To many, the science of geology can seem incomprehensibly technical and achingly slow. But when Emily Cahoon started her Ph.D. work in the field, the volcanologist, geochemist and igneous petrologist would uncover a treasure trove of mysteries that are as intriguing as they are dazzling — literally.
Cahoon’s studies centered on the Picture Gorge Basalt, a north-central Oregon portion of the much larger Columbia River Basalt Group. These vast basalt flows that erupted some 16-17 million years ago flowed over a major portion of the Pacific Northwest and gave rise to some of the region’s most epic landscapes as well as our state gem, the sunstone.
Columbia River basalts are divided into sub-units based on their location, age and geochemistry. The Picture Gorge Basalt, which Cahoon jokingly refers to as “the runt” of the Columbia River flood basalt family because of its smaller eruptive volume, was first identified near John Day. Through gathering rock samples and conducting geochemical analysis, Cahoon hoped to fill in some of the data gaps in what distinguishes that particular sub-unit.
“We’re trying to understand where they were erupting out of because the magma that fed these eruptions was basically the same body of magma that feeds Yellowstone, you know, is under Yellowstone National Park today,” she said.
Looking for a novel approach to the topic, Cahoon wondered if the presence of relatively large sunstone crystals in Columbia River lava flows might hold clues about where the magma that fed these basalt flows originated.
“That’s big because where the actual magma chamber was located during these eruptions is still very much a topic of debate in the scientific community,” she said.
In 2016, Cahoon’s advisor handed her a map from a gemological publication with an “X” on it, designating the location of a sunstone mine called the Ponderosa Mine. The mine is located in remote northern Harney County and outside the current boundaries of any of the Columbia River flood basalt areas. If the chemistry of the lavas there matched the ones Cahoon was analyzing, her advisor told her, it might mean some redrawing of the Picture Gorge map area.
State gemstone opens door to geologic discovery
Oregon’s state gemstone is the sunstone. What makes the stones unique is that they contain copper, a metal that is not normally found in these feldspar crystals. The copper inside the sunstone crystals can lead to a dazzling variety of colors from clear to pink to red and even green and teal. When the copper flecks in some crystals show a shimmery gold color, they’re referred to as “schiller.”
Sunstone mining in Oregon has been centered around the tiny town of Plush, which has a population around 60, in south-central Oregon’s Lake County. Geologists had long assumed that the gemstones occur there as a result of the nearby Steens Mountain sub-unit of the Columbia River basalts. But because the “X” on Cahoon’s map marked a location outside the known boundaries of the Steens Mountain sub-unit, she wondered whether that long-standing assumption was correct.
“Where the Ponderosa Mine is located has kind of been a gap on geologic maps,” she said, meaning it was not inside any of the recognized Columbia River basalt flow areas. “So, the first thing I did was geochemical analysis [of the local rocks] and determined that they’re actually part of the Picture Gorge Basalt.”
The new findings suggested the Picture Gorge Basalt was not only larger, but also erupted earlier and for longer than previously thought.
“We used to have this kind of conspicuous gap where no [Columbia River basalt] lava had been identified, and now we’re sitting on them,” Cahoon said last August on a visit to the Ponderosa Mine.
It got her thinking.
“So here at the Ponderosa Mine, this is the northernmost location of where sunstones have been found,” she said. “The rest of the mines are about 80 miles south, close to Steens Mountain, down in the Plush area.”
Cahoon headed for the sunstone mining area near Plush to conduct the same geochemical analysis on the rocks there. What she found up-ended the earlier assumption that sunstones are contained in lava flows associated with the Steens Mountain sub-unit.
“Sure enough, those rocks that host sunstones down in Plush are also Picture Gorge basalt and not Steens,” she said.
As fascinating as these revelations were geologically, Cahoon’s attention was drawn to the shiny sunstones literally lying on the ground under her feet.
“I saw these sunstones and was like ‘This is so cool!’ And for a geologist, gemstones are one of those things that every kid—” she stammers, still excited by the memory. “That’s kind of how you get interested in geology.”
But for Cahoon, sunstones’ coolness factor had a deeper significance.
“Usually if it’s cool, somebody’s looked at it, right?” she said.
Cahoon was in for yet another surprise.
“I remember, you know, going back and digging through some literature, going on Google Scholar and trying to figure out what’s been done,” she said. “And most of what I could find was on sunstone as a gemstone.”
These gemological articles discussed characteristics such as the stone’s hardness, its value as a cut gem or where they could be found in Oregon.
The one geology-focused study she found discussed how sunstone colors were influenced by the copper’s state of oxidation.
“I was kind of shocked to figure out that we don’t know how they form at the most basic level,” Cahoon recalled.
Now a research associate at Oregon State University, Cahoon is applying her geology skills to unraveling sunstone mysteries such as how, when and why the copper that makes them unique comes to be inside the stones in the first place.
“There’s still so much to learn,” Cahoon admits. “Trying to understand how sunstones fit into the volcanic history of Oregon and the Columbia River basalts is pretty cool. But also, just being basically on the cutting edge of how a gemstone forms is incredible and just a lot of fun.”