STEADY-STATE emanations of hydrothermal vent fluid along the mid-ocean ridges give rise to plumes rising 100-300 m above the sea floor, identified by anomalies of temperature, conductivity, chemistry and particles. Recently, a quite different type of hydrothermal plume created by a brief but massive release of high-temperature hydrothermal fluids was discovered by Baker et al. at a Juan de Fuca vent field1-3. This gigantic thermal plume was termed a 'megaplume'. In December 1987, we found a large hydrothermal plume showing anomalies of temperature, methane and manganese above the North Fiji Basin spreading axis. The transience of this plume at 173°30′ E, 18°50′ S was confirmed by our return visit to the site in November 1988. The discovery of megaplumes at two sites in very different tectonic environments indicates that this type of hydrothermal activity may make an important contribution to thermal and chemical fluxes from the mid-ocean ridges.
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