The Geologic Story
Today, the calm beauty of Crater Lake obscures the violent forces that formed it. Crater Lake lies inside the top of an ancient volcano known as Mount Mazama. This dormant volcano is just one in a chain of huge cones that extends along the crest of the Cascade Range from 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 part of the Pacific "Ring of Fire."

The volcanoes of the Cascade Range are the visible evidence of what geologists call "plate tectonics." The earth's surface is actually broken up into many huge plates, all floating on top of the earth's molten interior. As these plates slowly drift, the continents and adjacent sea floor either move apart or push into one another. The Cascade Range was formed when a plate carrying oceanic crust pushed into what is now the northwestern United States. This crust was forced under the less-yielding continental plate. Tremendous pressures were exerted on the crust, causing it to melt and turn into magma. Volcanic eruptions eventually propelled the magma onto the surface of the earth (where it is then called lava). Over a period of millions of years, a huge mountain chain was created.

Mount Mazama began to grow almost half a million years ago. As time passed, lava flowed from several volcanic vents, overlapping and building an irregularly-shaped mountain. By 8,000 years ago, Mount Mazama may have stood 12,000 feet (3,660 meters) high, one of the highest mountains in the Cascade Range. Mount Mazama's most violent eruption occurred about 7,700 years ago. A column of hot gas and lava was ejected high into the air, and fell to the earth as fragments of frothy white pumice and volcanic ash. Ash from this eruption can still be found in the soil as far away as Montana and Alberta, Canada, 1,000 miles (1,600 km) away. In all, 12 cubic miles (50 cubic km) of material poured out of the erupting volcano, emptying the magma chamber beneath the mountain. As underlying support for the mountain was lost, the walls of the volcano began to collapse inward, creating an enormous basin or "caldera." The mountain that had taken hundreds of thousands of years to build probably collapsed in just a few days.

For many years after the collapse of Mount Mazama, minor eruptions continued inside the newly formed caldera. These relatively recent flows created Wizard Island, which projects above the lake's surface, and Merriam Cone, which is underwater. About 5,000 years ago, a small eruption formed a lava dome just east of Wizard Island, which is also under the surface of the lake.

The caldera had already begun to fill with water as the first two cones formed. Scientists estimate that it may have taken 300 years for rain and snow to fill the caldera to its current depth of 1,943 feet (592 meters).

Are volcanic eruptions likely to happen again at Crater Lake? Another eruption as big as the caldera-forming event is unlikely within the next several thousand years. No volcanic activity has occurred at Mount Mazama in the last 5,000 years. Studies of the lake have found no evidence of magma movement beneath the earth's surface, and there have been no earthquakes of the kind associated with volcanic activity.

On the other hand, there is every reason to expect some kind of volcanic activity someday in the place where it has been occurring for almost half a million years.

Weather
Weather plays a dramatic role at Crater Lake National Park. Winter especially shapes the landscape. Snow begins to accumulate each year in October and doesn't melt in most places until mid-July. Snowfall averages 533 inches (1,354 cm) annually. By early spring, ten to fifteen feet (4 meters) of snow on the ground is typical.

While snowfall is common in the Cascade Mountains, Crater Lake National Park is one of the snowiest areas in the entire Northwest. The National Park Service began recording weather information here in 1926. The winter of 1932-33 holds the record for total snowfall, with 879 inches (2,233 cm).

Why so snowy?
The major weather patterns for this area start in the Pacific Ocean. Storm events grow in strength and moisture as they swirl over the North Pacific. Wind patterns move the storms inland. After crossing the Coast Range, storm clouds descend into the Rogue and Willamette Valleys, dropping about 30 inches (76 cm) of rain. As storms move eastward, the high mountains of the Cascades force the warm, moist air up, causing it to cool and condense into clouds. With enough moisture in the clouds, rain or snow is produced.

Crater Lake is shaped by its long winter season. Without the snow, and the precious water it contains, there would be no flowers, no streams, and ultimately no Crater Lake.