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 zon ation from initially erupted calc-alkaline basaltic andesite to final primitive basalt of tholeiitic affinity. Six chemical-stratigraphic gr oups have been recognized and mapped. The eruptive sequence was sample d 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. Wel l-grouped mean directions of magnetization were obtained for 41 sites in the Giant Crater lava field. Mean directions of magnetization deter mined for the lava field are nearly identical. The likelihood of any e xtended time interval for the eruption of the different lava types is extremely small, and the data suggest an eruptive event of less than 3 0 years duration, analogous to historic Hawaiian eruptions. However, t he average of groups 1-4, which cannot be distinguished paleomagnetica lly from each other, is slightly different statistically from that of the average of groups 5 and 6, which have similar directions. A time g ap of 10 +/- 5 years is inferred between eruption of group 4 and 5 lav as based on analysis of the probability of the observed angular differ ence of 1.27-degrees +/- 0.84-degrees between their mean directions an d by comparison of this angular difference to calculated field directi ons with similar declination and inclination determined from spherical harmonic models of the geomagnetic field for the time period 1945-199 0. About 200 oriented cores were also collected from predecessor and s uccessor basaltic lava flows on the upper flanks of the volcano. Toget her with remanent directions from lavas of the Snake River Plain the d ata define a clockwise loop of secular variation.