A volcanic eruption on an island off the coast of New Zealand spewed steam and ash thousands of feet into the air Monday, leaving at least five people dead as tourists were touring the remote and desolate landscape.

Hours after the eruption, White Island, home to New Zealand’s most-active volcano, was still too dangerous for rescue crews to begin searching for survivors. At least 18 people had been taken off the island after the eruption, some with severe burns, according to the Associated Press. Many were still missing.

The Pacific Northwest is home to 10 of the 18 volcanoes in the United States classified as “very high” threats, by the U.S. Geological Survey. Two of the top three, Mount Hood and Mount St. Helens, are visible from downtown Portland.

But there are some significant differences in the volcanoes that make up the Cascade range and the one that erupted in New Zealand, said Wendy Stovall, a volcanologist at the Cascades Volcano Observatory.

The White Island volcano has what is called an “open system,” Stovall explained, meaning that the magma beneath the surface has an open route to the atmosphere. Mount Hood, Mount St. Helens and the other Cascade peaks are all “closed-system” volcanoes, meaning there is a hard cap between the subsurface molten rock and the atmosphere. Essentially, closed-system volcanoes act like bottles of magma with a cork in the top.

For a peak like Mount Hood to erupt, the magma would have to break through that cork, which would produce earthquakes and release volcanic gas that people like Stovall would be able to detect with monitors stationed on the slopes.

“Monitoring equipment for closed systems allows us to see changes in the system much more quickly than in an open system,” she said. While closed-system volcanoes do offer more signs of an imminent eruption, many scientists have pushed for an expansion of monitoring efforts on the region’s volcanic peaks.

There were some signs the volcano on White Island was getting ready to blow ahead of Monday’s eruption. The GeoNet agency, which monitors volcanoes and earthquakes in New Zealand, raised the alert level on the island last month, after an increase in the amount of sulfur dioxide gas was observed, according to the Associated Press. It also said that over the previous weeks, volcanic tremors had increased from weak to moderate strength.

Stovall also noted that most of the Cascade volcanoes in the Pacific Northwest are on public land and there are a network of local, state and federal agencies that not only monitor the quakes, but keep detailed hazard maps and evacuation plans in case of an eruption.

-- Kale Williams

kwilliams@oregonian.com

503-294-4048

@sfkale

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