Geologic mapping and lava sampling were carried out after the discovery of large bursts of hydrothermal fluids (megaplumes) over the southern Juan de Fuca Ridge in 1986 and 1987. Our investigations of the northernmost section of the Cleft segment have discovered: (1) semicontinuous low-temperature venting and one major high-temperature vent site along 17 km of the neovolcanic zone and (2) very glassy, lightly sedimented sheet flows and pillow mounds superimposed on older terrain over about 24 km along the northern-most part. The pillow mounds are documented to have erupted between 1981 and 1987. The occurrence of the megaplumes during this same time period strengthens the hypothesis that megaplumes are caused by sea-floor extension events. Although the basalts from the entire length of the neovolcanic zone of the Cleft segment appear to have been derived from the same mantle source, a systematic northward increase in Mg number along the segment within the neovolcanic zone indicates less shallow-level differentiation to the north, possibly related to the development of new magma chambers during the recent phase of sea-floor spreading that has occurred there.

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