For many villages around the base of Guatemala's Volcan de Fuego, June 3 was a market day. Just before noon, the volcano erupted. This wasn't anything out of the ordinary; the volcano had erupted earlier this year, after all, and the volcano's exhaust of ash and gas that twirled around its peak was, according to reports compiled by The Washington Post, "almost peaceful."

This cloud hid something, however, and three hours later, communities were struggling to evacuate before their residents were caught up in a quick-moving flow of mud, boulders, ash and lava that they could not outrun.

Volcanic threats can vary

It's been a season of volcanoes inflicting damage on their environments. Hawaii's Kilauea erupted multiple times May, resulting in a slow-moving and searingly hot river of destruction that hasn't killed anyone but has caused havoc in the landscape. Laze, a portmanteau of lava and haze, has also been a danger as the lava plunges into the ocean and releases plumes of hydrochloric acid.

As of June 6, Volcan de Fuego, Spanish for volcano of fire, has claimed at least 99 lives and almost 200 are reported missing, per CNN. The danger from the Guatemalan volcano is not the lava so much as it is the pyroclastic flow of lava and the aforementioned mud and boulders. This flow of volcanic material looks less like the bright orange flow of lava we think of and instead "looks more like a roiling slurry of wet concrete," according to the U.S. Geological Survey (USGS).

A lahar flows down the slope of Volcan de Fuego in Guatemala A pyroclastic flow slides down the slope of Volcan de Fuego in Guatemala. It reached temperatures of 1,300 degrees Fahrenheit (1,000 degrees Celsius). (Photo: Johan Ordonez/AFP/Getty Images)

Where the lava flow in Hawaii is sticky and slow, a pyroclastic flow moves quickly, sometimes reaching hundreds of kilometers an hour in speed as it slides down the slope of the volcano. Between a lack of a warning from emergency authorities and the swiftness of the flow's movements, residents of the villages near Volcan de Fuego simply did not have an opportunity to find safer ground.

Hilda Lopez, a resident of the San Miguel Los Lotes village, told AP that her mother and sister were still missing as of June 5, and that her father-in-law was almost certainly caught up in the lahar.

"We were at a party, celebrating the birth of a baby, when one of the neighbors shouted at us to come out and see the lava that was coming," she said. "We didn't believe it, and when we went out the hot mud was already coming down the street."

"My mother was stuck there, she couldn't get out," she added while weeping into her hands.

Layers of dangers

A lahar flows down from the peak of the Volcan de Fuego The Volcan de Fuego is a stratovolcano, a peaked volcano formed over the centuries by layers of ash, rock and lava. (Photo: Johan Ordonez/AFP/Getty Images)

The difference in the dangers between Volcan de Fuego and Kilauea comes down to the different types of volcanoes they represent. Kilauea is a shield volcano, meaning it's broad and domed in shape, and its lava oozes out of its fissures, according to National Geographic. Volcan de Fuego is a stratovolcano, perhaps the more recognizable between the two types, with its conical appearance. Like its name implies, the stratovolcano is made up of layers (strata) of lava, ash and rock.

When a stratovolcano erupts, there's not a great deal of slow-moving lava to be found. Instead, the rock layer is smashed to particles, literally. Tiny particles of rock dust combine with hot ashes and gas to form a cloud made up of this unpleasant mixture.

This cloud can become quite heavy and dangerous in its own right. One BBC report indicates Volcan de Fuego's cloud turned the early afternoon light into darkness, and a rescue team that was searching for a missing person before the eruption had to rely in flashlights to make their way back to a command post. Rocks and ash fell onto people, enough that the AP report says many people looked like moving statues.

The village of San Miguel Los Lotes after it was covered in ash and hot mud during the Volcan de Fuego eruption. The village of San Miguel Los Lotes was covered in ash and then inundated with a flow of hot mud during the Volcan de Fuego eruption. (Photo: Johan Ordonez/AFP/Getty Image)

The cloud of a stratovolcano will eventually collapses under its own weight. This collapse is what starts the pyroclastic flow, and the materials of the cloud mix with lava blocks and the surrounding environment, tumbling down the volcano's conical slopes and into the areas at its base.

The danger doesn't end there. Even once a stratovolcano stops erupting, as de Fuego has, loose rocks and volcanic debris are still hanging around the slope. Heavy rainfalls can wash all this material down the slope in a sort of volcanic mudslide called a lahar. It can also resemble the wet concrete river described by the USGS if there is only a little bit of rain. If there is heavy rainfall, the result can be flash floods of volcanic debris.