yellowstone, whose hotspot now sits right below the northwest corner of the United States. 

Now, you might’ve heard Yellowstone described as a "supervolcano," which could cause colossal destruction if it erupted again. But "supervolcano" isn't a scientific term; most geologists kinda roll their eyes at it. 

It's like their personal Nickleback. Look at this supervolcanoooo.

Technically, a "supervolcano" is a volcano that's had at least one supereruption, like the one that formed Lake Taupō. 

What counts as a "supereruption" 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 around a 6 on the VEI. A supereruption is VEI of 8, meaning it ejects at least 1000 cubic kilometres of ash, pumice, and debris. 

That's enough volcano-stuff to fill 400 million Olympic swimming pools. The 100-metre butterfly would be... just so different. 

But could this really happen at Yellowstone? Whole towns and cities covered in ash? Emergency services stymied? Global agricultural collapse? 

In theory, yes. But the last supereruption at Yellowstone was around 640,000 years ago. And geologists say there's an "exceedingly low" chance that a supereruption 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, supereruptions in the past, doesn't mean its future blasts will be as hefty. 

Even if a supereruption 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's more magma down there than we initially thought. But importantly, it's spread across many smaller underground pockets. It's not linked up enough to erupt in one big boom. 

So, you have geologists' blessing — you can take a Yellowstone supereruption off your Things to Worry About list. 

But just because supereruptions 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 a hundred kilometres 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 through 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 pyroclastic flows, and landslides and mudslides called lahars. 

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 Mauna Loa by bombing it. 

Which... didn't really work. 

Truth be told, there's no way to stop a volcano from erupting. There's n9 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 rising ominously from places it shouldn't. 

They also base forecasts on volcanoes' recurrence intervals, or the average times between past volcanic eruptions. This can give us clues to when a peak might erupt again. 

And when it comes to disaster preparedness, there's 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 about 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. 

[Papier-mâché volcano finally goes off] Sweet!

Next time, we're talking about another geological hazard: earthquakes. See you then. 

Thanks for watching this episode of Crash Course Geology, which we filmed at our studio in Indianapolis, Indiana, 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.