On Sunday, Guatemala's Volcán de Fuego — "Volcano of Fire" — erupted violently, spewing ash nearly four miles into the air and burying villages under an avalanche-like phenomenon called a pyroclastic flow.
The disaster's fatalities had risen to 62 by Monday afternoon, with hundreds more injured. The death toll is expected to climb, and the country's disaster agency said 3,265 people had been evacuated.
In the aftermath, some residents criticized local authorities, asking why there was no advance warning and why officials waited so long to tell them to leave. The 12,000-foot "Volcano of Fire" has been erupting on and off since 2002, and Guatemala has a national institute — INSIVUMEH — committed to actively monitoring the country's volcanoes.
The PBS NewsHour spoke with a panel of volcanologists about why this eruption was so devastating. They said despite the destruction so far, the worst may be yet to come.
Why did Fuego's pyroclastic flow kill so many?
Fuego's lethal eruption took the form of a pyroclastic flow, the same searing cloud of debris that cooked and choked the city of Pompeii after Mount Vesuvius exploded in 79 AD.
On its surface, a pyroclastic flow looks like a falling cloud of ash. But if you could peer into the cloud, you would find a really hot and fast-moving storm of solid rock, said Janine Krippner, a volcanologist at Concord University who studies pyroclastic flows.
"It's not really like anything else on Earth," Krippner said. People are familiar with avalanches of rock or landslides, but pyroclastic flows move much more quickly, traveling more than 50 miles per hour. The upper part of the pyroclastic flow resembles a grainy sandstorm, but it is filled with hot gases, whose temperatures range from 400 to 1,300 degrees Fahrenheit.
"The bottom [of this cloud] is a jumble of chaotic [lava] rocks. It's large boulders that are breaking up into smaller pieces," Krippner said. "They can knock trees down like matchsticks and destroy houses. They can send cars flying. They're incredibly dangerous."
To understand how pyroclastic flows are made, Krippner said, look no further than a bottle of Coca-Cola.
While people tend to picture magma as a liquid, it actually contains a large number of gases, too. But you can't really see the gas in the lava because it is under so much pressure — like soda in a bottle.
"When you reach the surface with magma, there is a lot less pressure because there's a lot less rock, so the gas comes out," Krippner said. "During a violent eruption, that gas expands rapidly, forming bubbles in the magma. That then explodes, blowing the magma apart like shaking a bottle of coke and then opening the top."
But instead of foam, Fuego released sprays of solid rock.
Krippner said Fuego's latest eruption produced a larger-than-average pyroclastic flow, given it spread more than 10 kilometers downslope of the volcano crater. This may explain why it took so many by surprise.
Pyroclastic flows are also very different threats than the ones posed by the lava flows in Hawaii, which move sluggishly, said Erik Klemetti, a volcanologist at Denison University in Ohio.
"It's definitely not vog [volcanic fog]. These pyroclastic flows just erase everything in their path," Klemetti added.
Klemetti said water also compounded the danger. When combined with water, from rain and the river channel, the ash in the pyroclastic flow creates lahar, the technical term for volcanic mudslides. These mudflows move slowly but carry lots of strength — like a flood of concrete moving down the mountain.
Eruptions can be predicted weeks, months or even years in advance, thanks to decades of advances in volcanic monitoring. But two circumstances may have created blind spots in forecasting Fuego's eruption on Sunday, said volcanologist Diana Roman of the Carnegie Institution of Washington.
First, officials had only installed a single seismometer on the volcano.
This monitoring system is rudimentary compared to those on other volcanoes, where scientists may use seismometers — to detect earthquakes — along with gas meters to measure venting. Modern volcanic monitoring also employs pressure sensors and satellites to detect deformations in the ground.
"One of the issues with Fuego is that it's very difficult to access. It's very difficult to keep instruments going up there," Roman said, though its just 10 miles from one of Guatemala's biggest tourist destinations.
Volcanologists can predict eruptions from active volcanoes, but it requires comprehensive monitoring of things like the number of earthquakes, the types of earthquakes, increases in gas emissions and deformation.
The second blind spot in Fuego forecasting was potentially caused by its constant activity. Fuego typically has several large explosions per year and many small eruptions, occurring as often as several times a day. It erupted for about 20 hours in January of this year, and forced a minor evacuation of 100 people in 2015.
"One of the things we're looking for is an increasing amount of unrest. If a volcano is continuously active, it's harder for us to see if it's gone from a quiet state to a heightened state," Roman said.
It's also unclear what Fuego will do next. Roman said sometimes volcanoes like Fuego will produce a large eruption and then become quiet, and other times, they have additional events. Regardless, she said, the rainfall and the subsequent lahar could continue to cause damage and threaten lives.
What's up with all of these eruptions and earthquakes?
But experts say this isn't necessarily a reason for extra concern.
"On any given week, globally on average, we can have as many as 15 to 20 volcanoes in some state of eruption or unrest," said Charles Mandeville, coordinator of the U.S. Geological Survey's Volcano Hazards Program. Most of these eruptions don't cause any harm and go unnoticed by the media.
But even when scientists spot an eruption weeks in advance, such as they did with Kilauea, the threats can be difficult to predict. Kilauea has been erupting for a month, yet new evacuation orders are being issued on almost a daily basis. Volcanic fog from Hawaii has now spread as far as the Marshall Islands, 2,300 miles away. Hawaiian officials are now arresting people who loiter near lava flows or refuse to evacuate.
Mandeville said part of the unpredictability revolves around Kilauea's fissures. Older fissures, which had stopped spewing lava, have now reopened unexpectedly. He said Kilauea has spewed more than 3 million cubic meters of lava — enough to fill 1,200 olympic swimming pools — and this may only represent 2 to 3 percent of the molten rock that could erupt.
While eruptions like those at Fuego and Kilauea are fascinating, Krippner said people should always try to recognize their risks and avoid getting too close.
"We saw those videos yesterday of people standing on a bridge, watching a pyroclastic flow coming toward them. They could have been killed," Krippner said. "It's so important that opportunities like this are taken to inform people of what these processes are, how dangerous they are and what people need to do to stay safe."