Dylan J. Darling / The Bulletin

Over the past 2,000 years, volcanoes in the Cascade string from Northern California to northern Washington have rumbled awake on an average of twice a century.

Whether the next awakening will be at one of Central Oregon's signature peaks, calderas or cones is uncertain, but possible.

“Volcanoes can wake up very fast without warning,” said Seth Moran, a seismologist with the U.S. Geological Survey at the Cascades Volcano Observatory in Vancouver, Wash.

The USGS classified three volcanoes near Bend — Crater Lake, Newberry and South Sister — as “very high threat” volcanoes in a 2005 report that accounted for their eruptive pasts and their proximity to populated areas. The report then suggested to increase monitoring to a level that matched the threat posed by the volcanoes.

While the USGS has increased the amount of monitoring equipment at all three since the report, including the recent completion of eight new monitors at Newberry about 20 miles south of Bend, its scientists are still just building background data to understand each volcano.

“Every volcano has a different heartbeat,” Moran said.

Increases or missteps in that beat may warn of an impending eruption, he said. Along with looking at the most recent data, scientists continue to study the solidified lava flows, ash piles and other volcanic debris created by the last eruptions.

A retired U.S. Forest Service engineer, Bob Jensen, has been helping a USGS scientist map old lava flows from Newberry since 2005.

“It gives you what the volcano has done to know what it might do in the future,” Jensen said.

And he's sure Central Oregon's volcanoes will erupt again.

“It is going to happen ... just, when?” Jensen said.

Like Newberry's lava flows, other signs of Central Oregon's volcanic past are all around the landscape. They include the Three Sisters, Mount Washington and Mount Jefferson.

While there has been volcanic activity at the Three Sisters in the past 2,000 years, Willie Scott, a geologist with the USGS at the Cascades Volcano Observatory, said Washington and Jefferson have been quiet for more than 10,000 years.

Farther north in Oregon, Mount Hood erupted in 1790, he said, marking the most recent eruption in the state.

The last eruption in Central Oregon, from Newberry Volcano, was 1,300 years ago.

The eruption resulted in the Big Obsidian Flow in the Newberry Caldera. About 75,000 years ago a lava flow from Newberry oozed to where Bend's downtown is located.

That history demonstrates the dangers an eruption could pose to a town, Scott said.

“Bend is one of the few cities in the United States that has a credible lava flow hazard,” he said.

Along with lava the land that now holds Bend has also been covered in as many as 10 feet of ash from volcanic eruptions, which have long since been buried by more than 100,000 years of geologic change, Scott said.

More recently, starting in 1997, churning magma deep beneath the Earth's crust created a bulge on the western side of South Sister. Scientists first detected the bulge about a decade ago and have been tracking it since, Moran said. The magma pushed about 150 square miles of the mountainside up several inches but, for now, its development appears to have stalled.

It's unclear, he said, whether the bulge is a harbinger that an eruption is brewing within the mountain or if it is just the result of regular inner rumblings.

Warning signs

The USGS has monitoring systems on the bulge and now around Crater Lake and Newberry Volcano. The point of the systems, which track the slightest of earthquakes and smallest of geologic shifts, is to catch the early signs of a nearing eruption so that people nearby may be warned.

Volcanoes can vary vastly in how much time there is between the first signal of an eruption and the eruption itself, Moran said.

Washington's Mount St. Helens, which erupted in 1980, gave a week's warning via earthquakes before its eruption started, he said. In 1989, Mount Redoubt in Alaska first started rumbling 24 hours before its eruption.

Other volcanoes have taken a year to erupt, and still others have started to shake only to go quiet again without ever erupting.

Along with Central Oregon's volcanoes, the USGS has installed monitors on Mount St. Helens and Mount Rainier and will be putting them on Mount Hood in the next few years.

While Mount St. Helens, which has already had another eruption in 2004, is considered the most likely of the Cascades volcanoes to blow again, there is potential elsewhere.

The Three Sisters, Mount Bachelor and Mount Hood are similar mountains to Mount St. Helens and Mount Redoubt. Scientists are simply uncertain which might be next.

“We are uncertain enough that we want to keep an eye on all of these volcanoes,” Scott said.

Eruption risk

The U.S. Geological Survey categorizes volcanoes based on their history, proximity to towns and monitoring systems. Here's how Central Oregon's volcanoes rate:

Very high threat: Crater Lake, Newberry, South Sister

Moderate threat: Mount Bachelor, North Sister

Low threat: Belknap, Blue Lake Crater

Very low threat: Davis Lake, Mount Jefferson, Mount Washington

Cascade eruptions during the past 4,000 years