Physical volcanology of a sub-Plinian and phreatomagmatic eruption at Okmok Volcano, Alaska: implications for explosive mafic volcanism

Abstract

The Middle Scoria Unit (M.S.) of Okmok Volcano is a basaltic andesite (̃54 wt. % SiO₂) fall deposit that is stratigraphically located between two caldera-forming eruptions (̃12,000 and 2050 y B.P.). Stratigraphy, geochemistry, vesicularity, and crystallinity data imply that the M.S. erupted in three phases that began with an initial phreatomagmatic phase followed by a sub-Plinian phase caused by rapid ascent of magma. The sequence culminates with short-lived Vulcanian eruptions that demonstrate a strong magmatic influence and a possible decrease in ascent rate. The M.S. covers ̃300 km² and averages about 2 m thick. Estimates from this work suggest a total eruptive volume of 1.4 km³, an eruption column ̃9- 14 km high, and a mass discharge rate of ̃8 x 10⁶ kg/s. These values correspond with the low end value of other sub-Plinian and phreatomagmatic eruptions. The changes seen in the eruptive phases of the M.S. are a result of variations in the magma dynamics and access of external water, likely from snow or ice. Evidence for recent magma-water interaction has also been observed within the current 10 km summit caldera. The occurrence of relatively large, explosive, non-caldera forming mafic eruptions indicates a notable hazard exists at Okmok Volcano.