Volcanic Hazards: The Dangers Volcanoes Create

Which volcanic hazards kill the most people

A widely cited analysis of volcanic fatalities since 1500 (Auker et al., 2013) found that the danger is very unevenly distributed. The hazard people fear most — flowing lava — accounts for only a tiny fraction of deaths, because it usually moves slowly enough to escape.

The lesson is clear: the hazards that travel fast and far from the volcano — flows, mudflows, and waves — are the ones that matter most for survival. Understanding them is the foundation of any volcano safety plan.

Pyroclastic flows: the number one killer

A pyroclastic flow (or pyroclastic density current) is a ground-hugging avalanche of superheated gas, ash, and rock fragments. They form when an eruption column collapses or a lava dome fails, and they are almost impossible to outrun:

• Speed: commonly 100 km/h, with extremes exceeding 700 km/h.
• Temperature: 200°C to over 700°C — hot enough to kill instantly.
• Reach: tens of kilometers from the vent.

Pyroclastic flows destroyed Pompeii in AD 79 and, in 1902, obliterated the city of Saint-Pierre on Martinique when Mount Pelée erupted — killing roughly 28,000 people in minutes. Of the town's population, only a handful survived. These flows are the signature hazard of Plinian and Vulcanian eruptions and the reason stratovolcanoes are so feared. There is no defense except distance: evacuate before the eruption.

Lahars: volcanic mudflows

A lahar (an Indonesian term) is a fast-moving slurry of volcanic ash, rock, and water — essentially flowing wet concrete. They form when eruptions melt snow and ice, when crater lakes break out, or when heavy rain mobilizes loose ash, sometimes years after an eruption.

Lahars follow river valleys and can travel more than 100 km, burying everything in their path. The 1985 eruption of Nevado del Ruiz in Colombia was only a modest VEI 3, but it melted the summit glacier and sent lahars down onto the town of Armero, killing about 23,000 people — the worst volcanic disaster of the late 20th century. It is a stark reminder that a small eruption can be catastrophic if water and population are in the wrong place. Communities downslope of glaciated volcanoes like Rainier face this exact threat.

Volcanic ash

Volcanic ash is not soft like fireplace ash — it is pulverized rock and glass, abrasive and heavy. Its hazards are wide-ranging because ash travels the farthest of any hazard, carried by wind for thousands of kilometers:

• Roof collapse: wet ash is dense; just a few centimeters can collapse a roof, a major cause of injury in ashfall zones.
• Aviation danger: ash melts inside jet engines and can stall them. The 2010 Eyjafjallajökull eruption in Iceland grounded over 100,000 flights and cost the airline industry billions.
• Health: fine ash irritates lungs and eyes; an N95 mask offers meaningful protection.
• Infrastructure: ash short-circuits power lines, contaminates water, and clogs machinery.

Because ash spreads so widely, far more people experience ash fall than any other hazard. Preparedness — masks, water, and sealing your home — is covered in our safety guide.

Volcanic gases

Volcanoes constantly release gases, even when not erupting. Most is harmless water vapor, but several gases are dangerous in concentration:

• Carbon dioxide (CO₂): heavier than air, it pools invisibly in low areas. In 1986, a sudden release from Cameroon's Lake Nyos — a crater lake saturated with volcanic CO₂ — suffocated more than 1,700 people and their livestock overnight.
• Sulfur dioxide (SO₂): forms acid rain and volcanic smog ("vog"); large eruptions inject it into the stratosphere, cooling the climate.
• Hydrogen sulfide and fluorine: toxic; fluorine coats grass with a poison that has killed livestock after Icelandic eruptions.

The climate effect of SO₂ is dramatic at scale: the 1991 Pinatubo eruption lowered global temperatures by about 0.5°C for over a year.

Lava flows

Despite its fearsome image, lava is the least deadly major hazard. Most flows advance slowly enough — walking pace or less — for people to evacuate. The danger is to property, not usually life: lava buries and burns everything it touches but can be escaped on foot.

The 2018 eruption of Kīlauea in Hawaii destroyed more than 700 structures, yet caused no direct fatalities. The main exceptions are unusually fluid flows, such as those from Nyiragongo in the Democratic Republic of Congo, whose lava lake can drain and race downhill far faster than normal, threatening the city of Goma.

Tsunamis and secondary hazards

Some of the deadliest volcanic events happen offshore. Volcanic tsunamis are generated when eruptions, caldera collapse, or flank landslides displace seawater:

• The 1883 eruption of Krakatoa generated tsunamis up to 40 meters high that killed roughly 36,000 people — the vast majority of that disaster's deaths.
• In 2018, the collapse of Anak Krakatau triggered a tsunami that killed over 400 people in Indonesia with no warning.

Other secondary hazards include volcanic earthquakes, ground deformation, acid rain, and long-term famine — the 1815 Tambora eruption caused widespread crop failure far beyond Indonesia. These cascading effects are why volcanic risk is assessed across whole regions, not just the slopes of a single mountain.