Volcano and Earthquake Hazards in the Crater Lake Region, Oregon
Reawakening of Mount Mazama
The long history of volcanism at Mount Mazama strongly suggests that this volcanic center will be active in the future. The record of past eruptions shows us how the volcano behaved before the system was perturbed by the climactic eruption and caldera formation. Eruptions of Mount Mazama were more frequent than those of the monogenetic volcanoes around it. As the volcano grew, the focus of activity migrated in a west-northwest direction. Some eruptive episodes were much longer and produced a far greater volume of materials than others. Likewise, the lengths of repose periods must have varied considerably. Most of the vents that produced the lavas of Mount Mazama were within the area circumscribed by the present caldera. Vents for silicic magma that tapped into the Mazama system are mainly within 2 km and all within 11 km of the caldera rim.
Wizard Island and the other postcaldera volcanoes (fig. 3) are evidence of renewed activity of Mount Mazama following its climactic eruption. Postcaldera volcanism is common at calderas worldwide (Newhall and Dzurisin, 1988). As all postcaldera volcanism was restricted to the caldera, and given the eruptive history of Mazama with its west-northwest vent migration, we anticipate that the most likely site of the next eruption probably will be within the western part of the caldera. We have no basis for estimating a finite probability of volcanic eruptions from a reawakened Mazama system because of the dramatic changes that occurred as a result of the climactic eruption. Judging from the overall eruptive history of Mount Mazama and the surrounding region, renewed volcanic activity within or very near to the caldera is at least as likely as the birth of a new volcano within Regional Hazard Zone RH (around one chance in 10,000, or 10-4, or a 30-year probability of about one chance in 330, or 3×10-3; see Probability of a future volcanic eruption). We do not have sufficient information to evaluate the significance of the 5,000 year repose period since the last eruption in terms of its possible effect on the probability of future volcanic eruptions. Future eruptions within the caldera may be explosive (see Potential hazards from an eruption beneath Crater Lake). Eruptions outside of the caldera and fed by the Mazama system might produce andesite lava andtephra with hazards akin to those of regional volcanism (see Regional volcanism). Alternatively, the Mazama system might generate slowly emplaced, viscous dacite to rhyodacite domes that may be preceded or accompanied by explosive eruptions (see Hazards of silicic eruptions outside the caldera).