Two of Earth’s great volcanoes are erupting right now, but one of them is far deadlier than the other. While lava from Hawaii’s Kilauea Volcano has swallowed up huge swaths of the Big Island — including the Mayor’s house — the eruption has fortunately not claimed any lives. Sadly, the same can’t be said for Guatemala’s furiously exploding Volcán de Fuego, which has killed over 60 people and injured hundreds more. Its deadliness stems from what’s inside it.

These volcanoes are equally formidable in their own right, but, as volcanologist Janine Krippner, Ph.D. tells Inverse, Volcan de Fuego has a much more dangerous type of magma than Kilauea. What it all comes down to is its stickiness, or viscosity, which in turn determines the behavior of magma’s far more explosive element: its gas.

“Magma is very, very gassy stuff,” she says. “As it comes closer to the [Earth’s] surface, a lot of that gas starts to come out.”

When it comes to a volcanic eruption, runnier lava — like that pouring out of Kilauea — is much less dangerous because it allows gas molecules to easily escape from it. Thicker, stickier lavas trap those gas molecules within, which creates a buildup of pressure. And when that pressure gets too high, things blow up.

“It’s like shaking up a bottle of Coke and then taking the cap off, where the gas bubbles expand rapidly and they blow the magma apart,” says Krippner. “And that’s the volcanic ash that we see, as well as the pyroclastic flow.”

As Inverse wrote previously, pyroclastic flows aren’t just molten rock. They’re made of lava, mixed in with chunks of lava blocks, pumice, ash, and volcanic gas. The rocks within the flow can move at rates of up to 50 miles per hour, and the flow itself moves even faster.

“They’re like an avalanche of solid rock material that goes racing down the volcano,” says Krippner.” “It’s moving along the ground extremely rapidly. You cannot outrun this. If you’re stuck in a valley or something like that, it’s much too quick.” One of the most devastating eruptions of all time, the Vesuvius eruption that swallowed the entire city of Pompeii in 79 CE, was characterized by its pyroclastic flow.

Both Kilauea and Fuego have technically been erupting for a long time, but there’s one key difference that volcanologists can’t fully explain: These eruptions are huge. “Fuego does have what we call ‘paroxysms,’ or larger eruptions than usual,” Krippner explains. “It’s not completely abnormal for Fuego to produce larger eruptions, but this is much, much bigger than anything we’ve seen recently.”

We can predict to some extent when a volcano will erupt, but there’s not really a good way to tell how big the explosion will be. As the situation in Guatemala illustrates, this gap in our knowledge about volcanos can have tragic consequences.

“That’s what’s so bad about this eruption,” says Krippner. “It’s much larger than usual and so by the time it was realized how big it was, it was too late.”