11 Hazards of Silicic Eruptions Outside the Caldera

Volcano and Earthquake Hazards in the Crater Lake Region, Oregon

 Hazards of Silicic Eruptions Outside the Caldera

Silicic magma generated in a reawakened Mazama system might erupt outside of the caldera but within a few kilometers of the caldera rim. It is impossible to forecast this type of event or to determine where such an eruption would be most likely because of the drastic reorganization of the magmatic plumbing system that took place when the caldera formed. We believe the probability of such an eruption is less than for an eruption within the caldera. Nevertheless, it is worthwhile to describe the types of phenomena that could be expected.

Silicic eruptions typically begin with hydromagmatic explosions as the slowly rising magma heats groundwater in the shallow subsurface. Such explosions will be most likely in poorly-drained areas. Commonly, explosive magmatic eruptions follow the vent-clearing, hydromagmatic phase. Both phases can produce pyroclastic surges, flows, and falls. Factors controlling the runout distances of surges and pyroclastic flows are described above (see Proximal hazard zones for explosive eruptions). Heavy rainfall on fresh pyroclastic deposits may feed lahars (see Hazards of lahars (volcanic debris flows) and their runout flows).

Viscous magma that has lost its original gas may eventually extrude from the vent to form a lava dome or slowly-moving flow. Relatively small pyroclastic surges or flows may form as hot lava blocks fall from the unstable dome or flow front. The smallest domes are only a few tens of meters in diameter. Larger silicic lava flows are exemplified at Crater Lake by the preclimactic rhyodacite flows such as Cleetwood, Llao Rock, Grouse Hill, and Redcloud Cliff. They were preceded by plinian eruptions that resulted in tephra fall as far as several hundred kilometers from their vents. These eruptions were fed by the climactic magma chamber, which contained a large volume of silicic magma at the time. The present magma system is unlikely to have accumulated such a large volume of eruptible magma in the 7,700 years since the climactic eruption (see Another large volume or caldera-forming eruption?). Consequently, we do not consider eruptions of this magnitude to be likely in the next few thousand years. A smaller silicic eruption within a few kilometers of the caldera cannot be ruled out. We have not designated a specific hazard zone for this type of eruption on plate 1 because it would approximately coincide with Proximal Hazard Zone PB.

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