This story originally appeared on Time.com.
Dozens of people have died, hundreds are missing and many more homes were destroyed after separate volcanic eruptions in Hawaii and Guatemala that made headlines worldwide in recent months. And although those volcano eruptions are not part of a trend, experts have said, the volcanoes in question are far from the only dangerous ones in the world.
When assessing which volcanoes are the most dangerous in the world, however, several factors come into play, including population density surrounding active volcanoes, the types of magma that emerge during eruptions and each volcano’s eruption history. For example, an explosive volcanic eruption in a remote area is not as dangerous as one in a populous region that requires mass evacuations. And volcanoes that go a long time without erupting pose a higher risk for eruption because of mounting pressure inside them.
Magma content is one way to assess a particular volcano’s danger level, according to Stanley Mertzman, a geosciences professor at Franklin & Marshall College in Lancaster, Pa. Mertzman says experts will look at the level of silica, a rock-forming compound that affects the thickness of lava and a volcano’s overall shape, according to the USGS. Silica can indicate how dangerous an eruption can be because it tends to crystallize into chains in cooling lava, increasing its viscosity, in turn creating thicker magma that can make a volcano erupt more explosively.
“The degree of danger increases dramatically as silica content increases,” Mertzman tells TIME. “The higher the silica content, the greater the viscosity — the magma becomes stickier, thicker, much harder to stir.”
Rising silica levels in magma also give way to pyroclastic flows, a quick-moving mixture of lava, ash, gas and rock pieces that can be deadly because they cause asphyxiation, Mertzman says. “Picture a hot avalanche of material.”
Guatemala’s Fuego volcano eruption on Sunday, which killed at least 75 people, formed a pyroclastic flow.
Unlike Fuego, the lava flow from Hawaii’s Kilauea volcano, which has been erupting for a month, is a type of basalt. It poses little danger to people, unless they get in the way — thousands of people have been evacuated in the last month and Kilauea has destroyed several homes.
Regions where volcanic eruptions can be deadly include Indonesia, the Philippines, parts of South America and various volcanoes in the U.S. Below, see some of the world’s most dangerous volcanoes, based on their volcanic history, population densities and magma type.