VOLCANIC HAZARDS FROM MAFIC ACTIVITY IN THE CENTRAL OREGON CASCADES

Mafic activity has dominated Holocene volcanism in the central Oregon Cascades. Typical eruptions have produced scoria cones, extensive lava flows, and tephra blankets, all of which would pose hazards to surrounding communities. To improve our understanding of these hazards, we have compiled new maps of tephra deposits from five mafic vents in the central Oregon Cascades (Sand Mt., Blue Lake crater, Collier Cone, Yapoah Crater, and 4-in-1 Cone). Maps are generated from measured tephra thickness in road cuts, pits, and lake sediment cores. Tephra is correlated to source vents based on physical characteristics and geochemistry. Mafic tephra deposits are often limited to tens of kilometers from the vent, but eruption duration can be months to years and even decades. For example, Paricutin, Mexico, erupted continuously from 1943 to 1952 and covered ~195 km² with >25 cm of tephra. Cerro Negro, Nicaragua, has been intermittently active for ~140 years; the last major eruption continued for ~3 months and covered ~90 km² with >0.5 cm of tephra. By comparison, the Sand Mt. chain appears to have been intermittently active for ~1500 years and a single eruption deposited >70 cm of tephra 13 km from the main vent. This suggests at least one of the Sand Mt. chain eruptions was comparable to Paricutin and could easily have deposited trace amounts of tephra on modern urban areas. The possibility of a mafic eruption producing tephra continuously for months to years has not been fully considered in hazard assessments for communities such as Bend, OR. Such an eruption would have long-term impacts on water sources, transportation routes, and recreation. Hazards posed by mafic lava flows have also been underestimated. Recent mafic activity in the Cascades has repeatedly altered the flow of major rivers such as the McKenzie and Deschutes. The low viscosity of mafic lava and wide distribution of vents increase the likelihood of lava and surface water interaction occurring in developed areas. Current regional hazard assessments do not adequately prepare urban areas for threats associated with mafic eruptions. More comprehensive assessments could be made by addressing the range of threats posed by mafic activity to urban areas (e.g. Magill and Blong, 2004).