1. Department of Geological Sciences, University of Florida, Gainesville, Florida 32611, USA
2. Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK
3. Woods Hole Oceanographic Institution, Geology and Geophysics Department, Woods Hole, Massachusetts 02543, USA
4. Department of Geology and Geophysics, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA
5. Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA
6. US Geological Survey, Mineral Resources Team, Denver, Colorado 80225, USA

Correspondence to: Michael R. Perfit¹ Email: perfit@geology.ufl.edu.

Abstract

Lava erupts into cold sea water on the ocean floor at mid-ocean ridges (at depths of 2,500 m and greater), and the resulting flows make up the upper part of the global oceanic crust¹. Interactions between heated sea water and molten basaltic lava could exert significant control on the dynamics of lava flows and on their chemistry. But it has been thought that heating sea water at pressures of several hundred bars cannot produce significant amounts of vapour^{2, 3, 4, 5} and that a thick crust of chilled glass on the exterior of lava flows minimizes the interaction of lava with sea water. Here we present evidence to the contrary, and show that bubbles of vaporized sea water often rise through the base of lava flows and collect beneath the chilled upper crust. These bubbles of steam at magmatic temperatures may interact both chemically and physically with flowing lava, which could influence our understanding of deep-sea volcanic processes and oceanic crustal construction more generally⁶. We infer that vapour formation plays an important role in creating the collapse features that characterize much of the upper oceanic crust and may accordingly contribute to the measured low seismic velocities in this layer.