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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We thank D. Clague and J. Head for reviews that improved the final manuscript, the Advanced Imaging Laboratory at WHOI for technical support and the crew of icebreaker Oden for technical support at sea. This research was funded by the National Aeronautics and Space Administration, the National Science Foundation, and the Woods Hole Oceanographic Institution.

Author Contributions R.A.S. was the chief scientist of the Arctic Gakkel Vents Expedition and wrote the paper. H.S., T.M.S, S.H. and C.W. collected the dive imagery. T.M.S., S.H., C.W. and A.S. analysed the dive imagery. M.J. processed the bathymetric data. H.N.E., C.K., U.H., E.H., M.J., B.L., J.L., C.M., K.N., T.S., V.S, C.S., M.T., L.U. and P.W. provided technical and scientific support at sea. All authors discussed the results and provided input to the manuscript.

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  1. Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA

    • Robert A. Sohn
    • , Claire Willis
    • , Susan Humphris
    • , Timothy M. Shank
    • , Hanumant Singh
    • , Clayton Kunz
    • , Christopher Murphy
    • , Peter Winsor
    •  & Adam Soule
  2. Marine Science Institute, University of Texas at Austin, Port Aransas, Texas 78373, USA

    • Henrietta N. Edmonds
    •  & Lucia Upchurch
  3. Swedish Polar Secretariat, 104 05 Stockholm, Sweden

    • Ulf Hedman
  4. Alfred Wegener Institute for Polar and Marine Research, Bremerhaven 27570, Germany

    • Elisabeth Helmke
    • , Julia Linder
    • , Vera Schlindwein
    •  & Maria Tausenfreund
  5. Johns Hopkins University, Baltimore, Maryland 21218, USA

    • Michael Jakuba
  6. Göteborg University, Goteborg 40530, Sweden

    • Bengt Liljebladh
    •  & Christian Stranne
  7. AIST, Tsukuba Central 7, 1-1-1 Higashi, Japan

    • Ko-ichi Nakamura
  8. Ocean Research Institute, University of Tokyo, Minamidai, Nakano, Tokyo 164-8639, Japan

    • Taichi Sato
  9. Department of Geology and Geochemistry, Stockholm University, 106 91 Stockholm, Sweden

    • Martin Jakobsson


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Correspondence to Robert A. Sohn.

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