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Mount Mazama’s Final Eruption

The cataclysmic eruption of Mount Mazama 7,700 years ago started from a single vent on the northeast side of the volcano as a towering column of pumice and ash that reached some 30 miles (50 km) high. Winds carried the ash across much of the Pacific Northwest and parts of southern Canada. So much magma erupted that the volcano began to collapse in on itself. As the summit collapsed, circular cracks opened up around the peak. More magma erupted through these cracks to race down the slopes as pyroclastic flows. Deposits from these flows partially filled the valleys around Mount Mazama with up to 300 feet (100 m) of pumice and ash. As more magma was erupted, the collapse progressed until the dust settled to reveal a volcanic depression, called a caldera, 5 miles (8 km) in diameter and one mile (1.6 km) deep.

Crater Lake

Soon after the cataclysmic eruption, eruptions from new vents within the caldera built the base of Wizard Island and the central platform. As eruptions continued, rain and snowmelt also began to fill the caldera. For the next few hundred years, eruptions from these new vents kept pace above the rising water level. Lava flowed into the deepening lake, creating benches on the flanks of the growing cones that tell scientists how deep the lake was during these eruptions. One set of eruptions from a crater on the west edge of the central platform formed lava tubes or channels that sent lava far out onto the caldera floor.

Later eruptions from a vent in the northern part of the caldera, just south of present-day Cleetwood Cove, built Merriam Cone. The erupting Merriam Cone probably never reached the lake surface. The ever-deepening lake eventually drowned the central platform volcano as well. Only Wizard Island managed to grow high enough to stay above the waterline. The last eruptions at Wizard Island took place when the lake was about 260 feet (80 m) lower than today. All of this activity occurred within 750 years after the cataclysmic eruption. The water level continued to rise until reaching near present-day levels, where it encountered a thick layer of porous deposits in the northeast caldera wall. These deposits stabilize lake levels like an overflow drain in a bathtub.

Sometime after these eruptions, the Chaski Bay landslide, the top of which is still visible above the southern shore of Crater Lake, slumped into the caldera and ran up onto the edge of the central platform. Other landslides within the caldera were deflected by the base of Wizard Island and the central platform, burying explosion craters and other features on the lake floor with even more debris.

The last known eruption at Crater Lake occurred when a small lava dome erupted under water on the east flank of the base of Wizard Island about 4,800 years ago. Since that time, the volcano has remained quiet, allowing as much as 100 feet (30 m) of sediment to accumulate on the lake bottom.

The Future

The long history of volcanic activity at Crater Lake suggests strongly that this volcanic center will erupt again. The most recent eruptions occurred on the lake floor in the western part of the caldera. Future eruptions are more likely to occur in the same area than farther east. Interaction of magma and lake water at shallow levels (less than 100 feet or a few tens of meters) could generate explosions that throw large rocks and ash out beyond the caldera walls. The largest explosions could produce pyroclastic surges, hot, rapidly moving clouds of gas and ash, which could move out a few miles from vents along the margin of the lake. Eruptions in deeper water are less likely to be explosive or affect areas around the rim. An eruption similar to the one 7,700 years ago is unlikely because large volumes of magma are not available to cause such an eruption.

Landslides from the caldera walls can cause waves that flood shoreline areas, but one that could cause overtopping or failure of the caldera walls and catastrophic draining of Crater Lake is unlikely. At its lowest point, the caldera rim is more than 500 feet (165 m) above the lake level. Only major volcanic activity could cause the caldera walls to fail and result in flooding of lowland areas below Crater Lake.

Crater Lake lies within a region where the Earth’s crust is being gradually stretched. The faults that accommodate this extension could produce damaging earthquakes in Crater Lake National Park today. The West Klamath Lake fault zone consists of numerous faults, one of which passes 1/2 mile west of Rim Village. Although earthquakes as large as magnitude 7.0 are possible, scientists believe that they occur only about once every 3,000 to 10,000 years. Such earthquakes could cause landslides large enough to generate dangerous waves on Crater Lake. Smaller earthquakes are much more likely, but they are unlikely to trigger large landslides.

Probing the Depths

For more than 100 years, scientists have marveled at Crater Lake’s great depth and sought to unravel the mysteries hidden beneath its surface. Clarence Dutton of the USGS led the first expedition to determine the depth of the lake in 1886. With a lead weight and piano wire, Dutton’s party made 168 soundings from a rowboat and reported a maximum depth of 1,996 feet (608 m). The National Park Service made additional soundings of this type between 1938 and 1940.

It wasn’t until the advent of sonar that the nature of the caldera beneath the lake became clearer. In 1959, the US Coast and Geodetic Survey obtained more than 4,000 echo soundings and provided a more accurate estimate of the maximum depth of Crater Lake ö 1,932 feet (589 m). Contours of these data revealed the principal features on the floor of Crater Lake ö the central platform, Merriam Cone, a small lava dome on the east side of Wizard Island, and the Chaski Bay landslide ö for the first time. About the same time, Crater Lake ranger-naturalist C. Hans Nelson collected dredge samples that showed a variety of post-7,700-year eruptive activity. In 1979, Nelson returned to the lake as a USGS marine geologist and used acoustic imaging techniques (like making a CAT scan) of the lake floor to discover that as much as 250 feet (75 m) of sediment had accumulated since caldera collapse. Nelson’s study found that beneath the sediment were many depressions located along the caldera-collapse ring fractures. These crater-like structures were probably formed by steam explosions as water began to accumulate within the still-hot debris shortly after the cataclysmic eruption.

A later USGS party mapped areas where large amounts of heat escaped from the lake floor. In 1988 and 1989, scientists from Oregon State University, the National Park Service, and the USGS explored parts of the lake floor with a manned submersible, paying special attention to the warm areas identified earlier. Several dives were devoted to exploring the volcanic features on the lake floor and to sampling caldera-wall outcrops. The exploration with the submersible suggested the need for remapping the lake floor with modern techniques to provide a bathymetric (depth) map for interpreting the postcaldera geologic history and as a base for further study of the lake itself.

The new bathymetric mapping was completed in the summer of 2000 by scientists from the USGS, the National Park Service, the University of New Hampshire, and C & C Technologies. Using a high-resolution acoustic mapping system mounted on a special boat, they collected millions of accurately located echo soundings during a 5-day survey. The information gained from this latest survey not only provided a new maximum depth for Crater Lake ö 1,949 feet (594 m) ö but also resulted in a detailed map of features on the lake floor. By combining the new bathymetric data with past decades of other research, scientists now have the clearest picture yet of events that happened since the massive eruption 7,700 years ago that destroyed Mount Mazama and created Crater Lake.

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