Construction of Ultraslow-Spreading Oceanic Crust: New Insights on Volcanic Processes and Deposits From High-Resolution Mapping at the Mohns Ridge

The volcanic activity at ultraslow-spreading ridges is less understood compared with that at faster-spreading ridges. Studies of year-to-year changes along the faster-spreading ridges have provided important information regarding the size and frequency of eruptions. However, ultraslow-spreading ridges produce less frequent eruptions, limiting the possibility to study short-term changes in the seafloor morphology to understand longer-term volcanic processes. Therefore, a different approach is needed to estimate the size and frequency of volcanic eruptions at the slowest spreading ridges. Here, we use meter-scale bathymetric maps and backscatter data together with visual observations and geochronology of both basalts and sediments to study the construction of three axial volcanic ridges (AVRs) along the northern half of the ultraslow-spreading Mohns Ridge. Our study finds that most eruptions produce low-effusion rate pillow lavas (82% of the volcanic terrain). We define “lava flow units” as mappable building-blocks of the ARVs, each with a coherent morphology, which may be emplaced during multiple eruptions, but we envision over a relatively short time span (years to decades). These units vary in size from individual hummocks to larger edifices (0.42 × 10⁶ to 38 × 10⁶ m³). Moreover, we estimate the eruptive frequencies per AVR to be on a hundred-year scale. These spatial-temporal constraints in AVR volcanism offer insight into long-term magma flux and spatial focusing along magma-starved ultraslow spreading ridge systems.