Fountains of lava, whiffs of toxic gases, acidic plumes of vaporized seawater and blankets of ash: those are just a few of the dangers that volcanoes have delivered in recent weeks, with Guatemala’s Fuego Volcano and Hawaii’s Kilauea Volcano each producing its most powerful eruption in decades.

Dozens of people have died and thousands more have been evacuated in the area around Fuego as a result of its June 3 eruption. Like Kilauea, which in early May 2018 began a violent episode in an eruption that has carried on for nearly 35 years , Fuego was hardly at rest before this latest explosion. It often spews lava and ash several times in a year and can have many small eruptions in a single day.

But this time the Fuego eruption was different. Damage came fast and forcefully in a chaotic mixture of rock, gas and ash known as a pyroclastic flow – creating scenes of destruction bearing little resemblance to the images of creeping lava that have emerged from Kilauea. The contrast offers a reminder that the ways in which volcanoes become dangerous can be as varied as the places and communities where they rumble to life.

Stanford University geologists Gail Mahood (emerita) and Don Lowe, professors of geological sciences in the School of Earth, Energy & Environmental Sciences , have both studied volcanoes up close. They discussed surprising mysteries that remain for scientists around volcanic hazards, what and how researchers can learn from Fuego and other volcanoes after the ash settles and some of the science behind volcanic threats.

What differences caused Fuego to erupt so violently compared to Kilauea?

DON LOWE: We’ve just seen in Guatemala this small town buried in hot ash. Eruptions in the Americas tend to produce enormous amounts of ash, dust, rubble and angular blocks, and the volcanoes often have high, steep cones. Dense clouds of hot ash and rubble rumble down those slopes like freight trains and just gain momentum as they go. Rain and storms can mobilize loose rubble on these steep slopes and create cold debris flows. Hawaiian volcanoes tend to erupt less violent rivers and fountains of lava.

GAIL MAHOOD: All volcanoes in Central America and in the Cascades in North America are different from Hawaii in that they are related to subduction zones, places where one plate squeezes under another. They are much more explosive partly because magmas they erupt have more water dissolved in them – up to 10 times as much as in Hawaiian volcanoes.

Think of magma like a bottle of champagne. When you pop the top, you lower the pressure and that CO ₂ gas that was dissolved in the champagne forms bubbles and comes out.

Water, CO ₂ , sulfur gases, fluorine and chlorine are dissolved in magma when it is stored at high pressure deep in the Earth. But if magma rises quickly, those volatile elements come out of solution in bubbles that grow so fast the bubble walls break. It’s like a magma foam breaking into pieces and just flying apart.

Magmas in Hawaii might have water making up only half of a percent by weight. If you have 4 or 6 percent water in a magma as we see in Central America, you have that much greater potential for explosive eruptions.

Volcanoes in subduction zones also have more viscous, stickier magmas, which provide more resistance as the bubbles grow. As a result, pressures inside the bubbles can get much higher. So there are more bubbles breaking because of more water, and when the bubbles finally do break, they do so with greater force.

How do pyroclastic flows form after the eruption of a volcano like Fuego in Guatemala?

MAHOOD: These can form directly from an explosive eruption, or they can form by lava that comes out and cools a little bit, gets stuck and fills the vent. Then maybe there’s an earthquake, or new magma pushes it from below, and that lava plugging the vent comes out in a cascade of hot blocks. Those continue to effervesce and produce ash. People around Fuego are largely being killed by pyroclastic flows.

You’ve studied how volcanoes can trigger dangerous flows of not only lava and ash, but also thick, viscous mixtures of particles and water known as debris flows. Can you describe an example of how this type of flow begins, and what can be done to minimize harm once it’s underway?

LOWE: In 1985, debris flows following the eruption of Nevado del Ruiz volcano in Colombia killed about 20,000 people in a town called Armero, 60 kilometers (37 miles) downslope. These flows originated when hot ash, like the stuff coming out of Fuego, landed on a glacier around the summit. A minor puff of ash melted just a small part of that glacier and sent huge volumes of water cascading down canyons.

We learned by looking at older deposits in roadcuts around this town that this had been a common process in the past. All the elements were there that should have made a planner recognize that this was not a good place for a town. Better geological studies would have shown that the area had suffered many similar catastrophes in the past. In fact, hazard maps created in the months before the eruption showed Armero would be in the path of any mudflows (a type of debris flow) triggered by the volcano. But those maps were not widely distributed.

We still don’t fully understand how debris flows work or how some of them can travel so far over very low slopes. One promising theory is that water gets sucked under the main flow like hydroplaning on your tires. Another theory centers on how particles interact in the flow. Do they just kind of carry along passively in the fluid? Maybe particles in a debris flow behave like gas molecules in a balloon – they collide with one another, exert pressures and help keep themselves suspended.

These details are important to understanding how far debris flows can go, how much stuff they can carry, how quickly they form – all of which are relevant to whether you build towns around volcanoes, deciding how far away they need to be and evaluating the danger of settlements and villages that are already there.

Disaster officials in Guatemala have said Fuego’s June 3 eruption affected more than 1 million people. How does this compare to some of the biggest eruptions in history?

MAHOOD: This is not a big eruption by any stretch of the imagination. One of the big problems in Guatemala and many other places – in Indonesia and the Philippines, for example – is the large population packed on and around volcanoes of this type. Moderately small eruptions can kill a lot of people.

Fuego is a very active volcano. Presumably, what’s happening this time is it’s been a little more explosive than typical, so these pyroclastic flows are making their way farther down the volcano. Instead of going down the flanks of the pointy, conical volcano and kind of petering out, they’re getting out into the flanks and spilling out into villages.

Is it possible to anticipate whether and when a given eruption will produce this type of hazard?

MAHOOD: We can often predict that there’s an eruption coming. What’s harder to predict is the exact nature of the eruption and the time of onset.

To map volcanic hazards, volcanologists go into the field and map the footprints of an eruption: ash that fell from high in the air, deposits from pyroclastic and debris flows, and lava. Ash can cover tens of thousands of square miles, but the particles are cold by the time they land so they are catastrophic only close to the vent of the volcano, where they can be thick enough to collapse roofs.

Back in the lab, we use carbon or argon dating to learn about how frequent each type of eruption has been in the past. Increasingly, we’re also analyzing crystals that grew in the magma before an eruption. They act as tiny recorders of temperature, pressure and gas content, so they can help us reconstruct the ascent of the magma and its storage conditions.

Best of all is if these analyses can be integrated with geophysical studies of seismicity or deformation around the volcano. Fuego is difficult to observe because it is heavily forested, and once you get to the top it’s covered in clouds. Geophysicists have gotten very good at predicting eruptions at Kilauea in Hawaii and Mount St. Helens in Washington, because they have watched so many eruptions. We know very well the signs that magma is moving through the crust at Kilauea: the summit deflates and earthquakes change in style in a particular way. The only thing that is not certain is when an eruption like the ongoing one at Kilauea is going to stop.

Is there any way to adapt to threats from volcanic eruptions?

LOWE: We’ve settled areas without much concern about natural disasters and dangers. However, once a disaster occurs, we need to try to limit future growth in that area and in places that face similar risks. We may not see a truly catastrophic eruption in our lifetimes. But in our children’s or grandchildren’s lifetimes, there will inevitably be eruptions that wipe out major population centers. We need to realize how important it is to look further in the future than just tomorrow.

Donald Lowe is also Max Steineke Professor in the School of Earth, Energy & Environmental Sciences.