The 5 Criteria That Make a Volcano Really Dangerous
🌋 The Checklist Behind History’s Deadliest Eruptions
At any given moment, there are around 20 volcanoes actively erupting.
But thankfully, not all of them are ticking time bombs like Yellowstone.
With nearly 1,500 volcanoes scattered across our continents, it’s impossible to monitor each one closely at all times.
Some are clearly more dangerous than others, that’s for sure, but what makes one volcano really more dangerous than another?
That’s exactly what this article is about:
5 concrete criteria that help assess how dangerous a volcano really is.
#1 Proximity to Populated Areas
This first one actually has nothing to do with the volcano itself.
After all, the volcano chose its spot waaaay before humans decided to settle nearby.
But when we talk about how dangerous a volcano is, we’re obviously talking about how dangerous it is to us, our societies, and our lives.
A volcano erupting in the middle of the Siberian plains, with no one for hundreds of kilometers around, isn’t really a big concern (although we’ll come back to that in a later criterion)
But a volcano like Mount Vesuvius, which is located just 22 km from the center of Naples with a population of nearly 4.5 million, that’s a whole different story.
For your general knowledge, that’s what we call “vulnerability.”
Because a risk only exists at the intersection of a hazard (like a volcanic eruption) and a vulnerability (like human lives).
And just like in any multiplication:
The higher one of the factors, the higher the result
(and in our case, the risk)
That’s why the biggest, tallest, most powerful volcano on Earth isn’t necessarily the most dangerous one. It all depends on its proximity to populated areas
#2 Eruption Type
Not all eruptions are the same, and a single volcano can produce very different kinds of eruptions.
But here’s the key takeaway for this section:
Lava is actually the least dangerous part of a volcanic eruption.
Effusive Eruptions
Since we’re talking about lava, let’s start there.
Effusive eruptions are characterized by long, slow-moving lava flows.
They look incredibly impressive on camera (perfect for drone shots) but they rarely pose a serious threat to human life.
Simply because they give you time to evacuate.
Now, if you don’t evacuate and lava swallows your house… yeah, there’s no way you’re walking out of that.
Volcanoes like Kīlauea in Hawaii are well-known for this type of eruption.
Phreatic / Phreatomagmatic Eruptions
These eruptions happen when magma comes into contact with water.
If you’ve ever poured cold water onto a hot pan, you’ve seen a mini version of what happens.
They’re not as violent as the next type we’ll discuss, but the danger lies in their total unpredictability.
And when it comes to risk management, unpredictability is your worst enemy.
A recent example is the Whakaari / White Island eruption in 2019, which killed 22 people visiting the volcano.
The Netflix documentary “The Volcano: Rescue from Whakaari” is well worth watching.
Explosive Eruptions
The most dangerous thing about a volcanic eruption is, without a doubt, the pyroclastic flow.
These are mixtures of hot gas, ash, and rock that race down the slopes at speeds over 100 km/h (62 mph) and reach temperatures up to 700°C (1300°F).
In other words: no one outruns that.
They’re usually triggered when the eruptive column (that towering plume of ash and gas) collapses or when a lava dome violently explodes.
Explosive eruptions are always the deadliest of all, and they’ve caused the most catastrophic volcanic disasters in History:
- 🌋 Mount Tambora (1815) — 250,000 deaths
- 🌋 Krakatoa (1883) — 36,000 deaths
- 🌋 Mount Pelée (1902) — 30,000 deaths
So… when assessing how dangerous a volcano is, looking at how it erupts is obviously one of the most important criteria.
#3 Secondary Hazards
You thought eruption type was the only deadly danger a volcano could bring?
Far from it.
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Volcanoes can be the starting point of a deadly chain reaction, triggering disasters that are sometimes even worse than the eruption itself.
Let me start by introducing you to lahars.
They’re a mix of volcanic ash and water (from rain, melting snow, or crater lakes) that turns into a fast-moving, concrete-like mudflow.
But since it’s volcanic ash rather than mud, we call it lahars.
But their danger and destruction is just as great as a mudslide, i.e. a terrestrial tsunami!
One of the most tragic examples is the 1985 eruption of Nevado del Ruiz.
The eruption itself wasn’t massive, but the heat melted the mountain’s glacier. That triggered a huge lahar that rushed down the valley, buried the town of Armero, and killed 25,000 people.
Speaking of tsunamis, volcanoes can cause those too. And there are two ways it can happen:
First, a volcanic landslide collapsing into a body of water (like a lake or the ocean), generating a giant wave.
➤ This happened in 1980 with Mount St. Helens, when an entire side of the volcano collapsed into Spirit Lake — it triggered a wave nearly 180 meters high.
Second, a massive volcanic explosion, underwater or on an island, that violently displaces water.
➤ That’s exactly what happened during the 1883 Krakatoa eruption. The island was blown apart, creating a tsunami over 30 meters high.
So when assessing the danger of a volcano, never stop at the eruption itself and always ask: What can happen next?
#4 Eruptive History
Looking at the past to anticipate the future.
It’s definitely not an exact science, but these are trends we simply can’t afford to ignore.
Has the volcano been unpredictable over the last few millennia or relatively calm?
Does it have a history of sudden explosions or just gentle lava flows?
And most importantly: what’s its eruptive cycle?
A volcano generally erupts when excessive pressure builds up in its magma chamber. In simple terms: when it’s full, it blows.
And the time of this accumulation can be “regular” in the life of the volcano (regular on geological scales we can talk about thousands of years of differences…).
A volcano that erupts every 1,000 years or so and hasn’t erupted since 1,500 can be considered dangerous if it’s still active.
This criterion is also closely tied to the type of past eruptions:
A volcano with an explosive history isn’t treated the same as one with a calm, effusive background.
That’s why these criteria should never be looked at in isolation, they need to be connected and compared.
Take Yellowstone , for example. Its last eruption is believed to have occurred around 640,000 years ago.
But its estimated eruption cycle is 725,000 years. So “technically”, there’s nothing to worry about.
However… past behavior never guarantees future outcomes.
#5 Magma Volume
For this final factor, let’s stick with the example of Yellowstone, which is perfect for explaining it.
Why is it so famous and so feared?
Its size.
(And also because it’s in the US we won’t lie…)
More specifically: the volume of magma it holds beneath the surface.
Because a volcano isn’t just a smoking crater. It’s a massive underground system, and the amount of magma it stores gives us a glimpse of the destruction it could bring.
This last criterion is, without a doubt, about the volcano’s true potential, and that naturally comes down to the volume of magma it contains.
The more magma a volcano holds, the more devastating its eruption can be.
Simple as that
Final Words (For Quick Glancers)
- Proximity to Populated Areas: A volcano is only a threat if there’s something — or someone — to threaten.
- Eruption Type: Not all eruptions are born equal: some flow like honey, others explode like bombs.
- Secondary Hazards: Lahars, tsunamis, ash clouds — the eruption may not be the deadliest part.
- Eruptive History: A volcano’s past is one of the best clues to understanding its future behavior.
- Magma Volume: The more magma beneath the surface, the more devastating the eruption can be.
Now that you know what makes a volcano truly dangerous, the next question is:
How do we keep an eye on them before it’s too late?
I break it down in this article on ⬇️
Thank you for taking the time to read my work!
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