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Volcanoes – Windows into the Deep Earth · Talking Science

0:00
You know, it is, it's pretty wild to think about,
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but the absolute deepest hole humanity has ever
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managed to drill into the Earth reached about
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12 kilometers down. Right. The Kola super deep
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borehole, 12 kilometers. Exactly. 12 kilometers.
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And I mean, to put that into perspective for
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everyone listening, the distance to the very
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center of the Earth is roughly 6 ,000 kilometers.
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Yeah. It's a massive difference. It is. That
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12 kilometer hole is basically just a literal
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scratch on the surface of our planet. The vast,
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overwhelming majority of the Earth's interior
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is just completely, totally inaccessible to us.
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It really is a profound realization when you
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stop and think about it. We have mapped the surface
0:41
of the moon in incredible detail, right? We have
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rovers driving around on Mars right now. Yeah,
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sending back high def photos. Exactly. But the
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ground right beneath our own feet remains essentially
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an alien world. We just we simply cannot reach
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it with our current technology. And honestly,
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we likely never will. Which is exactly why understanding
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volcanoes completely changes how you see the
1:02
planet. We're doing a deep dive today into how
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they actually work. And, you know, our mission
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is to sort of demystify the whole process. Yeah.
1:09
Assuming. zero prior science background. We're
1:11
just going to break it down. Right. Because volcanoes,
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they aren't just mountains that occasionally
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breathe fire. You know, they are literal windows
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to the center of the earth. They deliver material
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from depths we could never even dream of digging
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to. They're pretty much our only physical messengers
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from the deep earth. They bring that deep, hidden
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unknown right up to the surface for us to look
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at. They do, but to really interpret what these
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messengers are telling us, we actually have to
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start by tearing down a very, very common mental
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image. Oh, yeah. Yeah, of what actually sits
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beneath a volcano. I think we all have a very
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specific picture in our heads, right? movies
1:50
or middle school science diagrams. Definitely.
1:53
The classic underground cavern. Exactly. You
1:56
picture a volcano and underneath it, you imagine
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this giant glowing underground swimming pool
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of liquid magma. Just, you know, sloshing around
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in a massive cave waiting to erupt. Right. The
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underground lake of fire. It's a striking image,
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but modern geophysics paints a totally different
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picture. Oh, really? Yeah. Magma chambers are
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rarely these big tanks of purely liquid rock.
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Instead, they are much closer to a hot... Mostly
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solid crystalline mush. A mush. Yeah, a mush.
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The melted liquid part is actually just distributed
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through the tiny gaps and pores between the solid
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crystals of rock. Wait, so it's less like a cavern
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filled with liquid lava and more like, I don't
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know, one of those slushies you get at a gas
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station. That is actually a really great analogy.
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Because it's mostly icy crystals. Right. With
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just enough liquid syrup in between to hold the
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whole thing together. Exactly. A very, very hot
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slushie. That is a highly accurate way to visualize
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it. Underneath a volcano, this mush just sits
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there, mostly solid and totally immobile. So
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how does it erupt then if it's just stuck there?
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Well, eruptions usually happen when that mush
3:02
gets mobilized. Often, what triggers it is a
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fresh batch of incredibly hot, truly liquid magma
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arriving from much deeper down in the earth.
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It shoots up into the chamber, heats up the slushy,
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destabilizes the entire system, and... and that
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forces the material up to the surface. Okay,
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so tracing this back a step then, where is that
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fresh, hot liquid magma coming from the first
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place? A good question. Because, I mean, the
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Earth isn't just a liquid ball on the inside,
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right? The mantle, that massive, thick layer
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just beneath the crust, that's actually solid
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rock. It is solid rock, yes. It's incredibly
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hot, way above its normal melting point at the
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surface, but it remains solid. Because of the
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pressure. Exactly. Because of the immense crushing
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pressure of all the rock resting on top of it.
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It does flow, but it flows incredibly slowly.
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Like how slowly? Like warm plastic over millions
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of years. Wow. So it is absolutely not a liquid
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ocean of magma down there. To get liquid magma
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to fuel a volcano, you have to actively find
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a way to melt that solid mantle rock. And there
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are a few specific ways the Earth actually manages
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to melt that solid rock. Right. Because changing
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the state of rock under that much pressure must
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take immense geological forces. It does. There
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are basically three primary mechanisms, and they
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dictate the exact kind of volcanoes we end up
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seeing on the surface. Okay, what's the first
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one? The first is called decompression melting.
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Decompression melting. Right. Because that deep
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rock is under enormous pressure, the molecule...
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are just forced to stay packed together in a
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solid state, even though the temperature is insanely
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high. Right. The heat wants to melt it. but the
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pressure won't let it. Exactly. So if you simply
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reduce that pressure, the rock can suddenly melt
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without adding a single degree of extra heat.
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Oh, it's like taking the lid off a pressure cooker.
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Yeah. The heat was already in there, but the
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pressure was keeping everything contained. That's
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the core of it, yeah. This happens mostly where
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tectonic plates are pulling apart from each other.
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When the plates separate, the pressure on the
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mantle right underneath them is relieved. We
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see this at mid -ocean ridges, deeply underwater,
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and at continental rift valleys. Like the one
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in East Africa. Exactly, like in East Africa.
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It is actually the most voluminous form of volcanism
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on the planet. Really? I feel like you never
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hear about it. Because almost all of it happens
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unseen in the pitch black on the seafloor, so
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it rarely makes the evening news. Okay, well,
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pulling plates apart to drop the pressure makes
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perfect logical sense. What about the most famous
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volcanoes, like the ones on the Pacific Ring
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of Fire? Ah, subduction zones. Yeah, because
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those happen where tectonic plates are crashing
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together, right? Pushing the pressure up. How
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does an increase in pressure result in melting
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rock? It doesn't, actually. And that leads us
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to the second mechanism, which involves adding
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water. Adding water. Yeah. At subduction zones,
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one tectonic plate is shoved underneath another,
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and it dives deep down into the Earth. Right.
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As that huge slab of ocean floor gets pushed
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down, it carries water with it. Wait, hold on.
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I have to pause on that. Sure. Because that sounds
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completely backward to literally everything we
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experience in daily life. Yeah. I mean, if I
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want to stop something from burning or melting,
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I throw water on it. Right. Water puts out fires.
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Yeah. Water cools things down. How on Earth...
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Does adding water melt solid rock? I know. It
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is deeply counterintuitive until you reframe
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how you think about the water down there. We
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aren't talking about water acting as a coolant.
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We are talking about water acting as a chemical
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catalyst. A chemical catalyst. Yes. The water
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is actually chemically locked inside the minerals
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of the rock itself. And as that descending plate
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gets deeper and hotter, it literally sweats out
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that locked in water into the solid mantle wedge
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sitting right above it. Oh, wow. It sweats it
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out. Yeah. And when you introduce water into
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the chemical structure of superheated solid rock,
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it substantially lowers the melting point of
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that rock. Oh, I see. Okay. It's like throwing
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salt on an icy sidewalk. Exactly. The salt doesn't
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heat up the ice. It just alters the chemistry
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so that the ice melts at a lower temperature.
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The water is doing the exact same thing to the
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rock deep in the earth. That is a brilliant comparison.
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The water hits the mantle, lowers its melting
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point, and the solid rock promptly begins to
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melt into magma. That is wild. This specific
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process is responsible for the Pacific Ring of
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Fire. Almost every famous volcano you can name,
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Mount St. Helens, Mount Fuji, Krakatoa, they
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were all born because the ocean floor is literally
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sweating water into the deep earth. The ocean
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floor sweating water into the mantle to create
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liquid rock. That is such a wild visual. It really
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is. So that's decompression and water. There's
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a third way to melt rock too, right? Yeah. One
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that doesn't rely on plate boundaries at all.
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Right. The third mechanism is called a hot spot.
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A hot spot. Yes. This is when a plume of unusually
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hot, buoyant material rises up from the very
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deep mantle way down near the Earth's core. It
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acts almost like a geological blowtorch. It is
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so intensely hot that it simply melts its way
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right through the tectonic plate sitting above
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it. Just burns right through? Exactly. Hawaii
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is the classic textbook case here. Right. And
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because the tectonic plate is constantly slowly
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moving over time, but the blowtorch plume underneath
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stays in one fixed place. You get a line of volcanoes.
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Spot on. The volcano directly over the hotspot
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is active, but as the plate carries it away from
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the heat source, it dies out and a new one forms
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behind it. Which perfectly explains why Hawaii
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is a chain of islands. Oh, that makes so much
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sense. Yeah. The Hawaiian emperor seamount chain
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is literally a geological record of the Pacific
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plate moving over a stationary hotspot for tens
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of millions of years. Wow. Yellowstone is another
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prime example of a hotspot. but it sits directly
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over a massive plume erupting through a thick
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continental plate instead of a thin oceanic one.
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Which, that changes the chemistry of the magma
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entirely, doesn't it? It absolutely does. And
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this brings us to a really crucial distinction
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for you guys listening. Once the rock is melted,
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whether by dropping pressure... adding water,
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or using a blowtorch. Why do some volcanoes just
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gently ooze rivers of lava that tourists can
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practically walk right up to while other volcanoes
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violently, catastrophically explode? Yeah, the
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difference between a gentle ooze and a massive
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explosion all comes down to magma composition,
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specifically a substance called silica. Right,
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silica, which is just all that silicon and oxygen.
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binding together. Exactly. And when you have
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high silica content, those molecules link up
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into extensive chemical chains and networks,
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right? It basically turns the magma into a highly
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viscous, sticky paste. Yep. But when the silica
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is low, the magma stays incredibly runny, almost
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like syrup. Right. And we call that runny, low
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silica magma basalt. Okay, basalt. The stiff,
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high silica magma is usually called rhyolite
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or andesite. But here's the thing. Viscosity
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alone doesn't cause an explosion. You also need
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immense pressure. So where does the pressure
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come from? That is where dissolved gases enter
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the equation. Magma contains massive amounts
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of dissolved gases, primarily water vapor and
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carbon dioxide. Okay. Deep underground, the immense
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pressure keeps those gases perfectly dissolved
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in the liquid rock. They are invisible. But as
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that magma rises toward the surface, the pressure
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drops. It's the soda bottle effect. Like, while
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the cap is on tight, a warm fizzy soda looks
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perfectly still because the pressure is keeping
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all that carbon dioxide dissolved in the liquid.
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Right. But the second you twist the cap and release
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the pressure, all those gas bubbles... instantly
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form out of solution and race to the top. Now
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combine that fizzy soda effect with the thickness
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of the magma we just talked about and you unlock
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the physics of a volcanic explosion. Oh, I see
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where this is going. Yeah. If you have runny,
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low silica magma, like in Hawaii, the bubbles
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form, but because the liquid is so thin, they
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easily slip right through it and escape into
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the air. The gas vents out naturally. Exactly.
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You get beautiful, dramatic lava fountains and
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glowing rivers. The internal pressure of the
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earth is released gracefully. But if you have
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that thick, highly networked, high silica magma,
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the bubbles form. But they can't travel through
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the paste? No, they can't. They get completely
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trapped. They're completely locked in. But the
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gas is still expanding as the magma continues
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to rise and the pressure drops further. So unimaginable
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pressure builds up inside the magma itself, pushing
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outward against this stiff, unyielding rock.
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Eventually it reaches a breaking point. And it
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just blows. The rock fails catastrophically.
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The magma is torn apart from the inside out by
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its own expanding gas. It fragments instantly
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into tiny razor -sharp pieces of ash and pumice
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and is blasted out of the vent at supersonic
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speeds. See, this completely reframes the danger
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for me. The difference between a beautiful lava
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flow and a mountain -destroying explosion has
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very little to do with how hot the magma is.
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Almost nothing to do with temperature, yeah.
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It is entirely about whether the expanding gas
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can escape gracefully or whether it gets trapped
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in a high silica paste until the whole system
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ruptures. And this highlights exactly why subduction
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zone volcanoes, the ones on the Ring of Fire
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created by water, are so incredibly dangerous.
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Because they have more silica. Yes. When magma
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rises through thick continental crust, it melts
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some of that crust and absorbs vast amounts of
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silica. So subduction zones produce exactly that
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sticky gas -charged magma that is prime for violent
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explosions. Which means we really need to rethink
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volcanic hazards entirely. Because, you know,
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you hear volcano and you instinctively think
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lava. Of course. But lava flows are basically
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the least of your worries in an explosive eruption.
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Absolutely. I mean, lava flows destroy property.
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Certainly they crush houses, bury roads and permanently
12:55
alter landscapes. But they don't really catch
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people off guard. No, they rarely kill people
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simply because they move slowly enough that you
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can usually just walk away from them. The deadliest
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direct product of an explosive eruption is something
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else entirely. The pyroclastic flow. Just the
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name is terrifying. A pyroclastic flow is essentially
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an avalanche of superheated gas, volcanic ash,
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and pulverized rock. Yes, it behaves like a heavy
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boiling fluid racing down the flanks of the volcano
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at speeds well over 100 kilometers per hour.
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Over 100 kilometers an hour. And the temperature
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inside that cloud is hundreds of degrees Celsius.
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So you can't outrun it. and you can't survive
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it. Not at all. The sheer density of it is the
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critical factor. Because it is so heavily laden
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with fragmented rock and ash, it is much denser
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than the surrounding air. So it doesn't just
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go up into the sky? No, it doesn't float up.
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Gravity pulls it violently down and it funnels
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directly into valleys and low -lying areas. There
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is absolutely no outrunning a pyroclastic flow
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and no surviving being caught in one. This is
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exactly what happened to Pompeii, wasn't it?
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Yes. This is the exact phenomenon that buried
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the Roman cities of Pompeii and Herculaneum in
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79 AD. But the threat to human life doesn't stop
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when the eruption ends, or even with the initial
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blast radius. There's a secondary hazard called
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a lahar. And those often travel much further
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than the pyroclastic flows. Lahars are terrifyingly
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destructive. What are they exactly? A lahar is
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a volcanic mudflow. During an explosive eruption,
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huge amounts of fine volcanic ash fall on the
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surrounding landscape, right? Right. If that
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ash mixes with a large volume of water, which
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could come from torrential rains or, very commonly,
14:37
from the extreme heat of the eruption instantly
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melting snow and ice at the volcano's summit,
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it turns into a heavy flowing slurry. And it's
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crucial to visualize this correctly. We aren't
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just talking about a muddy river here. The closest
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analogy for a lahar is fast -moving wet concrete.
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That is exactly what it's like. It has the mass
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and the density to carry entire boulders, uproot
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giant trees and completely sweep away bridges
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and reinforce concrete buildings as if they were
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made of paper. And just like pyroclastic flows,
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lahars funnel into river systems. But they can
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travel much, much further, striking settlements
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dozens of kilometers away, sometimes hours or
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even days after the actual eruption has peaked.
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I think the most devastating modern example of
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this was the 1985 eruption of Nevada del Ruiz
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in Colombia, right? Yes. The Armero tragedy.
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Armero is the textbook tragedy for this. Because
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from what I understand, the town wasn't even
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on the mountain. The people living there probably
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thought they were perfectly safe from a volcano
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that was nearly 50 kilometers away. They did.
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That geographical distance offered a terrible
15:40
false sense of security. The eruption itself
15:42
was comparatively modest actually. Really? Yeah.
15:45
But the heat melted a substantial portion of
15:48
the mountain's summit glacier. That immense volume
15:51
of water mixed with fresh ash and created colossal
15:55
lahars that roared down the river valleys in
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the dark. In the middle of the night. They hit
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the town of Armero in the dead of night, killing
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over 20 ,000 people. It remains a stark, heartbreaking
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reminder that you do not need to live on the
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slopes of a volcano to be in its direct path.
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It's just devastating. A lahar can wipe out a
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valley tens of kilometers away. But when we look
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at the really massive explosive eruptions, the
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impacts scale up exponentially. Like they can
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literally reach the stratosphere and alter the
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entire planet's climate. This is where a localized
16:28
geological hazard becomes a global meteorological
16:31
event. How does that happen? Well, when a very
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large explosive eruption occurs, it doesn't just
16:36
blast rock and ash. It injects millions of tons
16:39
of sulfur dioxide gas straight up into the stratosphere.
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Okay, sulfur dioxide. Once it reaches that altitude,
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the sulfur dioxide reacts with water vapor to
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form tiny sulfate droplets, which are known as
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aerosols. And the stratosphere is situated way
16:53
above where our normal weather systems operate,
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right? There are no rain clouds up there to wash
16:58
anything away. Precisely why it matters so much.
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Because there is no precipitation in the stratosphere
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to scrub these tiny droplets out of the air.
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They just persist. They just hang out there.
17:07
Yeah. And high altitude winds rapidly spread
17:09
them around the entire globe, creating this thin
17:12
chemical veil. And what does that veil do? These
17:14
droplets act like billions of tiny mirrors. They
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reflect a portion of the incoming sunlight right
17:20
back out into space before it can warm the Earth's
17:22
surface. So it literally blocks the sun. The
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inevitable result is global cooling, which can
17:27
last for a year or two until the aerosols finally
17:30
settle out. We actually saw a modern preview
17:32
of this when Mount Pinatubo in the Philippines
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erupted in 1991, right? It injected so much sulfur
17:41
into the stratosphere that it measurably cooled
17:44
the global average temperature by roughly half
17:46
a degree Celsius for a couple of years. We did,
17:49
but honestly, Pinatubo was mild compared to historical
17:52
extremes. Really? Oh, yeah. Mount Tambora in
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Indonesia, which erupted in 1815, was orders
17:59
of magnitude more powerful. That blast produced
18:02
an atmospheric veil so dense it caused what historically
18:05
became known as the year without a summer. The
18:09
downstream human impacts of Tambora are staggering
18:12
to think about. You had widespread crop failures
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across the entire northern hemisphere. It literally
18:18
snowed in New England in June. People were starving
18:20
in Europe because the harvests failed. And it
18:23
even bled into culture. Like Mary Shelley famously
18:26
wrote Frankenstein because she and her friends
18:28
were trapped indoors in Switzerland by the incessant,
18:31
gloomy, unseasonal weather. Right. I mean, a
18:33
mountain blowing its top in Indonesia directly
18:35
caused famine and influenced literature in Europe.
18:38
That is crazy. The global interconnectedness
18:41
is profound. Okay. Which brings us to the ultimate
18:43
extreme of explosive volcanism. Eruptions so
18:47
massive, they defy our normal concept of what
18:49
a volcano even looks like. The super volcanoes.
18:51
The term gets thrown around a lot in disaster
18:53
movies. Yeah. Usually alongside like a giant
18:56
countdown clock. It does. While supervolcano
18:59
is a bit of an informal media term, volcanologists
19:02
do use it to categorize eruptions at the very
19:04
top of the volcanic explosivity index. So what
19:07
qualifies as a supervolcano? Specifically, singular
19:11
events that eject more than 1 ,000 cubic kilometers
19:14
of fragmented material. 1 ,000 cubic kilometers.
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I can't even picture that. That volume is almost
19:20
incomprehensible. A single eruption of that size
19:23
would bury an area the size of a large country
19:26
in feet of ash. But the physical structure left
19:29
behind is actually fascinating because these
19:31
systems don't build towering, majestic, cone
19:34
-shaped mountains. Because they physically can't,
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right? Yeah. The eruption is so massive it empties
19:39
the underground magma chamber so comprehensively
19:42
and so quickly that the rock roof of the chamber
19:44
has nothing left to support its weight. Exactly.
19:47
The ground itself simply collapses downward into
19:49
the void. You don't get a mountain blowing its
19:51
top. You get the ground falling in, leaving a
19:53
vast, bowl -shaped depression in the earth called
19:56
a caldera. A caldera? Yes. Yellowstone is a classic
20:00
caldera system. Its main crater is roughly 50
20:04
by 70 kilometers across. You can literally drive
20:07
through it and barely realize you are inside
20:09
a volcano. Because it's so big. Right. And the
20:12
Togo Caldera in Sumatra, formed by an eruption
20:15
around 74 ,000 years ago, was even larger. Okay,
20:18
I am so glad we are landing on Yellowstone. I
20:20
constantly see headlines declaring that the Yellowstone
20:23
Supervolcano is overdue for a catastrophic eruption
20:27
that's going to... going to wipe out half the
20:29
continent. Let's apply some logic to this. Is
20:32
Yellowstone actually overdue? It is absolutely
20:35
not overdue. And we need to treat those headlines
20:37
with extreme skepticism. Thank you. People look
20:40
at the geological record. They see that Yellowstone
20:42
has produced massive eruptions on a rough average
20:45
of every six to eight hundred thousand years.
20:47
And they treat that average like a bus timetable.
20:50
But the Earth doesn't have a calendar on its
20:52
wall. It doesn't care that it's been 600 ,000
20:54
years since the last major event. Exactly. The
20:57
physics of an eruption are driven by thresholds,
20:59
not time. Thresholds. An eruption only occurs
21:03
when sufficient eruptable magma has accumulated,
21:06
and the internal gas pressure exceeds the structural
21:09
strength of the rock holding it in. right now
21:11
there is zero geophysical evidence that yellowstone
21:14
is preparing to erupt even with all the geysers
21:16
and earthquakes yes the ground deforms and there
21:19
are periodic earthquake swarms but that is entirely
21:21
within the normal baseline behavior for a restless
21:25
living caldera system that is slowly cooling
21:28
and shifting okay well that is deeply comforting
21:30
but it does raise a practical question since
21:34
we know volcanoes operate on physical thresholds
21:36
rather than a neat timetable Do we actually have
21:40
reliable ways of knowing when those thresholds
21:42
are being crossed? Can we predict when a volcano
21:45
is actually waking up? The science of forecasting
21:47
is advancing rapidly. While predicting the exact
21:49
timing of earthquakes remains incredibly difficult,
21:52
volcanoes are actually remarkably cooperative.
21:55
Really? How so? Well, as we discussed earlier,
21:58
eruptions begin with new magma moving up from
22:00
deep underground. That movement produces very
22:03
clear physical warning signs on the surface.
22:05
Right, because thousands of tons of liquid rock
22:08
cannot force its way upward without literally
22:10
shoving the existing rock out of its way. Exactly.
22:13
And that shoving fractures the crust, causing
22:16
highly distinctive swarms of small earthquakes.
22:19
Okay, so you watch for earthquakes. That's one
22:21
sign. The immense physical pressure of the rising
22:24
magma also causes the ground surface itself to
22:27
inflate and bulge upward. The ground actually
22:29
swells. It does. We can measure this deformation
22:33
with incredible precision. Scientists use GPS
22:36
stations on the mountain and satellites utilizing
22:38
radar interferometry, which basically bounce
22:42
signals off the ground to measure changes in
22:44
elevation down to the millimeter. That is amazing.
22:47
So you have the ground shaking, you have the
22:49
mountain literally swelling up like a balloon.
22:51
And I imagine the gases are changing too as that
22:54
magma gets closer to the surface and the pressure
22:56
drops. Yes. You measure dramatic changes in both
22:59
the composition and the sheer volume of gases
23:01
venting from the crater, particularly increases
23:04
in carbon dioxide and sulfur dioxide. So putting
23:07
it all together gives you a warning. Monitoring
23:09
these three indicators, seismicity, ground deformation,
23:12
and gas emissions, has led to incredible, genuine
23:15
successes in forecasting. Returning to the 1991
23:19
eruption of Mount Pinatubo, scientists tracked
23:22
those exact warning signs perfectly. And they
23:25
evacuated in time? They forecasted the eruption
23:27
in time to evacuate tens of thousands of people,
23:30
saving countless lives. Volcanology isn't perfectly
23:34
precise, and volcanoes can certainly be deceptive,
23:37
but the warning signs are real, physical, and
23:40
readable. It really makes you wonder why humans
23:42
have historically chosen to live so close to
23:44
them, given the risks. But there's a powerful
23:47
incentive, isn't there? There is. Because volcanic
23:49
ash breaks down into some of the most incredibly
23:51
fertile, nutrient -dense agricultural soil on
23:54
the planet. For thousands of years, the risk
23:57
of a potential eruption was far outweighed by
24:00
the immediate reward of abundant food. It's a
24:02
fundamental human trade -off. We build our societies
24:04
where the resources are, and we tend to possess
24:06
a very short -term memory when it comes to geological
24:09
time. Yeah, that makes sense. Let's bring this
24:11
reality home and look at a specific geographic
24:13
context, unlike Australia. The Australian mainland
24:17
doesn't have any currently active volcanoes.
24:19
It sits safely right smack in the middle of its
24:22
tectonic plate, far away from the subduction
24:24
zones of the Ring of Fire. But there is a massive
24:27
volcanic province stretching across western Victoria
24:30
and southeast South Australia. And geologists
24:34
are very careful not to call it dead. They call
24:37
it dormant. That's a crucial scientific distinction.
24:40
Dead implies the heat source is permanently gone,
24:43
that the engine is completely cold. Dormant simply
24:46
means it is sleeping. The most recent eruptions
24:50
in that Australian volcanic province happened
24:52
just a few thousand years ago. Which, to human
24:54
civilization, the few thousand years sounds like
24:56
ancient history. But to the Earth? To the Earth,
24:59
a few thousand years is barely a blink of an
25:02
eye. Geologically speaking, that is incredibly
25:04
recent. The underlying mantle dynamics haven't
25:07
vanished. There is every reason to believe magma
25:10
will eventually find its way to the surface in
25:11
Australia again. Just a matter of geological
25:14
time. Wow. Well, pulling all of this together,
25:17
we've gone from the deepest, most inaccessible
25:19
rocks to the upper reaches of the stratosphere.
25:22
We've learned that underneath a volcano sits
25:24
a mostly solid slushy of crystalline rock. Yep,
25:27
the slushy. And to melt it, the Earth has to
25:30
drop the pressure by pulling plates apart. turn
25:33
a blowtorch hotspot on it, or force water out
25:36
of a subducting plate to lower its melting point.
25:38
And we explored how the personality of the eruption,
25:41
whether it flows gently or explodes catastrophically,
25:44
is entirely dictated by how much sticky silica
25:47
is in the magma. Right. That silica determines
25:49
whether the fizzy, expanding gases can safely
25:52
escape, or whether they get trapped until the
25:55
rock is literally blown apart. And if it does
25:57
blow apart, you aren't running from a river of
25:59
lava. You are facing pyroclastic flows of superheated
26:02
ash hugging the ground. Lahars roaring down river
26:06
valleys with the force of wet concrete. And if
26:08
the eruption is massive enough, a veil of sulfate
26:11
aerosols that can plunge the entire globe into
26:13
a cold, dark year without a summer. It really
26:16
highlights how deeply interconnected the solid
26:18
Earth, the oceans, the atmosphere, and human
26:21
history truly are. It's all one massive system.
26:24
It is. I want to leave you all with a final thought
26:26
to mull over, building on where we started this
26:28
whole journey today. The next time you are out
26:31
for a walk or at a garden center, and you pick
26:34
up a piece of light, airy pumice, or a dark,
26:37
heavy basaltic rock, realize that you aren't
26:40
just holding a standard stone. No, you're not.
26:42
Think about the incredible physical journey it
26:44
took to get to your hand. You are holding a rare
26:47
direct messenger from a hidden, completely alien
26:50
world, thousands of kilometers beneath our feet.
26:53
It's a place humanity will never, ever be able
26:55
to visit. But every so often that alien world
26:58
reaches up to the surface just to remind us it
27:00
is there.