Vent processes during the 1912 eruption at Novarupta, Katmai National Park, Alaska. Progress report, (November 15, 1991--November 14, 1992)

摘要

Blocks of welded fragmental material ejected at Novarupta during the great eruption of 1912 provide evidence of the contents and development of the vent. Because they appear to represent material held at magmatic temperature for hours to days and then quenched at depth and ejected, they provide unusual information on the timing of processes of degassing, welding, and magma mixing. Two breccia types are distinguished by proportions of the three magmatic components. Type 1 breccia (Hildreth's ''vitrophyre'') is rhyolite- and andesite-rich (''volcanic inclusions'' in the glassy matrix were found to be 1912 andesite), contains abundant lithics, and is found throughout deposits of the eruption's second and third days. It corresponds to magmatic proportions being erupted toward the end of the first day, or Episode I. Type 2 is dacite-rich and poor in lithics, and occurs only at the surface. It corresponds to magmatic proportions erupted during Episodes II and III. A pyroclastic dike exposed in a bomb of Type 2 vent breccia is petrologically related to Novarupta lava. Water is strongly but not completely degassed from vent breccias (Type I breccia at 0.30 wt % H(sub 2)O and Type 2 breccia at 0.15 wt % H(sub 2)O even when bread crusted) and more thoroughly degassed from dome lava (rhyolite and andesite at < 0.10 wt % H(sub 2)O), but the pyroclastic dike retains significant water (averages 0.90 wt. % H(sub 2)O) and its host breccia likewise contains elevated water concentrations (0.30--0.40 wt % H(sub 2)O). The mafic component in Novarupta dome is derived from andesitic, rather than dacitic magma, and has crystallized substantially in response to mixing with its cooler host.