U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications
An understanding of the flow of mass and energy between the crust and the ocean requires an understanding of hydrothermal circulation within oceanic spreading centers. The most common expression of hydrothermal circulation found during the first decade of observations has been the steady emission of fluid from ridge axis hot springs marked by an at least decadal-scale stability in both composition and output [Campbell et al., 1988; Bowers et al., 1988]. The degree to which this class of hydrothermal activity is dominant over other types has obvious implications for the calculation of global hydrothermal budgets and our insight into fluid-rock interactions within the crust.
The recent and intriguing discovery of a "megaplume" apparently formed by the sudden and massive release of hydrothermal fluid on the Juan de Fuca Ridge (JDFR) [Baker et al., 1987] vividly demonstrated the existence of another class of hydrothermal activity: episodic venting on a very large scale. A cataclysmic release of hydrothermal fluids, ~108 m3 within several days time, implies the existence of previously unconsidered fluid flow processes in the axial crust and the inadequacy of steady state models of hydrothermal heat and mass discharge. Such events, if they are found to be widespread along the ocean ridge system, raise a challenge to the conventional view that hydrothermal emissions are supplied only by steadily discharging hot springs.
We report here additional evidence for the existence of episodic hydrothermal events. This evidence takes the form of a second megaplume found 13 months later and in the same general area of the JDFR as the first. We first review the similarities and differences between the two megaplumes and then address the issue of source conditions using a model of turbulent buoyancy-driven convective fluids. The model is used to infer the conditions of heat and mass flux required to form a megaplume-type feature. Finally, we discuss some hydrothermal and tectonic implications of these observations.
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