7,700 years go, give or take 1.5%, the 12,000 foot high stratovolcano Mt. Mazama catastrophically erupted, blasting so much ash and molten rock from its magma chamber that the whole edifice collapsed, creating an oblong caldera six miles long on the longer axis, four and a half on the shorter one, and nearly a mile deep. Between 4,000 feet and 8,000 feet of the summit of Mt. Mazama were gone, spread across a vast stretch of the western states and Canada.
What is left, of course, is the mis-named Crater Lake. It should be Caldera Lake. And while it may be mis-named, it is spectacular.
The caldera has filled with runoff and snowmelt to a depth of 1,949 feet – the deepest lake in North America – and the deep, clear water, combined with Rayleigh scattering, gives the water a nearly-unbelievable deep blue cast. Combine that with the dramatic caldera walls, towering 2,000 feet above the water in some places, and you have geology that is beyond beautiful.
After its collapse, Mt. Mazama remained volcanically active. The floor of the caldera rose about a thousand feet, and at three places smaller cones erupted. One of those cones remains above the water level in the lake and has the melodramatic name Wizard Island because folks think it disappears from some view points on the rim road [Spoiler alert: It doesn’t]. What’s interesting about Wizard Island is that, by radioactive dating, it last erupted just 250 years or so after the explosion that destroyed Mt. Mazama. And the platform in which the island sits is much larger than you’d think, covering about 20% of the caldera floor.
Mt. Mazama was about 450,000 years old, an immense pile of lava flows, ash and cinder slopes, basalt dikes and the rich assortment of rocks that make up the Cascade volcanos. The stratovolcano had been carved by water, wind and, especially, glaciers. Those carved slopes had in turn been re-filled with still more lava flows and ash and cinder falls. One such lava flow can be seen across the lake in this photo, at center left, where a lava flow filled and over-filled a valley on the slopes of Mt. Mazama. The collapse of Mt. Mazama revealed the messy understructure of the stratovolcano. In the previous photo, behind Wizard Island, an even larger lava flow filled what was likely a glacier-carved valley. The collapse of the caldera has left some of the lava flow looming over the water.
A study by the U.S. Geological Survey from 2000 revealed the complex structure of the caldera floor, hidden under all that water. Lava domes, submerged peaks hidden huge landslides, all buried under a hundred feet of windblown sediment, And there’s still hydrothermal activity at various spots on the caldera floor and walls. Mt. Mazama is dormant, not extinct.
Human time is less than a mayfly in comparison to geologic time. Mt. Mazama almost certainly has more volcanic activity in its future. The three quarters of a million people who visit Crater Lake National Park each year think they are seeing an unchanging site. They couldn’t be more wrong.
Wickersham's Conscience 
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