Hydrothermal plume distribution over the northern half of the Cleft segment in 2000 (top) and 2001 (bottom). Plume intensity, in terms of particle concentration, indicated by the color scale. Known vent sites show by inverted red Vs.

R/V Wecoma - CTD Cruise
Science News

Science Report - Friday, July 27, 2001
Position - 45 04.138' N /130 18.796' W
Chief Scientist Ed Baker

Even flying over an ocean basin drained and dry you might not recognize the largest volcano on the planet. Some volcanic features would be instantly recognizable--the cone-shaped Hawaiian volcanoes rise from the Pacific Ocean floor to thousands of meters above the level of our vanished sea surface. But volcanoes that look like volcanoes account for only a small fraction of the total amount of lava erupted each year from deep below the earth's crust. The earth's mightiest founts of lava are the sinuous midocean ridges that snake through every ocean basin. This "wound that never heals," as the pioneering oceanographer Bruce Heezen called it, is a linear volcano that marks where the earth's tectonic plates inexorably separate at about the rate your fingernails grow.

While midocean ridges have been recognized as volcanoes since the middle of the last century, only in the last decade have oceanographers begun to recognize what happens on the seafloor during an eruption.. While our work at Axial Volcano has been directed at the most recent eruption detected, the first studies of eruption events were made here on the Cleft segment of the Juan de Fuca Ridge. By good fortune (and wise planning) a NOAA Vents cruise was making hydrothermal measurements here in 1986 when scientists observed a hydrothermal plume far more enormous than any previously observed. Continued study demonstrated it was most likely the remnant of a seafloor lava eruption that propelled a short-lived but superheated gusher of hydrothermal fluid more than a kilometer above the seafloor.

Since that discovery, the Vents Program and other researchers have visited this area yearly to track the long term changes that are the legacy of the 1986 eruption. Unlike Axial, where we have seen clear differences each year since the 1998 eruption, changes here 15 years removed from the lava flow are subtle. The accompanying figure compares the distribution and intensity of plumes along the northern half of the Cleft Segment last year with that found this year. High plume intensities are found above known vent sites in both years. Small changes in the plume distribution are most likely caused by day-to-day variations in bottom currents that mix and disperse the hydrothermal discharge from each vent site. Some plumes are found far from the known vent sites; perhaps there are active sites yet to be discovered. Most remarkably, however, hydrothermal activity here remains robust and widespread fully 15 years after the last lava eruption. Thus the heat source that drives hydrothermal circulation here must be deeper and larger than that at Axial Volcano, where the plumes have declined sharply just 3 years after the 1998 eruption.