/https://tf-cmsv2-smithsonianmag-media.s3.amazonaws.com/filer_public/36/69/36698a0a-640f-428f-8f74-645e7bd147c1/gettyimages-2169784114.jpg)
A Massive Flow of Rock and Mud Tumbled Down Mount Rainier Centuries Ago. Scientists Pinpointed Its Date to Better Understand the Volcano’s Complex Threats
In a recent study, researchers dated a historic lahar called the Electron Mudflow to the summer of 1507, establishing a way to determine when past geologic events around the Puget Sound occurred
:focal(2774x2067:2775x2068)/https://tf-cmsv2-smithsonianmag-media.s3.amazonaws.com/filer_public/36/69/36698a0a-640f-428f-8f74-645e7bd147c1/gettyimages-2169784114.jpg)
In schools just south of Seattle, thousands of students and staff streamed out of their classrooms this past April and followed evacuation routes to higher ground. Their communities lie in the shadow of Mount Rainier, an ice-capped, active volcano standing more than 14,000 feet tall. They were practicing for what would happen if a deadly torrent of water and debris rushed down the mountain, giving them as little as 30 minutes to reach safety.
This type of catastrophic event is known as a lahar, an Indonesian term for a volcanic mudflow. When a volcano erupts, the resulting avalanche of hot rock can rapidly melt ice into water, which mixes with loose volcanic debris and barrels downhill. As a lahar picks up speed, it bulldozes trees and anything else in its path, becoming a churning river of wet, cement-like earth and debris, burying valleys wall to wall. Lahars usually accompany eruptions, offering some heads up to downstream residents, but landslides on the slope can create sudden, “no-notice” lahars.
Because of all the snow and ice atop Mount Rainier, along with the mountain’s geology, the volcano is especially vulnerable to lahars. In the past 6,000 years, about 11 lahars from the peak have been large enough to travel at least 30 miles. While lahars are not common, they are inevitable over geologic time, and now, with more people and permanent structures in harm’s way, such episodes are becoming more likely to trigger destruction.
“When you say ‘debris flow,’ the average person thinks, ‘Well, that’s something where you can just bring a front-end loader in and clean it off the road,’” says Patrick Pringle, an emeritus geologist at Centralia College. “These flows from volcanoes are much different. They can be colossal and have huge, huge run out.”
The last major Mount Rainier lahar, seemingly a no-notice lahar, was witnessed by the Indigenous people of the Puget Lowlands centuries ago, before European colonization of the region. Oral histories from the Puyallup Tribe, transcribed by 20th century anthropologists, tell of a debris-filled flood that buried the valley along the Puyallup River, near what’s now Orting, Washington.
Alongside oral histories, lahars leave their mark in buried forests and deep, distinct layers of unsorted sediment and rocks. The late geologist Rocky Crandell started mapping lahar deposits around Mount Rainier in the 1960s, and he named this most recent one the Electron Mudflow, after the nearby hamlet of Electron, Washington, which is less than nine miles from Orting.
A group of scientists recently examined samples of trees buried by the Electron Mudflow to pinpoint its date, down to the season. That date could help researchers understand the mysterious cause of this lahar, and their methodology could help find exact dates of other past geologic events in the Pacific Northwest, like huge landslides that could be linked to elusive earthquakes. They published their results in the February issue of the scientific journal Geology.
Pringle, one of the authors of the new paper, started investigating lahars four decades ago. He began his career soon after Mount St. Helens erupted in southern Washington in May 1980. The eruption triggered catastrophic lahars, sending a thick slurry from the volcano’s crater into surrounding river valleys and destroying homes and bridges. As Mount St. Helens is just one in a chain of active volcanoes that run up the spine of the Cascade Mountains, the disaster prompted more research into the threat of lahars in the region.
Pringle carved out a niche for himself by looking at other forests buried by past mudflows—ironically, he says, in areas where new developments were going up. Throughout the 1990s and 2000s, he assembled a large collection of samples from the Electron Mudflow. When ground was broken on a new subdivision, gas station or school building around Orting, Pringle would often show up with his increment boring tools, chainsaws and handsaws to collect slices of buried trees. As crews dug into deposits from the lahar, ranging from 6 to 15 feet thick, they’d come across huge, buried stumps of broken Douglas firs as wide as ten feet—a construction nuisance but a scientific treasure.
/https://tf-cmsv2-smithsonianmag-media.s3.amazonaws.com/filer_public/76/7e/767efa03-f868-42eb-a9f8-10352040fbfe/big_stump_orting_930916pat.jpg)
In a sea of rock, sediment and other inorganic material, the roughed-up logs and stumps didn’t just show the violence of past lahars, which was extreme. They also offered researchers a chance to establish better dates for these flows.
By studying rock layers, radiocarbon dating the wood and examining some tree ring records, researchers had previously been able to guess the date of the Electron Mudflow. Crandell first estimated that it swept down the mountain 600 years ago, then Pringle and other researchers pushed that date closer to 500 years ago. It only recently became possible to ascertain a more precise date thanks to new tree ring records from similar elevations in the Pacific Northwest. In general, tree rings yield more information in places with extreme conditions. At high elevations, for instance, cold winters and short growing seasons often produce distinct annual growth rings. But Orting, where the buried forest samples were collected, sits close to sea level.
“For tree ring dating, we want lots of variability in ring width and variability that’s shared among trees,” says Bryan Black, the lead author of the paper and a tree ring researcher at the University of Arizona. “Working at low elevation sites, that’s a challenge. The trees are happy. They’re growing very regularly, and so I think that’s in part why this date had been out there so long and hadn’t been finalized.”
/https://tf-cmsv2-smithsonianmag-media.s3.amazonaws.com/filer_public/dc/8a/dc8af852-344f-46e6-846f-a72347d71d74/leelamm_whitehawk971204ele28-cu.jpg)
Black made a breakthrough when he and his colleagues analyzed samples of Douglas firs on Vancouver Island—some that lived for close to 1,000 years before a logging company felled them in the 1980s. The samples are now in a collection at Columbia University. The scientists established precise ages for these trees and, from that, a master chronology, or a record that can link widespread climate events to exact calendar dates.
Tree ring records are sometimes compared to barcodes. For example, a series of especially wet winters in the Puget Sound area would leave the same distinct pattern of growth rings on trees from Vancouver Island and trees from near the Puyallup River. Once researchers align those “barcodes”—a technique called “wiggle matching”—they can count the remaining rings to the outermost layer of bark to determine the exact year a tree died.
“The dating of it was subtle, in that we also had to assemble all these puzzle pieces over the years of the right chronologies of the right species and the right elevations,” Black says. “Even though it was subtle and challenging, it turned out to be a very compelling result.”
The scientists were able to date the Electron Mudflow to the late summer of 1507. While many of the trees had been heavily abraded by the lahar, some still had their outer layers preserved on the downhill side of their trunks, giving the researchers insight into the time of year the lahar occurred. “We could tell it had done its springtime growth, the so-called early wood, and began its summertime growth, the late wood, at the time it was killed,” Black says.
Did you know? Dating past geologic events with tree rings
By comparing Douglas fir tree rings from around Puget Sound, Black and his colleagues previously pinpointed the date of a massive double-fault earthquake that struck the Seattle region to a six-month period between 923 and 924.
Though the authors of the study didn’t set out to find the cause of the Electron Mudflow, the more precise date and season may offer some new clues to what sparked the event. Other lahars have been linked to volcanic blasts, but so far, no ash layer has been found that would correspond to a 1507 eruption at Mount Rainier. And landslides are more typical in the winter, when the region gets doused by soil-loosening rain. Summers in the region are usually dry.
“Knowing the seasonality of the Electron Mudflow—in the summer—kind of helps us figure out what it isn’t,” Pringle says. “It doesn’t seem to be a big rainstorm event.”
Mount Rainier’s geology offers additional clues. Hot, acidic fluids chew through the volcano’s rock and break it down into clay—the smallest, most time-worn particles in soil. But this material isn’t very good at holding a mountaintop together. “Clay is very slippery. It doesn’t hold a lot of friction, and so that makes a pile of sediment unstable,” Pringle says.
The material that came down the mountain during the 1507 lahar was rich in clay, which suggests the mudflow was caused by melting ice that saturated the groundwater within the volcano over long time scales.
“Mount Rainier is a big pile of rotten volcanic rock with an ice cube on top, and it’s heated from below. That’s a recipe for sending off mudflows every now and then,” says David Montgomery, a geologist at the University of Washington who was not involved in the study. This explanation also seems to align with the researchers’ timeline: “The big, deep-seated landslides that are fed by groundwater will happen in the spring or summer,” he adds.
/https://tf-cmsv2-smithsonianmag-media.s3.amazonaws.com/filer_public/c5/37/c537c7dd-2d50-42a5-a80b-75739aa4eacc/gettyimages-2219667677.jpg)
With an exact date of the Electron Mudflow, scientists can also use all those samples of buried trees to cross-date other geologic phenomena that occurred at low elevations in the region. Black says the team now has a really well-replicated chronology that extends 400 years back from that 1507 event, which scientists had been missing for low elevations in the area.
“That gives us a really strong, low-elevation chronology we can use to date other events in that time period,” Black says. He adds that they’re turning their attention next to the Bonneville Landslide, which may have been triggered by an earthquake and was so large that it dammed the Columbia River sometime in the 15th century.
Montgomery is hopeful that this new date for the Electron Mudflow might prompt more appreciation for the threat of lahars. “The fact that they could actually put not only a date, but a probable season on it, that’s so cool—we never get to say that in geology,” he says. “It may cause people to pay more attention to the possibility of these things happening in the future. It makes it that much more concrete and a little less geologically abstract.”
The landscape around Mount Rainier is now heavily developed, with entire towns sitting on top of past lahar sediments, in need of evacuation plans in case one strikes again. The drill that occurred in local school districts in April was the largest of its kind in the world. Pringle says that knowing the exact dates of lahars, and thus their frequency in the past, can influence the long-term forecasts geologists create to understand when the next event is likely to occur. But importantly, those forecasts are not exact predictions, which can make the threat of such events challenging to communicate to the public.
“There’s institutional memory loss of these things,” Pringle says, as previous events fade into history. “Things that happen over long periods of time, it’s harder to take them seriously, because people forget.”