1. Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA
2. Marine Science Institute, University of Texas at Austin, Port Aransas, Texas 78373, USA
3. Swedish Polar Secretariat, 104 05 Stockholm, Sweden
4. Alfred Wegener Institute for Polar and Marine Research, Bremerhaven 27570, Germany
5. Johns Hopkins University, Baltimore, Maryland 21218, USA
6. Göteborg University, Goteborg 40530, Sweden
7. AIST, Tsukuba Central 7, 1-1-1 Higashi, Japan
8. Ocean Research Institute, University of Tokyo, Minamidai, Nakano, Tokyo 164-8639, Japan
9. Department of Geology and Geochemistry, Stockholm University, 106 91 Stockholm, Sweden

Correspondence to: Robert A. Sohn¹ Correspondence and requests for materials should be addressed to R.A.S. (Email: rsohn@whoi.edu).

Abstract

Roughly 60% of the Earth's outer surface is composed of oceanic crust formed by volcanic processes at mid-ocean ridges. Although only a small fraction of this vast volcanic terrain has been visually surveyed or sampled, the available evidence suggests that explosive eruptions are rare on mid-ocean ridges, particularly at depths below the critical point for seawater (3,000 m)¹. A pyroclastic deposit has never been observed on the sea floor below 3,000 m, presumably because the volatile content of mid-ocean-ridge basalts is generally too low to produce the gas fractions required for fragmenting 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 photographic and video images of 'zero-age' volcanic terrain on this remote, ice-covered ridge. Here we present images revealing that the axial valley at 4,000 m water depth is blanketed with unconsolidated pyroclastic deposits, including bubble wall fragments (limu o Pele)², covering a large (>10 km²) area. At least 13.5 wt% CO₂ is necessary to fragment magma at these depths³, which is about tenfold the highest values previously measured in a mid-ocean-ridge basalt⁴. These observations raise important questions about the accumulation and discharge of magmatic volatiles at ultraslow spreading rates on the Gakkel ridge⁵ and demonstrate that large-scale pyroclastic activity is possible along even the deepest portions of the global mid-ocean ridge volcanic system.

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