Abstract

SeaMARC II records show a 220 km2 area of unusually high reflectivity on the East Pacific Rise (EPR) near lat 8°S which we interpret to be a recently erupted lava field. This is the only segment of the EPR exhibiting high reflectivity over such a large area in our continuous SeaMARC II coverage from lat 8°N to 18°N and 3°S to 23°S. The lavas appear to issue from the axial neovolcanic zone between lat 8°15′ and 8°17′S. The lava field extends up to 18km off-axis and inundates scarps up to 100 m high. On the basis of measurements of scarp heights just outside areas where they are buried, we estimate the average thickness of the field to be 70 ±20 m. Thus, the volume of the lava field is 15 ±4 km3. In comparison, the average annual volcanic budget for the Earth is approximately 4-5 km3, and the largest historic basaltic eruption (from the Icelandic Laki fissure in 1783) was approximately 12.3 km3. When did the eruption occur? Comparative studies of acoustic reflectivity and photographed bottom characteristics elsewhere along the EPR suggest that the lava field has little or no sediment cover and is very young. This is also one of the only areas on the EPR where earthquakes occur that do not appear to be caused by transform faulting. If the earthquakes are related to the lava field, then major eruptions may have occurred in 1964, 1965, and 1969. A large helium plume near lat 13°S may be caused in part by eruptions of the lat 8°S lava field. This is consistent with estimates of currents at 2-3 km depth, which are such that a plume generated at lat 8°S would eventually appear off-axis at lat 13°S. Our observations suggest that this spreading segment, which is bounded by small ridge-axis discontinuities at lat 8°05′S and 8°47′S, may have been the most volcanically active segment along the EPR during the past 25 yr.

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