Explosive volcanism on the ultraslow-spreading Gakkel ridge, Arctic Ocean

Abstract

Roughly 60% of the Earth’s outer surface is comprised of oceanic crust formed by volcanic processes at mid-ocean ridges (MORs). Although only a small fraction of this vast volcanic terrain has been visually surveyed and/or sampled, the available evidence suggests that explosive eruptions are rare on MORs, particularly at depths below the critical point for steam (3000 m). A pyroclastic deposit has never been observed on the seafloor below 3000 m, presumably because the volatile content of mid-ocean ridge basalts is generally too low to produce the gas fractions required to fragment a magma at such high hydrostatic pressure. We employed new deep submergence technologies during an International Polar Year expedition to the Gakkel Ridge in the Arctic Basin at 85°E, to acquire the first-ever photographic images of ‘zero-age’ volcanic terrain on this remote, ice-covered MOR. Our imagery reveals that the axial valley at 4000 m water depth is blanketed with unconsolidated pyroclastic deposits, including bubble wall fragments (limu o Pele), covering a large area greater than 10 km2. At least 13.5 wt% CO2 is required to fragment magma at these depths, which is ~10x greater than the highest values measured to-date in a MOR basalt. These observations raise important questions regarding the accumulation and discharge of magmatic volatiles at ultra-slow spreading rates on the Gakkel Ridge (6- 14 mm yr-1, full-rate), and demonstrate that large-scale pyroclastic activity is possible along even the deepest portions of the global MOR volcanic system.