DURATION OF ERUPTION AT THE GIANT CRATER LAVA-FIELD, MEDICINE LAKE VOLCANO, CALIFORNIA, BASED ON PALEOMAGNETIC SECULAR VARIATION

Nearly 500 cores were collected from the postglacial Giant Crater lava field on the south flank of Medicine Lake volcano. The basaltic lavas form a continuous set of lava flows which display strong chemical zonation from initially erupted calc-alkaline basaltic andesite to final primitive basalt of tholeiitic affinity. Six chemical-stratigraphic groups have been recognized and mapped. The eruptive sequence was sampled at numerous sites both to determine the characteristic paleomagnetic direction of each chemical group and to estimate the duration of the eruption inferred from secular variation of the geomagnetic field. Well-grouped mean directions of magnetization were obtained for 41 sites in the Giant Crater lava field. Mean directions of magnetization determined for the lava field are nearly identical. The likelihood of any extended time interval for the eruption of the different lava types is extremely small, and the data suggest an eruptive event of less than 30 years duration, analogous to historic Hawaiian eruptions. However, the average of groups 1-4, which cannot be distinguished paleomagnetically from each other, is slightly different statistically from that of the average of groups 5 and 6, which have similar directions. A time gap of 10 +/- 5 years is inferred between eruption of group 4 and 5 lavas based on analysis of the probability of the observed angular difference of 1.27-degrees +/- 0.84-degrees between their mean directions and by comparison of this angular difference to calculated field directions with similar declination and inclination determined from spherical harmonic models of the geomagnetic field for the time period 1945-1990. About 200 oriented cores were also collected from predecessor and successor basaltic lava flows on the upper flanks of the volcano. Together with remanent directions from lavas of the Snake River Plain the data define a clockwise loop of secular variation.