The Cleft segment is the southern-most segment of the Juan de Fuca Ridge, from the intersection with the Blanco Fracture Zone at 44°30' N to 45°10' N where it overlaps with the Vance segment. The Cleft segment is of particular interest because, after yearly chemical and thermal monitoring since 1983, an extraordinarily large plume of hydrothermal effluent was discovered in 1986. The center of this so-called "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 a typical ridge crest vent field. 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 pre-existing hydrothermal system, probably from a fissure produced during an episode of seafloor extension. Independent evidence for this interpretation was discovered when subsequent surveys of the northern Cleft segment using multibeam sonar, side scan sonar and deeply towed camera revealed that a basaltic fissure eruption had occurred in the area between 1983 and 1987 (see Geology Cleft page for more information).

The discovery of the megaplume focused a broad research effort within the source area on the northern portion of the Cleft segment, which resulted in a special session at the Fall 1991 American Geophysical Union meeting and a special section of the Journal of Geophysical Research in the March, 1994 issue (vol. 99).

See also:

Flow over Ridges and Seamounts Models

updated 3/12/2003