The research vessel SONNE slipped out of Yokohama port this week, bound for one of the most remote corners of the ocean. Its destination: the Hess Rise, a T-shaped volcanic plateau stretching roughly 1,000 kilometers through the middle of the North Pacific, equidistant from Japan and Canada. The last scientists to study this geological wonder arrived here in 1980—more than four decades ago.
"The Hess Rise is so far from the mainland that we have scheduled eight days at sea just for the transit to the study area," says expedition leader Dr. Anke Dannowski, a geophysicist at GEOMAR Helmholtz Center for Ocean Research Kiel. For the past two years, German and Japanese researchers have been preparing for this moment, pooling expertise to answer questions that have remained unanswered for generations.
Oceanic plateaus like the Hess Rise form through extreme magmatic events, where lava erupts repeatedly in massive flows that stack upon each other, sometimes extending hundreds of kilometers and reaching hundreds of meters in thickness. The Middle Cretaceous period, between 115 and 90 million years ago, was particularly turbulent—during that era, at least twelve of the world's larger oceanic plateaus took shape. How exactly these structures emerged has never been fully understood.
During expedition SO320/1, the team will test three competing models for the Hess Rise's formation. One proposes it rose along the path of a migrating "Triple Junction" where three tectonic plates meet. Another suggests it formed at the ancient Pacific-Farallon spreading center, where new oceanic crust was born. The third describes it as an intraplate plateau, created entirely by a mantle plume—rising currents of hot material from deep within the Earth—independent of any plate boundaries.
The scientists also hope to investigate whether the same hotspot that created the neighboring Shatsky Rise roughly 30 million years earlier might have later flared up again to form the Hess Rise in a second pulse of volcanic activity.
To peer beneath the seafloor, the team will deploy up to 40 ocean bottom seismometers at depths ranging from 2,000 to 5,000 meters. These instruments sink autonomously to the ocean floor, where they will record seismic movements and pressure waves around the clock. The measurements will be complemented by ship-based gravimetry, a magnetometer towed behind the vessel, and detailed seafloor mapping.
Before departure, German and Japanese scientists gathered aboard the SONNE for a reception hosted by the German Embassy, trading notes with counterparts at JAMSTEC, Japan's marine research agency. Students from the German School Tokyo even toured the vessel, glimpsing the work and life that await the research team during their weeks at sea.
The SO320/1 expedition will be followed by a second cruise, SO320/2, which will collect actual rock samples from the seafloor. When combined with the geophysical data—particularly rock ages—the complete picture of the Hess Rise's origins will finally come into focus. "The investigation of Hess Rise will provide insights into the entire geotectonic evolution of the Pacific," Anke Dannowski said. After 44 years of waiting, that picture is finally within reach.