The Lake Inside the Volcano

The Earth's crust is dynamic, being composed of slowly moving masses called tectonic plates. The contact zones between these crustal plates are often the scene of dramatic geologic activity. The Cascade volcanoes result from the collision of two of these plates, the Juan de Fuca and the North American. Pushing against each other, these two plates create a subduction zone, where one plate is forced beneath the other. The light crustal material of the Juan de Fuca Plate melts as it is forced into the hot depths of the earth's mantle. The resulting molten material rises back through the North American Plate, eventually emerging as lava and forming the volcanoes of the Cascade Range.

About 420,000 years ago, the volcano we call Mt. Mazama began to form in the Cascade Range. Periodic eruptions built the mountain layer by layer, lava flow by lava flow. The mountain had several different vents, so it was probably never a symmetrical cone. Instead, it was a mass of overlapping small volcanoes forming one giant composite volcano. Geologists believe that Mt. Mazama possibly reached as high as 12,000 ft. above sea level.

Mt. Mazama's most violent eruption occurred around 7,700 years ago. This "cataclysmic" eruption began by ejecting a column of hot gas and magma high into the air. This ejected material fell back to the surface blanketing the local landscape with frothy pale pumice lava and volcanic ash.

As this eruption continued it also increased in magnitude, producing massive, fast moving flows of hot ash. The magma chamber deep below the mountain was losing more and more of its contents. This weakened the mountain's underlying support and allowed the top of the peak to collapse into the magma chamber in a very short time. The mountain which had taken hundreds of thousands of years to create metamorphosed in a just a few days.

The eruption's aftermath left a 4,000 ft deep caldera, a term adopted from Spanish which literally means "devil's bowl". Subsequent smaller flows helped to seal the caldera's bottom. Heavy snowfalls each year filled the caldera, eventually creating a lake with a depth of nearly 2000 ft. Today, the lake is in a state of equilibrium, neither significantly increasing or decreasing in depth. Its stability results from a balance between the incoming precipitation, and an outflow of seepage and evaporation.