What Happens during a Volcanic Eruption?

Step one: Boom!

You probably remember from elementary school that volcanoes are papier mache mountains filled with vinegar, red food coloring, and baking soda. And they are. But they’re also so much more.

If you have a slightly better memory, you might remember from elementary school that volcanoes are openings in the earth’s crust through which lava, rock fragments, hot vapors, and gases can escape. Most volcanoes are ancient by human standards, having existed for tens of millions of years. Recent estimates suggest that there are around 1,350 potentially active volcanoes in the world. The United States is home to 161. 

Some well-known US volcanoes include the Kilauea volcano in Hawaii, Mount St. Helens and Mount Rainier in Washington, and Mount Shasta in California. Maybe you live near one of those or near one of the other 157 volcanoes on lava nerds’ radar. We’re not here to tell you that the volcano near you is going to pop off, but we ARE here to tell you that it’s possible.


The 1980 eruption of Mount St. Helens killed 57 individuals.

Estimates suggest that 80,000 people and their homes are in Mount Rainier’s hazard zone.

But you’ll probably be fine?

If you have a REALLY good memory, you might remember from elementary school that volcanoes are grouped into one of three categories, depending on their relative risk of eruption:

  • Active volcanoes have erupted within the last 10,000 years, or are periodically erupting in the present.
  • Dormant volcanoes are not erupting and have not had recent eruptions, but are expected to erupt at some point in the future.
  • Extinct volcanoes are not expected to erupt again in the future.
Rule of Thumb

The United States is home to the highest number of active volcanoes in the world; many are in Hawaii and Alaska. Not to be outdone by the volcanoes, we also have the most tornadoes in the world, most of which do not occur in Hawaii or Alaska.

What’s Dangerous about a Volcanic Eruption?

Eruptions aren’t all the same, as you know from your baking soda and vinegar days. Pressure from gas and magma build below the crust until they’re able to escape through the nearest exit, AKA the volcano’s vent. Depending on the chemical composition of the magma, two things can happen when it reaches the surface:

  1. Low-viscosity (“thinner”) magma pours out of the rock opening and flows freely, allowing trapped gas to escape
  2. High-viscosity (“thicker”) magma resists flowing, remains stagnant and prevents trapped gases from escaping gradually. This leads to an explosive eruption.

Number two is what many of us think of when we imagine a volcano erupting. It’s a little bit like shaking a corked champagne bottle. As interior gases and liquid move to the top, pressure builds until the cork is blown off. Instead of bubbly wine, a volcano’s pop releases gas, ash, and lava. That’s right, it’s not magma once it’s out in the open; it’s lava.

Fresh lava, at an average temp of more than 2000°F, is insanely dangerous. Humans, structures, and wildlife are all highly flammable, so any lava encounter could result in severe burns or even spontaneous combustion. Just being near a river of lava could cause fatal heat exposure. Volcano eruption is also a wildcard disaster in that it can cause secondary disasters like wildfires, rock falls, and mudslides. Truly a top notch catastrophe.

The Most Common Eruption Hazard: Falling Ash

Stand aside, lava! Falling ash is the most widespread and frequent eruption-related hazard. Ash is composed of rock, minerals, and glass, in tiny particles no bigger than 2mm in diameter. During a type 2 eruption, ash plumes are ejected into the air. These clouds of ash are picked up by wind currents and can affect communities across hundreds (or even thousands) of square miles.


More elementary school memories for you to cherish: Most people killed in the Mount Vesuvius eruption at Pompeii were killed by ash. The ash also preserved their postures of terror at the point of death.

Ash fallout is tiny and sharp. These qualities make ash a formidable foe for human airways, lungs, and eyeballs. Additionally, ashfall that leaves a thick layer of residue behind can create a host of other issues:

  • Poor air quality for breathing
  • Collapsed roofs
  • Clogged air intake fans and air conditioning units in homes and cars
  • Contaminated water supplies
  • Potential damage to infrastructure and mechanical equipment
  • Danger to local wildlife and livestock

Worse yet, accumulated ashfall can create serious long-term problems for locals. (See our section on Post-Eruption Concerns for more.)

In sum, when a volcano erupts near you, you could burn to death, be smashed by falling rocks, or be inundated with ash… which can kill you or cause devastating health problems. The odds are not in your favor. So who really needs to prepare?

lava flow from volcano, what happens during a volcanic eruption
We know it’s a killer, but it’s just so pretty.

About the Authors

It takes a village! We are researching, writing and fact checking as a family. Collaboration is the name of the game, whether we’re running from a zombie horde or finding the best way to turn a complex concept into a deliciously digestible set of bullet points.

Katherine Esperanza is a Los Angeles based writer. When she's not conjuring new queer slice-of-life short stories, she's busy watching the newest films, out at queer shows, supporting queer artists, or just checking out the queer community as a whole.

A former international non-profiteer, small business owner, and co-op'er, Katherine is delighted to help introduce more leftist politics into the disaster preparedness/prepper sphere, which is currently far too right-wing.