To investigate an event of massive geological proportions, a team of Bowdoin researchers this summer scoured the rocks of northern Greece for some tiny bits of evidence. Associate Professor of Earth and Oceanographic Science Emily Peterman and her two student research assistants — Tessa Peterson ’20 and Katelyn Cox ’21 — spent three weeks in June looking for clues contained within minerals the size of sand grains and smaller.
They were trying to gain more insights into the subduction cycle, which is when two continents collide and the denser continent is forced down while the lighter one remains on top. Pulled in by gravity, rocks in the submerged continent sink deep into the Earth’s mantle.
But then something surprising happens. While most of the rocks will disappear forever into the Earth’s depths, a few fragments travel back to the surface again. The process takes roughly ten million years, and is one of the primary drivers of almost all of the topography that we observe on Earth.
While it’s largely understood why and how the rocks descend, their return is a bit more complicated. “The complexity stems from the fact that the rocks exhumed from ultrahigh-pressure conditions vary in composition, size, deformation, etc., so there must be multiple ways that the rocks can exhume to the surface,” Peterman said. Her research focuses on mineral evidence found on the Earth’s crust that bears a record of its travels down and back again.
Along with two researchers from the University of Massachusetts, the Bowdoin group began its work near the city of Xanthi at a riverbed with rich outcrops of exposed bedrock. Because this part of Greece does not attract many tourists, local people would often come by to chat. (And every day at 4 p.m., a herd of goats charged through their field site.)
“They had a lot of pride,” Peterson said, describing her conversations with locals. “They’d say, ‘you came to our mountains to look at our rocks?!’ We’d tell them that the rocks they’re living among have gone down to such depths and have such incredible geological secrets.”
The Bowdoin team ended up collecting about 150 pounds of rock, from the riverbed site and two other spots. It took that much rock to be sure to salvage at least twenty usable samples, inscribed by their long journey 100 kilometers to 150 kilometers into the Earth. “We’re trying to understand all of the processes that happened from the very start, all the way down to their return to the top,” Peterman said. “It gives us a better sense of how the Earth operates.”
What is especially exciting about Peterman’s research is that she and her students are revealing alternative minerals with which to study subduction — garnet and kyanite. Right now, scientists mainly rely on micro diamonds and coesite for telltale signs of subduction.
“They’re old minerals, but we are discovering new ways to recover information they preserve,” Peterman said. By learning how to read the historical records embedded in garnet and kyanite, Peterman could increase the number of places around the world where geologists study subduction.
At the moment, scientists’ understanding of subduction comes mainly from a few key localities in the world, including the German Swiss Alps, western Norway, Papua New Guinea, and parts of China. “Notably, nowhere in North American has proven to have this, but I am hoping to change that,” Peterman said. She added, “If we could expand the number of places where we can recover that record, we may come to very different interpretations about how much crust has gone down, and discover that maybe it’s not that uncommon for it to go down [and return], but it is uncommon for us to recognize it.”
The two Bowdoin students, who were funded by Peterman’s National Science Foundation grant, are each focusing on one of the two minerals. For her piece of the research project, Peterson is analyzing the textures and luminescence of kyanite, which change in reaction with other minerals and under exposure to ultra-high temperatures and pressures.
Cox is looking at how garnet composition tracks the pressure and temperature conditions recorded by the rock during the subduction cycle. Garnet “absolutely adores manganese,” Peterman said. “Any manganese in a rock, it will try to scavenge it” and sequester it away. So if a subducted garnet resurfaces, it will contain a rich core of manganese—and other clues from its past. Cox is doing computer modeling and high-precision geochemical analyses to figure out under what pressures and temperatures the garnet traversed.
Peterman said the work her team did this summer turned up so many discoveries that she anticipates writing seven academic papers. “It was a breakthrough summer,” she said.
