The Middle Scoria sequence: A Holocene violent strombolian, subplinian and phreatomagmatic eruption of Okmok volcano, Alaska

摘要: The Middle Scoria deposit represents an explosive eruption of basaltic andesite magma (54 wt. % SiO2) from Okmok volcano during mid-Holocene time. pattern dispersal and characteristics the ejecta indicate that opened explosively, with ash textural evidence for a limited degree phreatomagmatism. second phase produced thick vesicular scoria deposits grain texture, size it was violent strombolian to subplinian in style. third eruptive shift towards shapes are dense, blocky, poorly vesicular, intermittent surge layers, indicating later transitions between magmatic (violent strombolian) phreatomagmatic (vulcanian) styles. Isopach maps yield bulk volume estimates range 0.06 0.43 km3, ~ 0.04 0.25 km3 total DRE. associated column heights mass discharge values calculated isopleth individual layers 8.5 – 14 km 0.4 45 × 106 kg/s. tephras enriched plagioclase microlites have rapid ascent relatively high degrees effective undercooling. Those textures probably reflect accompanying phases eruption. In stages eruption, microlite number densities decrease include more tabular plagioclase, slowing rate. findings on consistent other mafic eruptions, show outside two large caldera-forming is also capable producing marked by varying