If Glacier Peak were to wake up tomorrow and start rumbling, it would be hard for scientists to tell. The National Volcanic Early Warning and Monitoring Systems Act, passed in February, sets out to change that.

Glacier Peak, a volcano in Washington’s North Cascades, is only monitored by one seismometer, which is a device designed to detect shaking. That makes it difficult to tell if shaking is from something volcanic — like an earthquake or a small explosion — or caused by something more ordinary, like a small rockfall, or a glacial “icequake.”

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Eighteen volcanoes are classified as “very high threat” in the United States; eight of them are located in the Pacific Northwest. And right now only Mount St. Helens is considered well-monitored. The other seven are Mount Rainier, Mount Baker and Glacier Peak in Washington, and Mount Hood, the Three Sisters, Newberry Volcano and Crater Lake in Oregon.

The volcanoes get their high threat ranking because of their likelihood of eruption and close proximity to groups of humans.

None of them currently have the type of surveillance that scientists say they would need to accurately predict volcanic eruptions and model potential hazards.

At a press conference Thursday in Vancouver, Washington, Sen. Maria Cantwell, who spearheaded the legislation with Alaska Sen. Lisa Murkowski, gathered with geologists and local public safety officials to discuss the impacts of the new legislation.

Dan Douthit, a spokesperson for the Portland Bureau of Emergency Communications, said eruptions from Mount St. Helens, Mount Hood, Mount Adams or Mount Jefferson (the latter two are considered high threat volcanoes) “could cause ashfall in the urban area, and also send mud and debris down the Columbia River, disrupt travel and the economy, and require the close coordination of government agencies throughout the region.”

An early warning system would give governments time to coordinate emergency response, he said, and mitigate the impacts of the travel disruptions.

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“This legislation gives visitors, land managers and organizations like ours peace of mind while spending time in this majestic and impactful landscape, and ultimately, it will save lives in the event of an eruption,” said Ray Yurkewycz, executive director of the Mount St. Helens Institute.

Washington’s Mount Rainier has about half the monitors of Mount St Helens. It’s of particular concern to Cantwell. “It is estimated that 150,000 people who live near that area could see the impacts of a lahar off of Mount Rainier, and they might only have as much as 40 minutes to respond to that type of incident.”

Accurately predicting eruptions is incredibly important. Even remote volcanic eruptions can cause lahars, which are massive flows of mud, debris and rock caused by melting glaciers and snow during eruptions. Those lahars can travel far downstream and destroy local communities. If a lahar started on Mount Rainier, for example, the town of Orting, Washington, would only have an hour to evacuate to high ground.

After the press conference, Seth Moran, a geologist with the Cascades Volcano Observatory, took Cantwell and reporters on a tour of the U.S. Geological Survey facility.

To demonstrate how more sensors are better than one, Moran gestured to a screen showing data coming in live from seven stations on Mount St. Helens. “This is what we’re kind of aiming at, Mount St. Helens has a pretty good network,” Moran said.

If something only shows up on one seismometer, it could be an anomaly — even the wind can give a false positive. “The important thing is when there’s an earthquake you’ll see it on all these stations,” Moran said.

Different types of tools can help scientists interpret a volcano’s rumblings too. Moran showed seismometer readings taken on March 11 at Mount St. Helens. Something had caused one of the crater seismometers to spike, in a way that seemed similar to the spikes caused by the sorts of small explosions volcanoes make when they move fluid around and wake up.

But they had other seismometers on the mountain, and tools besides seismometers in the crater. One, an infrasound sensor, can detect the pressure waves caused by sound. It didn’t hear anything: There was no explosion.

Similar things can happen on other monitored volcanoes in the Pacific Northwest. The movement of glaciers can show up on seismometers, and those same glaciers can generate icequakes. It can be hard to differentiate an icequake from the sort of small earthquake active volcanoes make — but icequakes are more localized, so having more than one monitor can help.

These tools aren’t just for predicting eruptions. Volcanoes don’t just build up. As we learned in 1980 from Mount St. Helens, they come down as landslides too. And while the destructive landslides of Mount St. Helens were associated with a volcanic eruption, they can also be triggered by ordinary rainfall and weathering. By placing more monitors on the Cascades’ steep peaks, geoscientists will be able to continue to measure active landslides and catch others.

In addition to authorizing funding for improved volcano monitoring, the bill creates a national Volcano Watch Office. Right now, volcanoes are monitored locally. But during the 2018 Kilauea eruption in Hawaii, the staff needed time off to sleep. Other volcano monitoring stations stepped in to fill the gaps. The idea is that the Volcano Watch Office could do the same.

Correction (March 22, 2019) — An earlier version of the story did not correctly describe what Congress has done to finance improved volcano monitoring. Congress has authorized funding for this initiative.