LAVA FLOWS FROM A MID-1980S SUBMARINE ERUPTION ON THE CLEFT SEGMENT, JUAN-DE-FUCA RIDGE

A series of lava flows with a total volume of 0.05 km3 were erupted in the mid-1980s along 17 km of the northern Cleft segment of the Juan d e Fuca Ridge. Observations from camera tows and submersible dives show that the new flows are all similar in appearance and consist entirely of pillow lava with a mixture of smooth and striated surface textures , suggesting a relatively uniform eruption rate approaching 1 m3/s at point source vents. The flows vary in size from small patches to large steep-sided ridges and were probably erupted from a dike intruded alo ng the ridge axis because they are aligned along a linear fissure/grab en system. Observations at north Cleft show that the physical appearan ce of new flows changes more rapidly than previously realized and that earlier qualitative dating of young lavas based on sediment cover and glassy surface texture were probably overestimates by an order of mag nitude. Sediment accumulation on the lavas is quite variable and local ly surprisingly substantial, mainly due to hydrothermal deposits that formed while the lava flows were cooling. Biological vent communities photographed on the new flows in 1989 show that vent animals can colon ize new vent sites rapidly but that warm water was still venting only in a few places. Nonvent animals are much slower to colonize the new f lows and rates of colonization observed at north Cleft may be useful f or making improved age estimates of young (<10 years) lava flows elsew here. The north Cleft eruption represents about 2% of the estimated av erage annual volcanic output along the global mid-ocean ridge, implyin g that many other submarine eruptions are occurring undetected.