Direct observation of a submarine volcanic eruption from a sea-floor instrument caught in a lava flow

Our understanding of submarine volcanic eruptions has improved substantiall y in the past decade owing to the recent ability to remotely detect such ev ents(1) and to then respond rapidly with synoptic surveys and sampling at t he eruption site. But these data are necessarily limited to observations af ter the event(2). In contrast, the 1998 eruption of Axial volcano on the Ju an de Fuca ridge(3,4) was monitored by in situ sea-floor instruments(5-7). One of these instruments, which measured bottom pressure as a proxy for ver tical deformation of the sea floor, was overrun and entrapped by the 1998 l ava flow. The instrument survived- being insulated from the molten lava by the solidified crust- and was later recovered. The data serendipitously rec orded by this instrument reveal the duration, character and effusion rate o f a sheet flow eruption on a mid-ocean ridge, and document over three metre s of lava-flow inflation and subsequent drain-back. After the brief two-hou r eruption, the instrument also measured gradual subsidence of 1.4 metres o ver the next several days, reflecting deflation of the entire volcano summi t as magma moved into the adjacent rift zone. These findings are consistent with our understanding of submarine lava effusion, as previously inferred from seafloor observations, terrestrial analogues, and laboratory simulatio ns(8-11).