摘要：

In September 1986, during an investigation of the thermal and chemical properties of the water column overlying the southern Juan de Fuca Ridge (JdFR), a plume of hydrothermal effluent of extraordinary size and heat content was discovered (Baker et al., 1987). The center of this 'megaplume' (and a subsequent one found in 1987) was several hundred meters shallower than the 'steady state' hydrothermal plume observed over the ridge and contained the equivalent of about 1 year's discharge from typical ridge crest vent field (Baker et al., 1989). A conductivity-temperature-depth (CTD) profile taken 1 month later at the site revealed that the megaplume was no longer present. Modeling of the dynamics of megaplume generation revealed that it was consistent with a sudden expulsion of fluids from a preexisting hydrothermal system, probably from a fissure produced during an episode of seafloor extension (Baker et al., 1989). Independent evidence for this interpretation was discovered when subsequent surveys of the northern Cleft segment using Sea Beam, side scan sonar and deeply towed camera revealed that a basaltic fissure eruption had occurred in the area between 1983 and 1987 (Chadwick et al., 1991; Embley et al., 1991; Fox et al., 1992). The discovery of the megaplume focused a broad research effort within the source area on the northern portion of the Cleft segment. This special issue is an outgrowth of a special session at the Fall 1991 American Geophysical Union (AGU) Meeting and includes results of several years of interdisciplinary field investigations focused on this area.

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