RADIOCARBON AGES OF LACUSTRINE DEPOSITS IN VOLCANIC SEQUENCES OF THE LOMAS-COLORADAS AREA, SOCORRO ISLAND, MEXICO

Extensive eruptions of alkalic basalt from low-elevation fissures and vents on the southern flank of the dormant volcano, Cerro Evermann, ac companied the most recent phase of volcanic activity on Socorro Island , and created the Lomas Coloradas, a broad, gently sloping terrain com prising the southern part of the island. We obtained C-14 ages of 4690 +/- 270 BP (5000-5700 cal BP) and 5040 +/- 460 BP (5300-6300 cal BP) froM lacustrine deposits that occur within volcanic sequences of the l ower Lomas Coloradas. Apparently, the sediments accumulated within a t opographic depression between two scoria cones shortly after they form ed. The lacustrine environment was destroyed when the cones were breac hed by headward erosion of adjacent stream drainages. This was followe d by the eruption of a thin basaltic flow from fissures near the base of the northernmost cone. The flow moved downslope for a short distanc e and into the drainages that presently bound the study area on the ea st and west. The flow postdates development of the present drainage sy stem and may be very recent. Our C-14 data, along with historical acco unts of volcanic activity over the last century, including submarine e ruptions that occurred a few km west of Socorro in early 1993, undersc ore the high risk for explosive volcanism in this region and the need for a detailed volcanic hazards plan and seismic monitoring.