SONNE SO320/2

The SO320 expedition comprises two legs, SO320/1 and SO320/2, and is set to carry out geophysical and geological work on the Hess Rise, a volcanic oceanic plateau in the north-western Pacific Ocean.

These oceanic plateaus are large-scale submarine magmatic areas (Large Igneous Provinces, or LIPs), and are among the least understood forms of volcanism on Earth. Many important questions regarding their formation, structure, and geodynamic relationships remain unanswered. Hess Rise, with an estimated volume of 9.1 × 10⁶ km³, is considered one of the largest oceanic plateaus in the North Pacific, yet it is also one of the least researched.

Extending over more than 1,000 kilometres in an east-west direction, Hess Rise can be roughly divided into a western ridge, a central platform and an eastern ridge. Based on earlier studies, Hess Rise formed in the Middle Cretaceous period (around 100–110 million years ago). Most models interpret Hess Rise as an oceanic plateau that formed in the southern hemisphere near the Pacific-Farallon spreading centre, probably at a 'triple junction' (where three tectonic plates meet). The involvement of a mantle plume (hot material rising from the Earth's mantle) in explaining the excessive volcanism is controversial.

According to classical plume theory, oceanic plateaus form when a large, mushroom-shaped plume 'head' arrives at the base of the lithosphere, triggering widespread and simultaneous plateau volcanism without the need for a spreading centre. Subsequently, plate movement over the narrow plume 'stem' creates a much narrower, linear, age-progressive hotspot trail.

The main objective of SO320 is to test competing models for the formation of the Hess Rise: 1) did it form along the path of a migrating 'triple junction'? 2) did it form at the Pacific-Farallon spreading centre? or 3) did it form as a purely intraplate plateau generated solely by a mantle plume? Regarding the first two scenarios, we also intend to examine whether the Hess Rise could have formed exclusively through shallow plate tectonic processes, without the involvement of a mantle plume. For example, it could have formed through the migration of a spreading centre over regions of the upper mantle with an anomalous composition, i.e. a higher melting degree. Regarding the third scenario, we will test whether the Hess Rise formed as a result of a second pulse from the same hotspot that is believed to have generated the nearby Shatsky Rise around 30 million years earlier.

Finally, we aim to investigate whether a late stage of alkali-basaltic volcanism occurred after the formation of the main plateau. This could potentially explain the presence of some solitary seamounts on the plateau. Such a late phase of volcanism is also evident in other oceanic plateaus, although the cause remains unclear. It is often speculated that gravitational expansion/faulting, or lithospheric stresses due to loading at the edges of the plateaus, may lead to decompression melting in the upper mantle or lithosphere. It is thought that the resulting small-volume melts utilise deep-seated faults for their ascent.

To address these geological questions during Leg 2, we will collect volcanic rock samples from all areas of the Hess Rise at water depths of up to 6,000 metres using chain-bag dredges. These samples can then be examined petrologically and geochemically after the cruise to gain insights into the origin, magma sources, and age of the volcanism. Sampling will be supplemented by ship-based gravimetry and extensive seabed mapping using the ship's multibeam echo sounder.