Expedition to Hess Rise in the Northwest Pacific Begins

Located in the middle of the North Pacific, between Japan and Canada, lies one of the world’s largest oceanic plateaus, the so-called Hess Rise. The plateau is roughly T-shaped and extends over a length of about 1,000 kilometers. Due to its distance from the nearest mainland, the research area at Hess Rise is difficult to access and has therefore been the destination of only a few expeditions to date. The last one took place in 1980. For the past two years, Hess Rise and its surroundings have once again become the focus of German and Japanese researchers.

Expedition SO320/1, led by the GEOMAR Helmholtz Centre for Ocean Research Kiel, is now testing various models on the western and northern parts of the plateau that explain how Hess Rise might have formed. The SO320/1 expedition will be followed by the SO320/2 expedition, which will also address this question.

“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.

Focus on the Formation of the Hess Rise

Oceanic plateaus are found worldwide, both on land and in the oceans. They form as a result of extreme magmatic events in which lava repeatedly erupts in the form of massive lava flows that accumulate on top of one another. Individual lava flows extend over several hundred kilometers and are tens to several hundred meters thick. The Middle Cretaceous period (115–90 million years ago) in particular was marked by exceptionally intense volcanism, during which at least twelve of the world’s larger oceanic plateaus were formed. How exactly these structures came into being has not yet been fully clarified. “The investigation of Hess Rise will provide insights into the entire geotectonic evolution of the Pacific,” says Anke Dannowski.

Three models that could explain its formation are to be tested during the expedition: According to the first scenario, the plateau formed along the path of a migrating “Triple Junction,” that is, where three tectonic plates meet. The second model assumes that the Hess Rise formed directly at the former Pacific-Farallon spreading center—the zone where new oceanic crust was formed. The third scenario describes the Hess Rise as a so-called intraplate plateau that formed independently of plate boundaries, solely due to a mantle plume. Plumes are rising currents of hot material from the deep Earth’s mantle. The researchers also aim to investigate whether the same hotspot could have formed the neighboring Shatsky Rise some 30 million years earlier and later became active again at the Hess Rise in a second pulse

Multiple research instruments in use

For the investigations, up to 40 ocean bottom seismometers (OBS) will be deployed on the seafloor at depths of 2,000 to 5,000 meters. These are deployed from the ship and sink to the seafloor in free fall. There, they operate autonomously and continuously record seafloor movements as well as pressure waves in the water. The measurements are supplemented by ship-based gravimetry, a magnetometer towed by the ship, and seafloor mapping using the ship’s own multibeam echo sounder. The data obtained in this way provides insights into the composition and deep structure of the Hess Rise. Based on the work of the expedition that has now begun, the subsequent SO320/2 cruise will specifically collect rock samples from the seafloor. Later, the geophysical results will be combined with the geological data, particularly regarding the age of the rocks.

Before the research vessel SONNE left the port of Yokohama, a reception hosted by the German Embassy provided Japanese and German scientists and technicians with the opportunity for an in-depth professional exchange on board. The following day, the German guests visited the Japanese marine research institute JAMSTEC (Japan Agency for Marine-Earth Science and Technology) in Yokosuka and its research vessel KAIMEI. In addition, students from the German School Tokyo/Yokohama came on board to get a sense of the work and life on a research vessel.

Expedition at a glance:

Name: SONNE Expedition SO320/1 “Hess Evolution”

Expedition Leader: Dr. Anke Dannowski (GEOMAR)

Duration: May 15, 2026–June 16, 2026

Start: Yokohama (Japan)

End: Honolulu (USA)

Expedition area: Northwest Pacific