HR: 0800h
AN: V51C-0709    [Abstracts]
TI: Late Pliocene-Holocene explosive volcanism and glacial history in Kamchatka: major calderas, oxygen isotope depletions, and deep sea sediment record
AU: Leonov, V
EM: lvl@kscnet.ru
AF: Institute of Volcanology, Piip Blvd 6, Petropavlovsk, 106999, Russian Federation
AU: * Ponomereva, V
EM: ponomareva@ksc.net
AF: Institute of Volcanology, Piip Blvd 6, Petropavlovsk, 106999, Russian Federation
AU: Bindeman, I
EM: bindeman@uoregon.edu
AF: Geological Sciences, 1272 University of Oregon, Eugene, 9703, United States
AB: We present results of a study of explosive silicic volcanism from Late Pliocene to Holocene caldera-forming eruptions in Kamchatka arc, northwest Pacific. Mid-Pleistocene to Holocene calderas are well expressed in the topography and are present both in the front and rear volcanic arc. Due to uplifts and erosion, calderas older than 1My are not preserved morphologically but thick intracaldera ignimbrite fills and reconstructed caldera outlines provide rough estimates on the large, >>100km3 eruptive volumes. Major tephra and ignimbrite deposits were dated by a variety of techniques: 14C, K-Ar, and Ar-Ar. Reconstruction of space images and extensive long-term field observations permitted identification of moraines belonging to at least two last glacial stages that are morphologically preserved and are intercalated with many caldera deposits. Older moraines may be also present but are more difficult to relate or date. Kamchatka was not covered by a peninsula-wide glaciation, but the majority of volcanic edifices and calderas served as centers of glaciation. Investigation of marine record of ash deposits from sites in the northern Pacific display increased explosiveness starting from 2.65 Ma that is close in time to the onset of the Northern Hemisphere glaciation. Thus, it appears that glacial unloading plays little role in causing explosive silicic volcanic eruptions, but the availability of meteoric water seems to promote hydration of the upper crust by low-d18O synglacial waters in calderas. Detailed ash record in the Sea of Okhotsk demonstrates more explosiveness in interglacial cycles, the record in the Pacific does not show this trend. Radiocarbon-constrained on-land record of the Holocene explosive eruptions suggests retardation of major caldera-forming eruptions by several k.y. after the onset of the deglaciation. Based on the Iceland's case, we suggest that deglaciation promoted basaltic volcanism that remelted hydrothermally-altered crust to form large silicic magma chambers, and that it took several k.y. to form large silicic magma chambers. Oxygen isotopic values of phenocrysts within major caldera- forming eruptions and intracaldera products and reconstructed magma d18O values are variable from 7 permil to 3.4 permil, but low values predominate in Pleistocene. Holocene magmas are both normal and low-d18O that may reflect memory effects of the last glaciation. This suggests that silicic volcanic rocks are derived from the upper hydro thermaly-altered crust, altered by synglacial meteoric waters.
DE: 8419 Volcano monitoring (7280)
DE: 8440 Calderas
DE: 8455 Tephrochronology (1145)
DE: 8488 Volcanic hazards and risks
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting