Cleft Segment
The first documented historical eruption on the mid-ocean ridge was on the Cleft segment. It was discovered by the NOAA/EOI Program with a combination of water column surveys, repeated SeaBeam surveys, and detailed geologic mapping with camera tows and submersible dives. In 1986 and 1987, unusually large and shallow hydrothermal plumes were found over the southern Juan de Fuca Ridge. Modeling of these megaplumes (or "event plumes") strongly suggested that some kind of rifting event had triggered massive expulsion of hydrothermal fluid from the seafloor. After extensive geologic mapping in this area, evidence was found that a volcanic eruption had indeed occurred in the same area and during the same time interval. Very fresh lavas were mapped with the PMEL towed camera system which correlate exactly with a line of depth changes detected between SeaBeam surveys in 1983 and 1987. Near-bottom observations suggest that only short-lived, low-temperature hydrothermal venting occurred on the new pillow lava mounds, contrasting with the sustained, high-temperature venting along a fissure zone that was the eruptive vent for a slightly older sheet flow to the south. This evidence led us to conclude that the pillow mounds were erupted from a dike or dikes injected laterally to the north from a magma body lying beneath the young sheet flow. Video clips of black smoker vents on the Cleft segment are available on the multimedia page.
Results from interdisciplinary studies at the Cleft segment were published in a special issue of the Journal of Geophysical Research (vol. 99, no. B3, March 10, 1994, p.4735-5024). The following are some selected scientific papers on the geology of the Cleft segment:
Chadwick, W. W., Jr., T.K.P. Gregg, and R. W. Embley, Submarine lineated sheet flows: A unique lava morphology formed on subsiding lava ponds, Bull. Volcanol., 61, 194-206, 1999.
Gregg, T.K.P. and W.W. Chadwick, Jr., Submarine lava flow inflation: A model for the formation of lava pillars, Geology, 24, 981-984, 1996.
Baker, E. T., J. W. Lavelle, R. A. Feely, G. J. Massoth, S. L. Walker, and J. E. Lupton, Episodic venting of hydrothermal fluids from the Juan de Fuca Ridge, J. Geophys. Res., 94, 9237-9250, 1989.
Chadwick, W. W., Jr., and R. W. Embley, Lava flows from a mid-1980s submarine eruption on the Cleft Segment, Juan de Fuca Ridge, J. Geophys. Res., 99, 4761-4776, 1994.
Embley, R. W., and W. W. Chadwick Jr., Volcanic and hydrothermal processes associated with a recent phase of seafloor spreading at the northern Cleft segment: Juan de Fuca Ridge, J. Geophys. Res., 99, 4741-4760, 1994.
Koski, R., and I. Jonasson, Sulfide deposits at the north Cleft segment: implications for the evolution of the hydrothermal system, J. Geophys. Res., 99, 4813-4832, 1994.
Milligan, B., and V. Tunnicliffe, Vent and nonvent faunas of Cleft segment, Juan de Fuca Ridge, and their relations to lava age, J. Geophys. Res., 99, 4777-4786, 1994.
Smith, M. C., M. R. Perfit, and I. R. Jonasson, Petrology and geochemistry of basalts from the southern Juan de Fuca Ridge: Controls on the spatial and temporal evolution of mid-ocean ridge basalt, J. Geophys. Res., 99, 4787-4812, 1994.
Embley, R. W., W. W. Chadwick Jr., M. R. Perfit, and E. T. Baker, Geology of the northern Cleft segment, Juan de Fuca Ridge: Recent lava flows, sea-floor spreading, and the formation of megaplumes, Geology, 19, 771-775, 1991.
Chadwick, W. W., Jr., R. W. Embley, and C. G. Fox, Evidence for volcanic eruption on the southern Juan de Fuca Ridge between 1981 and 1987, Nature, 350, 416-418, 1991.
last modified 11/09/01 by Bill Chadwick