Walking atop an explosive volcano, like White Island, is always dicey

Volcanologist Erik Klemetti has turned down opportunities to visit White Island, the New Zealand volcano that erupted Monday afternoon and killed at least five tourists.

That's because White Island, also known as Whakaari, is the top of an explosive volcano, and Klemetti didn't like the risk. (The rest of the volcano is submerged under the sea.) Whakaari is similar to the imposing Cascade volcanoes on the western coast of the U.S., which are liable to erupt violently, sometimes with relatively little notice.

"You need to be conscious that there could be an unexpected explosion," said Klemetti, who is an associate professor in the Department of Geosciences at Denison University. "And if you’re caught near it, you could be out of luck."

The short-lived eruption was powerful enough to eject volcanic ash some 12,000 feet into the sky. But this was still a relatively small eruption, explained Janine Krippner, a volcanologist at the Smithsonian Global Volcanism Program.

"This is an inherent risk with visiting volcanoes," said Krippner, a New Zealander who has visited White Island. "It doesn’t take big eruptions to be deadly."

"[White Island] is well-monitored, but surprises like this still happen," she added.

As of Tuesday at midnight in New Zealand, police and emergency responders flying over the island had not spotted any "signs of life," signaling a grim outcome for still missing visitors.

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Indeed, many tourists have for years walked by the volcano's boiling lake with no incident. "This time the timing's bad," said Klemetti. Such are the indifferent whims of nature.

"How many hundreds of thousands of people visit this place with nothing happening?" Krippner asked. "Some people are there at the wrong place and the wrong time."

It's difficult to know when a volcano like White Island, technically known as a stratovolcano, might have a brief blast. "There is very little way to tell that this was going to happen," explained Klemetti. Water might have drained into the scorching insides of this volcano, leading to an unpredictable steam-driven event, he said.

"I've always been skeptical of the tourism there," said Klemetti, noting the waivers he would have needed to sign to step foot on the volcanic summit.

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Before taking down its webpages and citing the current emergency, White Island Tours Limited, a tourism company that takes people to the volcano, advertised online that "visitors are able to get up close to roaring steam vents, bubbling pits of mud, hot volcanic streams and the amazing lake of steaming acid. The vivid hues of yellow and orange resulting from the abundant sulphur on the island make it a photographer’s dream."

Not all volcanoes are likely to erupt explosively and carry risks similar to White Island. For decades, millions of visitors have visited Kilauea at Hawaii Volcanoes National Park. Here, the volcano, called a "shield volcano," doesn't generally explode. This is because the magma — underground molten rock — beneath Hawaiian volcanoes is not as thick as volcanoes like White Island, so they don't trap gas and build up explosive pressure over time. Instead, Hawaiian eruptions tend to mean lava flows, which are not inherently deadly (unless you get too close, or don't get out the way of the slowly oozing rock).

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Eruptions like White Island's happen on Earth every day. It's a reminder that we inhabit a dynamic planet that's roiling with geological activity. "We have over 1,400 potentially active volcanoes," noted Krippner, with 40 to 50 erupting each day.

But venturing close isn't just a fun day out on an island, stressed Klemetti.

"It's really important when people visit active or non-active volcanoes to know what the specific dangers of that volcano are and to make that decision for yourself," said Krippner. But even then, she added, "That might not save your life."