Imagine the magma chamber of a volcano.  

What do you see? A big tank filled with frothing molten lava, like a pot of bubbling soup?  

“That’s what many people think,” said geochemist Kari Cooper, a professor of earth and planetary sciences in the College of Letters and Science at UC Davis. “It turns out that that’s mostly wrong.” 

Rather than picturing a magma chamber as a pot of boiling soup, a more apt analogy is a snow cone, according to Cooper. The icy treat is a mixture dominated by shaved ice, much like how a magma chamber is predominantly solid, in at least some cases, for roughly 90% of its total storage duration.   

Cooper, along with geochemist Adam Kent of Oregon State University, shared this landmark discovery in a letter published in Nature nearly 10 years ago. 

For decades, Cooper has analyzed the minerals found in volcanic eruption debris, using them to recreate the interior of volcanic magma chambers and understand the signs preceding potential volcanic eruptions.  

A fellow of both the American Association for the Advancement of Science and the Geological Society of America, Cooper has now won the American Geophysical Union’s Norman L. Bowen Award, which honors a mid-career or senior scientist for outstanding contributions to the fields of petrology, volcanology and geochemistry.  

“Personally, it’s very meaningful for me,” said Cooper, noting that the list of past Bowen awardees reads like a who’s who list of her scientific heroes. “Norman Bowen was one of the first people to say that magmas will crystallize in an orderly sequence of minerals. Every petrology student learns Bowen’s reaction series for crystallization, so it’s a meaningful award because it really taps into the history of this field.”   

Hot soup versus a snow cone 

Cooper’s and Kent’s landmark discovery hinged on the analyses of tiny crystals, a few millimeters across, found in volcanic eruption debris from Oregon’s Mount Hood. By combining two different techniques, they quantified both the total time the crystals spent in the reservoir and the proportion of that time when the magma was liquid-dominated and easily mobilized. 

“These crystals are like tiny black boxes and there are a whole bunch of them,” Cooper said. “So you can analyze 50 to 100 crystals that came from fist-sized rock and they can give you information spanning hundreds of thousands of years of history, and their chemistry can indicate if they were interacting with different magmas during their history, both during the ‘lifetime’ of individual crystals and comparing one to another.” 

“The crystals that we measured at Mount Hood spent less than 10% of their existence in this active mobile state,” added Cooper, noting that such liquid-dominated states could possibly indicate that an eruption is imminent. “That prompts a lot of other interesting questions, like how does a volcano’s magma chamber go from mostly solid to erupting within decades?”   

Pivoting in the name of discovery 

This work is based on uranium series geochronology, a method of obtaining crystal ages that Cooper started refining during her doctoral studies at UCLA. The method employs radiometric dating of intermediate daughter isotopes between uranium and lead (such as thorium and radium) found in the mineral crystals of volcanic rocks. While pursuing her doctorate, Cooper used the technique to analyze crystals erupted from Hawaiian volcanoes.  

“While my advisor had set up part of the technique that I needed to use, to get at really short timescales, I needed to use a different isotopic daughter pair within that system that hadn’t been set up at UCLA yet,” she said.  

Linking with a colleague at New Mexico’s Los Alamos National Laboratory, Cooper learned the necessary techniques for better analyses over several stints that totaled two years. She was close to the finish line of her project when disaster struck. The lab was destroyed by a fire.   

Without the controlled environment necessary to conduct her crystallization research, Cooper faced a roadblock. So, she pivoted, developing an alternative method to convert measurements of the abundance of different isotopes into a crystallization age.    

“For most geochronology dating systems, we use a stable or long-lived isotope of the daughter product to quantify the amount of daughter that was present when the crystallization clock started, but for the thorium-radium system, which is what I was working with initially, there is no longer-lived isotope of the daughter,” Cooper said.  

A proxy for elemental behavior 

Cooper needed a proxy for radium. While the element barium is its closest chemical analog, there were some discrepancies in prior studies that linked the amount of barium found in a mineral crystal to the original presence of radium. Cooper refined the method, using a theoretical framework designed to predict how elements behave during the mineral crystallization process for her project.      

“During crystallization, depending on the mineral and the temperature, the mineral will either take in or reject different elements,” she explained. “What this method allowed me to do was to calculate the theoretical behavior of radium in the crystal structure and compare that with the theoretical behavior of barium in the crystal, so I could correct for these differences when calculating the crystal age.”  

The adjustments enabled Cooper to successfully reconstruct the age of different mineral crystals found in volcanic debris.  

Reflections of a geochemical pioneer 

Cooper said her experiences pioneering this geochemistry technique reinforced the value of flexibility in the scientific endeavor.  

“It’s a rare research project that actually plays out the way you think it’s going to play out,” she said, “but sometimes that leads to more interesting discoveries.” 

Today, Cooper has gone on to combine uranium-series dating of crystals with different techniques to quantify the temperature experienced by the crystals. In one novel approach, she’s measured the ages of different types of minerals in the same rock, providing different “black boxes” within the same magma reservoir that record different events.  

Her research group’s recent work has focused on understanding whether the conditions leading up to eruption are different for large eruptions (including some of the largest on Earth at Yellowstone and the Taupo Volcanic Zone in New Zealand) compared to smaller more frequent eruptions. 

— Greg Watry, content strategist & writer for the College of Letters and Science at UC Davis