HOW EXPLOSIVE IS MOUNT SHASTA?

Mount Shasta is not considered to be a very explosive volcano because tephra falls and low-density (<1.2 gm/cm³) pyroclastic-flow deposits are relatively rare. For example, only two tephra falls are known in the Holocene—the Red Banks, which covers the eastern half of the volcano and a very localized, thin, younger tephra fall deposit related to the growth of Black Butte dome—both falls occurred in early Holocene time. However, remnants of low-density tephra falls and pumice flows, presumably older than or contemporaneous with the Red Banks tephra, are found around the volcano, but little is known about their distribution or timing. Here we combine field data with bulk geochemistry, mafic mineral assemblages, and microbeam analyses of glass, minerals, and Fe-Ti oxides (composition, thermometry, and fugacity) to characterize these remnants for correlation purposes. Four mafic mineral assemblages distinguish the remnant deposits, Red Banks and Black Butte: (1) hornblende only (Black Butte); (2) two pyroxene only (Red Banks tephra fall and remnant flow deposits found on the east and north flanks); (3) hornblende and orthopyroxene (found in remnant tephra falls on the west and south flanks), and (4) hornblende and two pyroxene (found in remnant tephra fall and on the south flank and remnant fall and flow on the north flank). Preliminary iron-titanium oxide, glass, and mineral analyses indicate reasonable correlation for group (3) deposits despite distinct bulk chemical silica contents. Correlation amongst deposits in Groups (2) and (4) is poorer. Glass, magnetite, and bulk chemistry overlap for clasts within group (4), but ilmenite chemistry and thus related oxide fugacity and temperature are distinct for north and south flank deposits. The Red Banks tephra and clasts within a two-pyroxene pyroclastic-flow deposit on the east side of the volcano (group 2) show unique differences in glass and Fe-Ti oxide chemistry (thus also temperature and fugacity). The Red Banks tephra, however, has a broad range in bulk composition; thus, more data are needed to tell whether differences noted here are significant or not. Correlation of these remnant low-density tephra and pyroclastic deposits is key to establishing the extent and volume of past explosive events, and hence, the potential explosivity and hazards of future eruptions.