PRELIMINARY VOLUME ESTIMATES OF PRIMARY HOLOCENE RHYOLITIC TEPHRAS AT MEDICINE LAKE VOLCANO, NORTHERN CALIFORNIA

Medicine Lake volcano, located in the southern Cascades ~55 km east-northeast of Mount Shasta, is a large rear-arc, shield-shaped volcano with an eruptive history spanning nearly 500 ky. The most recent eruptions at Medicine Lake volcano are the late Holocene explosive to effusive events at Glass Mountain (~950 yr B.P.) and Little Glass Mountain (~1000 yr B.P.), which began as sub-plinian to plinian eruptions of rhyolite pumice from fissure vents and culminated in the rhyolite-dacite flow of Glass Mountain and the rhyolite flow of Little Glass Mountain. New isopach maps of primary tephra deposits from these eruptions were completed in 2008 and are based on more extensive fieldwork and on a different interpretation of Little Glass Mountain tephras than previous work. The maps show a strong northeast-southwest trend for the Little Glass Mountain tephras as previously known, extending southwest in the direction of Mount Shasta. More than 2 m of Little Glass Mountain tephra were deposited proximal to its vents, although the deposit thins quickly to 1 cm within 2 km perpendicular to trend axis and within 20 km along trend. Tephras from Glass Mountain were not deposited along a single trend, but rather are found in lobes extending from the fissure vents to the west, north, and northeast. The maximum observed thickness of Glass Mountain tephra is greater than 9 m proximal to its vents, thinning abruptly to less than 1 cm of primary airfall within about 10 km of the vents. Beyond these limits, both the Glass Mountain and Little Glass Mountain deposits have been reworked by wind, water, and animal burrowing and exhibit varying texture and thickness, although the maximum pumice size varies little for as much as 10 km downwind from the primary deposits. Preliminary volume estimates for the primary tephra deposits are 0.058 km³ for Little Glass Mountain and 0.18 km³ for Glass Mountain.