HR: 10:20h
AN: V31G-01    [PDF]
TI: Eruption Dynamics and Conduit Processes of the Circa 240 A.D. KS1 Eruption of Ksudach Volcano, Kamchatka, Russia
AU: * Andrews, B J
EM: ftbja@uaf.edu
AF: Alaska Volcano Observatory, Geophysical Institute, University of Alaska Fairbanks, Fairbanks, Ak 99775
AU: Gardner, J E
EM: gardner@mail.utexas.edu
AF: Geological Science Department, The University of Texas at Austin, Austin, TX 78712
AU: Izbekov, P E
EM: pavel@gi.alaska.edu
AF: Alaska Volcano Observatory, Geophysical Institute, University of Alaska Fairbanks, Fairbanks, Ak 99775
AB: The 240 A.D. KS1 eruption of Ksudach volcano formed a 6-7 km3 caldera and dispersed 18-19 km3 of material over much of the Kamchatka peninsula. Previous workers have shown that the deposit consists of 15 km3 of tephra falls and 3-4 km3 of pyroclastic flows. Current studies of deposit stratigraphy and componentry, rock textures, compositions, and volatile contents give insight into dynamics and conduit processes of the eruption. KS1 deposits consist of interbedded white pumice falls and ash layers overlain by gray pumice fall and flow deposits. Two lithic-rich fall layers are present at the transition from white to gray pumice and ash. We interpret the changing character of the deposits with distance and height as a reflection of changing eruptive conditions. Early stages of eruption were dominated by buoyant (fall) behavior. During the main stage, falls and flows (non-buoyant behavior) were simultaneously erupted. The final stage involved caldera collapse followed by buoyant eruption of gray material. Column heights calculated from lithic distributions show that the column was highest and eruption most vigorous during the main stage of eruption, just prior to caldera collapse. Occurring just after collapse and at the shift from fall and flow to fall-dominated deposition, the white to gray transition provides an important timeline in the eruption. Whole-rock and microprobe analyses show the two pumice types to be nearly identical. Similarly, magnetite-ilmenite geothermometry shows that both types were erupted at ~895 degrees C. FTIR analyses of melt inclusions in phenocrysts indicate that both white and gray pumices had water contents of ~4 wt% prior to eruption and completely degassed during eruption. These data indicate that the difference between the white and gray pumices may be solely textural. Ongoing studies of textural differences between white and gray pumices, particularly with regard to bubble sizes, shapes and distributions, may reveal important differences between pre- and post-collapse eruptive processes.
DE: 8400 VOLCANOLOGY
DE: 8404 Ash deposits
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting