Profita reported in early February, 2022 that measurements show tiny earthquakes and accelerating ground uplift (meaning movement of magma upwards), but no sign of imminent eruption yet. Geoscientists are seeing new signs that magma is moving deep within the Three Sisters volcanic complex in Oregon’s Cascade Range. Equipment used by the U.S. Geological Survey to monitor volcanic activity has detected short bursts of tiny earthquakes across a 12-mile area where the ground has been lifting at a faster pace.
Jon Major, scientist in charge at the Cascades Volcano Observatory, said the activity is likely coming from pulses of magma moving about four miles underground.
“There’s no imminent threat,” he said. “Our interpretation is it’s probably related to ongoing small inputs of magma deep underground.”
If the volcano were close to erupting, Major said, the magma would be breaking more rock, generating bigger earthquakes, releasing gases and lifting the ground at a much faster pace.
“There would be a lot more signs that magma was making its way to the surface and poised for an eruption,” he said. “We’re not seeing any of that right now.”
The last time Three Sisters erupted was about 2,200 years ago. Major said it is likely to erupt again, but the latest activity is minimal compared to what scientists expect to see before an eruption. Over a 14-month period between June 2020 and August 2021, advanced satellite technology measured the ground lifting up a little less than an inch across a 12-mile area west of South Sister.
“Which is pretty amazing to think about,” Major said. “This is not the kind of change in the surface of the ground that you and I would stand out there and detect at all.”
Major said the recent rate of uplift is not as fast as scientists saw in the region in the 1990s, when the ground lifted several inches, but it is faster than they’ve seen in the past two decades. And it’s all happening in the same general location.
“We have been monitoring volcanoes up and down the Cascades to take the pulse of anything that’s abnormal,” Major said. “That way, if we detect any abnormalities that a volcano is going into a state of unrest we pick it up as early as possible.”
New technology has allowed scientists to detect miniscule changes in the ground level and nearly undetectable seismic activity, Major said.
“These satellites are sensitive enough that they can detect very small differences between satellite and ground surfaces,” he said.
USGS also has very sensitive seismometers that detected very small earthquakes in the same area in October, December and earlier this month.
“The magnitudes are well below magnitude 1,” Major said. “They’re very small earthquakes — barely detectable.”
This kind of activity might just be normal background noise for the Three Sisters Volcanic Complex, and it might be that scientists are now able to measure because of new technology, he said.
“This process has probably been going on for a long time but we haven’t necessarily had the technology to detect this kind of thing before,” Major said.
Scientists say periods of uplift in the Three Sisters volcanic cluster are likely a continuation of episodic movements of magma that have been occurring regularly for centuries or millennia. The complex’s three volcanoes — North Sister, Middle Sister and South Sister — were formed between 120,000 and 50,000 years ago.
When looking from Bend, Oregon, west toward the peaks of the Three Sisters. The hills in front of the Sisters are composed of geologically young rocks. At just 50,000 to 100,000 years old. Then look a little bit lower: In front of those peaks, just barely rising above the ground, lies a cluster of small hills. This is the Tumalo Volcanic Center (more). It’s the most visible remnant of an eruption that was two or three times older, and hundreds of times larger, than anything the Sisters, or even Mount St. Helens, ever produced. It’s also an eruption that quite literally built the City of Bend.
Today, they know the eruption was huge. If he had to ballpark the size of the eruption, Hill says to think of a cube of molten rock, one that’s 1.5 miles long by 1.5 miles wide by 1.5 miles high. That’s how much material the eruption created. Eruptions of that size were very uncommon in the Central Cascades. The massive explosion that turned Mount Mazama into Crater Lake was just a bit larger, but was caused by different geologic processes.
The Cascade volcanoes exist because of the Cascadia subduction zone just off shore. There, the dense, heavy Juan de Fuca plate sinks below the lighter North American plate. As it pushes down, it forms molten rock or magma. This rises to the surface, and creates volcanoes, including most of the Cascades. At the same time, farther inland, the ground is slowly stretching apart along a number of faults. One of those stretching faults extended all the way to the Cascades, and helped tear the ground apart at depth. Molten rock moved to the surface, where, geologists think, it started to melt the crust. That eventually created the Tumalo volcanic center.
“That’s why it created this large volume of unusually composed rock,” Hill said. “Most of the other places in the Cascades aren’t being pulled apart at the same sort of rate and character.”
This same process is still happening in the central Cascades. The young, gray, obsidian-like domes that one can see driving the Cascade Lakes Highway reflect the same processes that started 400,000 years ago, just at a much slower rate.
“And that,” said Hill, “is what makes the central Cascades even more special.”
[Volcanoes are part of Earth and a part of our past, and our future. Get used to the idea in case one of them blows tomorrow (here). As geoscientists have to say: "It's not if something will happen or not, but when!"].
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