Why Do Volcanoes Erupt?
— It's the Same as Opening a Fizzy Drink
A volcanic eruption isn't "magma blasting out because of heat." It's gas that was held under pressure suddenly turning into bubbles once that pressure is released. That's exactly what happens when you open a can of fizzy drink. The only difference is whether the contents are runny or thick.
An unopened bottle just looks like plain liquid. No bubbles in sight. The carbon dioxide is dissolved in the liquid — it's simply invisible.
Open the cap and you get a hiss, and bubbles appear. The contents haven't changed. Only the pressure has changed. With the squeezing force gone, the gas can no longer stay dissolved, so it turns to bubbles.
Now imagine the contents were thick molasses instead of water. The bubbles couldn't escape upward — they'd push the whole mass up with them. Shake it hard first, then open it, and it sprays out.
That's a volcano. The bottle is the ground, the contents are magma, and the dissolved gas is water and carbon dioxide.
Deep underground, everything is under intense pressure, so both water and carbon dioxide stay dissolved in the magma. As magma rises and pressure drops, they can no longer stay dissolved and become bubbles.
If the magma is runny, bubbles escape and lava flows quietly. If it's thick, bubbles get trapped and burst all at once once they hit their limit.
In other words, what decides how violent an eruption is isn't temperature — it's how "sticky," or viscous, the magma is. Let's look at this step by step.
Underground, the gas is "invisible"
Magma contains a few percent water and carbon dioxide. But deep underground, none of it exists as bubbles. Intense pressure keeps it squeezed and dissolved inside the magma.
The amount of gas a liquid can dissolve increases with the squeezing force. Fizzy drinks are bottled under high pressure precisely because otherwise the carbon dioxide wouldn't all fit.
As magma rises, the weight of rock above it keeps decreasing. Less squeeze means less gas can stay dissolved. The excess becomes bubbles.
From here things speed up. Once bubbles form, the whole mass of magma gets lighter, so it rises even faster. Faster rising means pressure drops further, which makes more bubbles. It becomes a runaway loop.
It's the cap coming off, and bubbles forming.
Whether bubbles can escape decides everything
This is where magma's viscosity becomes decisive. Viscosity depends heavily on the proportion of one particular component in the magma. Less of it means runny; more means thick.
Bubbles escape upward, so pressure never builds up. The eruption takes the form of lava flowing out, like a river of red. Hawaii's volcanoes are this type.
Bubbles can't escape and keep swelling from within. Eventually the magma itself is torn apart into fragments, blasting out with tremendous force.
Case B — "the magma is torn apart" — is the single most important moment in an eruption. What was liquid an instant earlier turns, in a flash, into a mixture of fine fragments and superheated gas. These fragments become volcanic ash if they rise into the sky, or a pyroclastic flow if they race down a slope.
Most Japanese volcanoes lean somewhere between the middle and the thick end. That's why Japan has to prepare for explosive eruptions.
A pyroclastic flow is superheated ash, rock fragments and gas racing down a slope together as one mass. It's said to reach several hundred degrees Celsius and can exceed 100 km/h.
Seeing it and then running is, in practical terms, not an option. It doesn't necessarily follow valleys — it can go straight over a ridge. The only real countermeasure is not being within its reach in the first place.
That's why evacuation must happen the moment a warning is issued, not after something visibly starts. The fact that things "still look fine" is the most dangerous part of this hazard.
What you can do before climbing, or where you live
- Always check the eruption warning level before climbingThe Japan Meteorological Agency issues one for each volcano. Don't climb a mountain whose level has been raised. "I came all this way" is the most dangerous excuse. File a climbing plan too.
- On the mountain, keep shelter and a helmet in mindVolcanic bombs — rocks tens of centimetres across — can fly in at high speed. Check the location of a helmet and of shelters or evacuation huts on a map before you climb. A sturdy rock overhang is a backup option.
- If you notice an eruption, get away from the crater and follow instructionsProtect your head and head upwind or downhill. If ash starts falling, protect your mouth and eyes with a mask and goggles (or a towel and glasses if that's all you have). Call 119 if anyone is injured. After that, follow information from local authorities and the Japan Meteorological Agency.
It's not burnt residue — it's fragments of glass and minerals formed when magma is shattered. The edges are sharp, and it behaves like this:
- It cuts your eyes and throat. Rinse with water rather than rubbing
- It gets heavier when wet. Roofs can collapse under the weight once it rains
- It gets into machinery. Car engines and air conditioning get damaged; aircraft can't fly
- It turns into mudflows in the rain. The danger continues with every heavy rain, even after the eruption ends
Once ash has settled, the standard advice is to hose it down or dampen it before collecting it, rather than sweeping it, because sweeping dry ash into the air makes it easy to inhale.
Something you can check in your kitchen (no fire needed)
- Get two clear glasses and fill one with ① water and the other with ② honey or syrup
- Blow gently through a straw into each and watch what happens to the bubbles
- In water, bubbles rise and vanish immediately. In syrup, bubbles stay trapped inside and swell
- Next, get a fizzy drink and compare it before and after opening the cap. Confirm no bubbles are visible before opening
- (Somewhere safe) shake it a little before opening, and check how the force changes too
The difference between water and syrup is exactly the difference between Hawaii's volcanoes and Japan's. Even with the same amount of bubbles forming, whether they can escape completely changes the outcome. If you shake and open a fizzy drink, do it outdoors or over a sink.
Summary
Volcanoes erupt because gas that had been dissolved under pressure underground turns into bubbles as the magma rises and pressure drops. It's the same principle as opening a fizzy drink. And what decides the violence isn't temperature — it's whether the bubbles can escape, which comes down to the magma's viscosity.
The type of eruption is decided underground.
That's why preparation differs from mountain to mountain.
Bubbles that form this way and freeze before fully escaping become pumice. Why a rock you can hold in your palm floats on water is explained in Why Does Pumice Float on Water Even Though It's a Rock?. And the story of how the pressure wave from a huge eruption circled the Earth several times is covered in How Far Can the Sound of a Huge Eruption Travel?.
Want to know more? — Terms, numbers, and links to the textbookWe label each level clearly, from junior-high science to topics still being researched
- JHSCovered in junior-high school science
- HSCovered in high-school "Earth Science Basics" / "Chemistry Basics"
- HS+Covered in high-school "Earth Science" / "Chemistry," or an advanced/sidebar topic in textbooks
- Univ.Not taught in high school — content from a university specialist course (volcanology)
- ResearchNot even taught as settled fact at university — something researchers are actively investigating
JHSTerms: the vocabulary of volcanoes
- Magma: molten rock underground. Once it flows out onto the surface, it's called lava.
- Volatile components: water, carbon dioxide and the like dissolved in magma. This is the "gas" in the main text.
- Viscosity: resistance to flow. In magma it changes hugely with the proportion of silicon dioxide.
- Pyroclastic flow: superheated ash, rock fragments and gas racing down a slope together. Considered the most dangerous volcanic hazard.
- Eruption warning level: a 5-level scale the Japan Meteorological Agency issues for each volcano, each level mapped to a recommended action.
HSChecking with a formula: how many fizzy-drink bottles' worth of pressure is 5 km underground?
The main text said "drop the pressure and it becomes bubbles." So how much pressure is actually down there? We can calculate this.
Pressure = density × gravity × depth
| Pressure | units of Pa (pascals) |
| Density | weight of the rock above. About 2700 kg/m³ |
| Gravity | 9.8 m/s² |
| Depth | units of m |
Same idea as your ears hurting at the bottom of a swimming pool. The weight of whatever is above you becomes pressure, directly. Underground, what's above is rock rather than water, so the numbers jump by orders of magnitude.
And the amount of gas a liquid can dissolve is proportional to pressure (Henry's law). At 1300 times the pressure, 1300 times the gas can dissolve. The answer to ① directly decides how much gas magma can hold dissolved.
| Depth | 5 km = 5000 m |
| Substitute | 2700 × 9.8 × 5000 = 132300000 Pa |
| Atmospheric pressure | about 101300 Pa |
| Multiple of atmospheric pressure | 132300000 ÷ 101300 ≒ 1306 times |
5 km underground is about 1300 times atmospheric pressure. A sealed fizzy-drink bottle is said to hold roughly 2–3 atmospheres inside. So this is being squeezed by roughly 500 times that force.
That's why the gas can stay dissolved. You don't see bubbles not because there's no gas, but because it's being squeezed.
As pressure drops, a gas's volume grows inversely. At 1/1300th the pressure, the volume is 1300 times bigger.
| Gas that became bubbles underground | say 0.1 L |
| By the time it reaches the surface | 0.1 × 1300 = 130 L |
Half a glass of gas swells into a bathtub's worth. And it keeps expanding all the way up, the whole time it's rising.
This is what the main text's "runaway loop" actually consists of. Expand → get lighter → rise faster → pressure drops further → expand more. There's nowhere along the way for it to stop.
※ In reality temperature also changes, and dissolution doesn't follow a simple proportion. This is a calculation for grasping the order of magnitude.
The main text said "you cannot outrun it." This too can be checked with numbers.
| Speed of a pyroclastic flow | sometimes cited as 100 km/h |
| Converted to m/s | 100 ÷ 3.6 ≒ 28 m/s |
| A person's full sprint speed | about 8 m/s (100 m in 12.5 s) |
| Speed ratio | 28 ÷ 8 = 3.5 times |
It closes in at 3.5 times a full sprint. Even if you spot it 1 km away, it reaches you in 1000 ÷ 28 ≒ 36 seconds. In that time you could run about 8 × 36 = 288 m.
"See it, then flee" isn't an option on paper. That's why evacuation has to happen the moment the warning level rises, not after something happens. The numbers dictate the order of action.
HSHow much dissolves is decided by pressure
The amount of gas that dissolves in a liquid is proportional to the force that gas exerts. This is called Henry's law. Bubbles appearing when you open a fizzy drink is this law in action.
| Water content of magma | roughly a few percent (varies by type) |
| Runny magma (low silicon dioxide) | about 45–52% / lava-flow eruptions |
| Intermediate magma | about 52–63% / common in Japan |
| Thick magma (high silicon dioxide) | about 63%+ / explosive eruptions |
| Difference in viscosity | varies by orders of magnitude between types (can be tens of thousands of times) |
※ These boundary values are typical benchmarks. Temperature and water content also affect viscosity.
What's interesting is that more water makes magma runnier. But once it rises and loses that water, viscosity suddenly increases. Magma that has partly degassed can actually get more prone to clogging.
HS+Why more silicon dioxide means more viscous
Inside magma, tetrahedra formed from silicon bonded to oxygen are the basic unit. When the proportion of silicon dioxide is high, these tetrahedra share oxygen with each other and link into long networks.
The more developed the network, the more bonds have to break for the material to flow, which increases resistance. That's the essence of "viscous." Adding water breaks up this network, so viscosity drops.
And the decisive moment of an eruption is fragmentation. Once the bubble fraction reaches roughly 70–80% of the volume, the films between bubbles can no longer hold, and what was a continuous liquid switches over into a flow of gas carrying flying fragments. The instant this switch happens, the eruption becomes explosive.
Univ.Why is a pyroclastic flow so fast?
The body of a pyroclastic flow is a mixture of superheated particles and gas. Rather than particles colliding directly with each other, the hot gas between them keeps the particles suspended as the flow moves. Friction drops enormously, so a dense fluid descends a slope with almost no resistance.
Because of this, it doesn't just fill valleys — its momentum can let it go straight over a ridge. "Safe because you're away from the valley" doesn't hold. Predicting how far it will reach uses numerical models combining eruption volume and terrain, but uncertainty remains.
The picture of what a magma chamber looks like has also been updated. It used to be imagined as "a room of liquid magma sitting underground," but the now-favoured picture is that it exists for long stretches as a crystal-rich, porridge-like mush, which rapidly liquefies just before an eruption due to changes in temperature and pressure.
ResearchPrediction still isn't good enough
- We still can't pin down "when, and how big." We can detect precursors like volcanic earthquakes, ground swelling, and changes in gas composition. But eruptions sometimes don't follow precursors, and predicting scale is even harder. Monitoring is possible; forecasting is not, as things stand.
- Steam-driven eruptions remain especially hard to predict. In this type, magma itself doesn't rise — groundwater is heated instead — and precursors can be tiny, sometimes showing no change right up until the moment. The 2014 eruption of Mount Ontake became a case that exposed this difficulty. It struck during a time of day with many climbers on the mountain, and many lives were lost. Since then, monitoring has been strengthened and information sharing with climbers has improved.
- The outlook for giant caldera eruptions is even less certain. These are extremely rare, but their impact would be widespread if one occurred. Research estimates frequency from past deposits, but we're nowhere near being able to say when the next one will happen.
- The real state of magma chambers is also hard to observe. We infer it from seismic waves and crustal deformation, but there's no way to directly see the "porridge-like" state several to over a dozen kilometres down, so models carry a wide range of uncertainty.
Since prediction isn't possible, preparation, and following warnings when they're issued, remains the most reliable countermeasure today.
Links to the textbook (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| JHS | Science — volcanoes and igneous rock / gas dissolution | Magma viscosity and eruption type, comparison with fizzy drinks |
| HS | Chemistry Basics — gas solubility (Henry's law) | Gas no longer staying dissolved as pressure drops |
| HS | Earth Science Basics — igneous rock and volcanic activity | Silicon dioxide proportion and eruption style |
| HS+ | Earth Science — magma properties / Chemistry — silicates | Network structure, viscosity, and conditions for fragmentation |
| Univ. | Volcanology / geophysics | Pyroclastic flow dynamics, picture of magma chambers |
| Research | Volcanology (unresolved) | Eruption forecasting, steam eruptions, caldera eruptions |
| — | Disaster-preparedness education | Eruption warning levels, evacuation shelters, dealing with ashfall |
- Explanatory materials on eruption warning levels and volcanic activity, and volcano monitoring, from the Japan Meteorological Agency (気象庁).
- Sparks, R. S. J. et al., Volcanic Plumes and related volcanology textbooks (physics of vesiculation, fragmentation, and eruption columns).
- Cashman, K. V., Sparks, R. S. J. & Blundy, J. D., Vertically extensive and unstable magmatic systems, Science 355, 2017 (updated picture of magma chambers).
- Evacuation plans for volcanic hazard warning areas and information for climbers, from the Cabinet Office (内閣府) and local governments.
- Information on dealing with volcanic ash, from the Volcanological Society of Japan (日本火山学会).
※ Values for temperature, speed and composition vary greatly between volcanoes and eruptions. This article uses commonly cited benchmark figures.
※ This article is a general-audience science explainer. For decisions about volcano-related action, follow the eruption warning levels issued by the Japan Meteorological Agency and the instructions of local authorities, fire departments and police. Check the latest volcano information before climbing, and file a climbing plan. The figures given here are benchmarks for understanding the underlying mechanism.