PALEOMAGNETIC DATA IDENTIFY POST-GLACIAL ERUPTIVE EPISODES AND THEIR DURATION AT MEDICINE LAKE VOLCANO, CALIFORNIA

Radiocarbon dating indicates that eruptive activity was strongly episodic at Medicine Lake volcano, N. CA Cascade Range (Donnelly-Nolan et al., 1991 JGR). Most of the 17 post-glacial eruptions scattered across this large shield volcano have been dated directly or indirectly using charcoal or wood found under lava flows, in or under tephra deposits, or in tephra-bearing lake sediments. The error on any calibrated radiocarbon age is a function of the laboratory analytical error, whether the ¹⁴C age is on the steep or flat portion of the calibration curve, and the nature of the sample (e.g. burned evergreen needles under tephra vs. charcoal from 20-cm-diameter roots at the base of 1-m-diameter tree molds in lava flows). Thus, a well-measured radiocarbon age can still have an uncertainty of many hundreds of years. In order to better define the actual ages, paleomagnetic data were collected on all 17 units. We then compared the resulting directions with the known archaeomagnetic and secular variation records for the western U.S., giving additional indirect radiocarbon age constraints. Making the assumption that units with similar radiocarbon ages and very similar remanent paleomagnetic directions constitute an eruptive episode, we define 4 episodes of volcanic activity: Episode 1 at ~10,600 ¹⁴C BP (8 mafic eruptions including primitive basalt in 200 yrs), Episode 2 at ~4400 ¹⁴C BP (2 dacitic eruptions in 100 yrs), Episode 3 at ~2900 ¹⁴C BP (2 mafic eruptions in 150 yrs), and Episode 4 centered at ~1100 ¹⁴C BP (5 mafic and silicic eruptions spanning 350 yrs). Within Episode 4, silicic eruptions occurred at ~1230, 1065, and 900 BP interspersed with mafic eruptions at ~1150 and 1130 BP. The most recent eruption, at Glass Mountain, produced 1 km3 of silicic magma beginning with rhyolitic tephra (¹⁴C age on burned needles = 910±60 BP) followed by dacite and rhyolite lava (¹⁴C age of burned tree in edge of flow = 885±40 BP). All products of the Glass Mountain eruption have overlapping paleomagnetic directions. Chilled basaltic inclusions are present in lavas of Episodes 3 and 4 indicating intrusion of primitive magma into the volcano during late Holocene time. Total time span of the 4 episodes is ~800 years, or 7.5% of post-glacial time, although the volcano was unlikely to be erupting even 1% of the time.