A Day in the Life of a Field Geologist in Japan

By Nicole Ferrie, Doctoral Student

Morning

Matsuyama, Shikoku Island, Japan — I put on my robe and wooden slippers and clunk down to the onsen, a natural hot spring. After catching the sunrise through the bamboo, I put on my field gear for the day. Breakfast is miso soup, rice, seaweed salad, and tofu with some coffee. I pile in the car with Hironubu Harada and Tsujimori Tatsuki of Tohoku University in Sendai, and Maureen Fineman, Ella Do, and Yusuke Hayashi of Penn State University. We meet the rest of the crew and grab the usual kelp onigiri and Jasmine tea that no field day is complete without. After that we’re off.

Afternoon

Waves lap at our feet as we scramble along slick rock, hammers at our fingertips ready to sample.

Japan’s geology is complicated and unique. It hosts an active subduction zone that runs into an exposed paleosubduction zone along the ancient plate interface, meaning rocks from deep underground are now exposed at the surface. We are sampling a continuous suite of rock for geochemical analysis ranging from around three kilometers to subarc paleo-depths at paleotemperatures of 220° Celcius to around 500°Celcius in the Shimanto and Sanbagawa Belts on Shikoku Island in Japan.

We are looking at the evolution of boron’s geochemical behavior in subduction zones. Boron is important to us for two reasons. One, boron is highly fluid-mobile and partitions between rock and fluid based on temperature. Two, boron is a volatile element that is essential to plant life. It’s also a part of the chemical compositions of our oceans, magmas, and the mantle.

We can use the samples we’re gathering to trace fluid flow in subduction zones by looking at the changing boron concentration with depth. Fluids are an integral piece of the puzzle in determining the magnitude and timing of earthquakes. We can also look at how boron concentration changes during progressive subduction to determine the distribution of boron within oceans, magmas and mantle as it cycles through the Earth.

We scramble along an outcrop of ancient mélange, giant sections of ribbon chert, sandstone, and mudstone beneath our feet. We cannot go as far as we would have liked to as the ocean has covered a part of our path. We turn back and head into town.

Yusuke leads us around tight turns on a small one-way road in the mountains of Shikoku, the Asemi River on our left and a mountain to our right. The river runs through a post-depositional fold increasing in metamorphic grade from chlorite to its peak oligoclasebiotite zone from south to north. This is a beautiful transect of the paleosubarc, and we get to work sampling metamorphosed pillow basalt, garnet rich rocks, and metamorphosed chert. After a long day, we head to dinner.

Evening

We walk through the curtains of an izakaya, a Japanese pub, exhausted. Peeling off our field boots, we slide onto our mats, secluded from the rest of the world by rice paper doors. I look around at everyone from all these different walks of life and remind myself how lucky I am. My job is to learn about subduction zones, hike over mountains and coasts, and work with people from all over the world.