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BEND, Ore. -- When Newberry Volcano south of Bend last erupted 1,300 years ago, gas trapped in sticky magma exploded from the caldera south of Bend, flinging pumice and ash more than 3,000 feet up into the air. As the volcanic gases settled down, flows of lava oozed out, quickly cooling into the glassy Big Obsidian Flow.

About 7,000 years ago, magma spread north from Newberry through a crack in the rock, erupting to form the Lava Butte cinder cone.

And with the magma still heating rocks under Newberry Volcano, geologists expect more eruptions to shake the area in the future.

It's still hot under there, said Julie Donnelly-Nolan, a research geologist with the U.S. Geological Survey's Volcano Science Center in Menlo Park, Calif. We have no reason to believe it's finished.

Because of this, the USGS is planning to add eight seismic monitoring stations next summer on Newberry Volcano, which the agency classifies as a very high threat.

But that doesn't mean new buttes or obsidian flows are going to pop up in Central Oregon next week, or even in the next century -- so people shouldn't sell their homes, Donnelly-Nolan said.

The chances it's going to happen any time in our lifetime is very small, she said, but when the volcano does start to rumble, it could cause a lot of damage. Imagine the disruption from just having a little lava flow across Highway 97, she said.

Newberry Volcano first erupted more than 400,000 years ago scientists -- can't determine the exact date because so many eruptions have covered the area with lava since then, Donnelly-Nolan said. The broad volcano is about 500 square miles in size, but the lava flows from Newberry cover more than double that. More than 400 cinder cones and vents dot the flanks of Newberry.

The caldera itself formed about 80,000 years ago, when Paulina Peak erupted much like Mount Mazama did in forming Crater Lake. A pool of magma exploded in an eruption of rocks, ash and gas, before the material settled into the crater.

The event would have dwarfed the 1980 Mount St. Helens eruption, said Bart Wills, Deschutes National Forest geologist.

Newberry Volcano is active because of movements of the Earth's crust, he said -- the Pacific Ocean plate is slowly pushing under the North American plate. As the Pacific plate edges deeper, the water in the rocks gets super heated and starts to melt the crust. The liquid hot material then seeps up through the crust to form magma chambers. That heat, at relatively shallow depths below Newberry, has drawn the interest of geothermal exploration companies, hoping to tap into the hot rocks and potentially steam to turn turbines and generate power. It's also what causes the hot springs in Paulina and East lakes.

And it causes two different types of volcanic eruptions, Wills said.

Features like the Big Obsidian Flow resulted when magma rich in silica exploded, he said. After gas trapped in the thick, viscous lava escapes in an eruption of ash and pumice, the lava oozes out, cooling quickly and tumbling over itself. Because of the high, sandlike silica content, it forms the glassy obsidian. The second type of eruption, seen at Lava Butte, is more common and involves basalt flows, said Daniele McKay, a Ph.D. student at the University of Oregon who studies the area's cinder cones.

Lava Butte is outside of the main volcano, but formed 7,000 years ago when lava spread out from magma chambers through a fissure in the rocks. The lava emerged at the butte as well as other vents along the rift, including Mokst Butte, which is southeast of Lava Butte across Highway 97, and the Lava Cast Forest, which is even farther to the southeast.

And those eruptions are driven by volcanic gas, she said.

That gas makes the eruption a little more explosive, she said.

It sends the lava up, like a fire fountain. Once the gas has escaped, the lava flows are fluid and can travel a long distance, McKay said.

And even a small eruption could send out a lava flow that could cause damage, Donnelly-Nolan said. In addition to blocking roads or railroad tracks, the lava could dam up the Deschutes River. That's what the Lava Butte Flow did 7,000 years ago, forming a lake behind the lava until the water burst through, rushing downstream toward Bend. Donnelly-Nolan is currently working to map all the different lava flows and rock formations at Newberry, to understand more about the volcanic processes.

If you don't understand what it's been doing, she said, then you can't argue very strongly for what it might do in the future.

Although eruptions sometimes come in clusters over time, it's difficult to tease out a pattern that could tell geologists when Newberry might erupt again.

But the USGS is setting up additional monitoring stations to be able to give an advance warning if Newberry does start acting up.

The plan is to place eight seismometers and Global Positioning Systems around Newberry, said Cynthia Gardner, scientist in charge at the Cascades Volcano Observatory in Vancouver, Wash. The seismometers will alert researchers if the magma starts moving, breaking rocks, while the GPS can tell if the ground starts moving.

We want to be able to detect those things to know that the volcano's showing signs of reawakening, Gardner said. It's a way we can give warning.

USGS considers Newberry to be a very high threat volcano, one of the top three in Oregon after Crater Lake and Mount Hood. The threat is based on the potential for an eruption as well as the proximity to populated areas, and the possibility for an eruption to disrupt air travel by sending ash into the air.

Currently, Newberry only has one seismometer, she said, which could detect movement but does not allow geologists to calculate the precise locations where the earthquakes are happening. With additional monitoring stations which cost between $20,000 and $30,000 each -- as well as new equipment to send the data back to the Cascades Volcano Observatory, the scientists should be able to keep a close eye on the actions at Newberry.

It's not that we have concerns that it's going to happen tomorrow, Gardner said. It's more that we have a concern it will happen someday, and we need to be really prepared for that when it happens.

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