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In April, 1991, we witnessed from the submersible Alvin a suite of previously undocumented seafloor phenomenaaccompanying an in-progress eruption of the mid-ocean ridge on the East Pacific Rise crest at 9°45'N-52'N. The volume ofthe eruption could not be precisely determined, although comparison of pre- and post-eruption SeaBeam bathymetryindicate that any changes in ridge crest morphology resulting from the eruption were < 10 m high.Effects of the eruption included: (1) increased abundance and redistribution of hydrothermal vents, disappearance ofnumerous vent communities, and changes in characteristics of vent fauna and mineral deposits within the eruption areasince December, 1989; (2) murkiness of bottom waters up to tens of meters above the seafloor due to high densities ofsuspended mineral and biogenic particulates; (3) destruction of a vent community by lava flows, mass wasting, and possiblehydrovolcanic explosion at a site known as 'Tubeworm Barbecue' in the axial summit caldera (ASC) at 9°50.6'N; (4)near-critical temperatures of hydrothermal vent fluids, ranging up to 403°C; (5) temporal variations over a 2 week interval inboth temperatures and chemical/isotopic compositions of hydrothermal fluids; (6) unusual compositions of end-membervent fluids, with pH values ranging to a record low of 2.5, salinities ranging as low as 0.3 wt% NaCI (one-twelfth that ofseawater), and dissolved gases reaching high concentrations (> 65 mmol/1 for both CO 2 and H2S); (7) venting attemperatures above 380°C of visually detectable white vapor that transformed to plumes of gray smoke a few centimetersabove vent orifices; (8) disorganized venting of both high-temperature fluids (black and gray smoke) and large volumes of cooler, diffuse hydrothermal fluids directly from the basaltic seafloor, rather than from hydrothermal mineral constructions;(9) rapid and extensive growth of flocculent white bacterial mats (species unknown) on and under the seafloor in areasexperiencing widespread venting of diffuse hydrothermal fluid; and (10) subseafloor downslope migration of magma normalto the ridge axis in a network of small-scale (1-5 m diameter) lava tubes and channels to distances at least 100-200 moutside the ASC.We suggest that, in April, 1991, intrusion of dikes in the eruption area to < 200 m beneath the ASC floor resulted inphase separation of fluids near the tops of the dikes and a large flux of vapor-rich hydrothermal fluids through the overlyingrubbly, cavernous lavas. Low salinities and gas-rich compositions of hydrothermal fluids sampled in the eruption area areappropriate for a vapor phase in a seawater system undergoing subcritical liquid-vapor phase separation (boiling) and phasesegregation. Hydrothermal fluids streamed directly from fissures and pits that may have been loci of lava drainback and/orhydrovolcanic explosions. These fissures and pits were lined with white mats of a unique fast-growing bacteria that was theonly life associated with the brand-new vents. The prolific bacteria, which covered thousands of square meters on the ridgecrest and were also abundant in subseafloor voids, may thrive on high levels of gases in the vapor-rich hydrothermal fluidsinitially escaping the hydrothermal system. White bacterial particulates swept from the seafloor by hydrothermal ventsswirled in an unprecedented biogenic 'blizzard' up to 50 m above the bottom. The bacterial proliferation of April, 1991 islikely to be a transient bloom that will be checked quickly either by decline of dissolved gas concentrations in the fluids asrapid heat loss brings about cessation of boiling, and/or by grazing as other organisms are re-established in the biologicallydevastated area

zenodo (cern european organization for nuclear research)

Volcanic eruption of the mid-ocean ridge along the East Pacific Rise crest at 9°45–52′N: Direct submersible observations of seafloor phenomena associated with an eruption event in April, 1991