Today, the calm beauty of Crater Lake belies the violent earth forces that formed the lake. Crater Lake lies inside the top of an ancient volcano known as Mount Mazama. This dormant volcano is just one in a group of huge cones that extends along the crest of the Cascade Range. This range extends form Lassen Peak in California to Mount Garibaldi near Vancouver, British Columbia. Four national parks and numerous national forests protect major portions of the Cascade Range, which is a part of the Pacific Ring of Fire.

Pushy Plates and Colliding Continents

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 plate and the North American plate. Pushing against each other, these two plates create a subduction zone, where one plate is forced beneath the other. Continental crust is thicker than oceanic crust and tends to be less yielding. The light crustal material of the Juan de Fuca Plate melts as it is forced into the hot depths of the earth's mantle. This melted rock is called magma. The resulting lighter and more fluid molten material rises back through the North American Plate, eventually emerging as lava and forming the volcanoes of the Cascade Range.

The Mountain That Used To Be

Mount Mazama began to grow as basaltic lava flows. These are very similar to the flows that form the Hawaiian Islands. Basaltic flows usually form gently sloping shield volcanoes. About 450,000 years ago the magma began to change in composition. The result was a series of more violent eruptions. The flows produced during this eruptive phase were less fluid and could not flow as far. The steep sided volcanoes of today's Cascade Range were born.

Mount Mazama is estimated to have been between 10,500 and 12,500 feet in elevation prior to its collapse, making it a near twin of Mount Shasta to the south. A visit to Mount Mazama would have been very similar to Mount Rainier today: beautiful glacier fields, hanging valleys, alpine meadows, riparian canyons, and tall forests.

The End Is Near

Mount Mazama's most violent eruption occurred about 7,700 years ago. A column of hot gas and magma was ejected high into the air. This ejected magma fell to the earth as fragments of frothy white pumice and volcanic ash. Ash from this eruption can be found as far away as Alberta, Canada and western Montana. Rhyodacite lava flooded the vent which was emitting these ejections. The vent soon overflowed and a 1.25 mile wide lava flow oozed a mile downslope. This flow was as much as 1,200 feet thick in some places.

The Eruption

Explosions on the north northeastern side of Mount Mazama produced fast moving flows of hot ash. Ejections of pumice built up to depths of 20 feet at the base of the mountain and were a foot deep as much as 70 miles away. Finally, as the magma chamber was emptying and the underlying support for the mountain was lost, the walls of the volcano began to collapse. The top of a mountain that was built over hundreds of thousands of years "disappeared" in just a few days. Some of the cliffs around the caldera (which comes from the Spanish word for "kettle" or "boiler") towered almost 4,000 feet above the surface of the newly formed crater.

The Aftermath

Minor eruptions continued inside caldera. About 6,000 years ago, these flows created Wizard Island, a volcanic cone which projects 800 feet above the lake's surface today. The crater atop Wizard Island is one of the best preserved vents in the Cascades. Only 300 feet wide and 90 feet deep, it ejected lava blocks as large as 6 feet in diameter. Even today, the remnants of a miniature lava flow are seen on the southwest section of the crater formed by semi-molten rock. Some time after the volcanic activity ceased, rain and snow began to fill the caldera. Over the centuries, a lake was formed to its present depth of almost 2,000 feet.

What the Future Holds

Even if Mt. Mazama were to never erupt again, Crater Lake would considerably change with the passage of time. Over centuries and milleniums, erosion could fill the lake with sedimentation, extended drought could diminish the lake level, even global warming would have an impact on the lake ecology. However, before these forces have a chance to come into play, the possibility of Mt. Mazama's reawakening always exists. More than likely, any volcanic activity would begin inside the caldera, resulting in cone building activity within the lake itself. Who knows, maybe one day there will be another Wizard Island, or even two lakes created by volcanic ridges! In any event, when Llao next stirs from his subterranean hibernation, we can be confident only of being surprised.