Mt. Mazama was once among a cluster of Pleistocene stratovolcanoes stretching along the Washington and Oregon coast. The magma chamber formed over a period of
15,000-40,000 yrs. During that period, eruptions of basalt,
andesite, dacite, and rhyolite occurred in the Mt. Mazama area.
During the few centuries preceding the climactic eruption, at least two large eruptions of rhyolite occurred in the area underlain by the magma chamber. Tephra from one of these eruptions extended into eastern Washington and western Nevada.
The climactic eruptions of 6850 yr ago occured over a number of months, and produced tephra-fall and two episodes of pyroclastic-flow deposits. Tephra deposits from this series of eruptions have been found in 8 western states and 3 Canadian provinces. The major episode of pyroclastic-flow in this series of eruptions extended to 60 km (40 miles). Following this series of eruptions, the caldera collapsed into the magma chamber, and Mt Mazama became dormant except for activity which formed a small cinder cone in the center. The collapsed caldera filled with water and became what we now know as Crater Lake.
The total volume of magma erupted during the climactic eruption was about 10 times larger than that produced during any other explosive eruption in the Cascade Range during postglacial times.
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