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Tephra Fall Hazards Around Crater Lake, Oregon

Map showing one-year probability of accumulation of 1 centimeter (0.4 inch) or more of tephra from eruptions of volcanoes in the Cascade Range. (Public domain.)

Tephra fall can occur from an explosive eruption from either Crater Lake or any of the volcanoes in the area. Based on information from volcanoes around the world we know that eruptions that follow long periods (thousands of years) of quiet at individual volcanoes can be quite large magnitude. Crater Lake (Mount Mazama) has not produced significant eruptive activity in the past approximately 5000 years, and a reawakening of the volcanic system could result in a variety of volcano hazards, including tephra fall. Eruptions within a few kilometers of the caldera cannot be ruled out, and tephra may spread for tens of kilometers in any direction, especially those downwind on the day(s) of the eruption.

A somewhat less serious hazard is the ejection of large ballistic blocks, tens of centimeters or more in diameter, to distances up to a few kilometers (miles). Blocks can be mixed with tephra and ash derived from magma if the eruption occurs due to the explosive interaction between water and magma in a hydro-magmatic or hydrothermal explosion. In such eruptions, ballistic blocks can be ejected at velocities ranging from less than 100 m/s (225 mi/hr) to nearly 250 m/s (550 mi/hr). At Crater Lake, blocks ejected by hydro-magmatic or hydrothermal explosions could easily overtop the caldera rim if the eruption occurred near the lakeshore.