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Crater Lake Revealed
Continued...

Calculating the Volumes of Features

With the lake floor's volcanoes, lava flows, and landslides identified, GIS was used to calculate the volumes of these features. The first step was to re-create the basement surface of Crater Lake before volcanism and landsliding reshaped the lake bottom. Fifty-meter contours were generated from the multibeam bathymetry data. These contours were then rearranged to loosely match a basement surface derived by a study of the lake bottom published in 1988. Polygons outlining each individual landform were taken from the geologic map of the lake floor and used to clip out both the modern and basement surfaces. Volumes were calculated using the CUTFILL command in ArcInfo Workstation. The volume of water in the lake was also calculated.

Learning More About the Lake's History

While virtually flying around the lake bottom, a series of submerged beaches were spotted just below the current lake level. These beaches were not evident in the data anywhere else, and no former beaches are known to exist above the current lake level. This indicates that Crater Lake has remained at or near its present water level for some time. Another sign that lake level has remained constant was a wave-cut platform found just below the present-day water level. Because no streams flow from Crater Lake, the question was asked, "Why has the lake remained at such a constant level?"

The geologic map contained the answer. Permeable sand and gravel deposited by ancient glaciers on the former Mount Mazama had been mapped on the walls of Crater Lake. These deposits continue below the water line, and the geologic contact between these deposits and underlying lava flows was mapped using the hillshade, slope, and acoustic backscatter maps. It appears that these porous glacial deposits function like a bathtub overflow—water flows out of the lake and into the subsurface. This mechanism has kept Crater Lake at its present level. This major discovery has led to a better understanding of the lake filling and geologic histories of Crater Lake.

Geologic History of Crater Lake
The young caldera approximately 7,700 years ago New volcanism underway just 80 years after the young caldera formed
The end of volcanism approximately 4,900 years ago The later occurrence of landslides that carried sediments into the lake

Lake Filling and Geologic Histories

Armed with the knowledge that lake level had remained relatively constant for some time and with the new GIS calculations for the volume of water in the lake, USGS scientist Manuel Nathenson calculated a hydrologic lake filling history for Crater Lake. These calculations were based on lowered precipitation rates subsequent to filling having caused the submerged beaches.

Defining the lake filling history was a major step in unraveling the geologic history of Crater Lake. Knowing where the lake level was at a given time helps date the eruptions and landslides on the lake floor. Ancient shorelines, where lava flows contacted the water's surface on two of the volcanoes located on the lake floor, indicate that—for at least part of their lives—they were erupting above the water line. Knowing when the water line was at the level of an ancient shoreline helps date these eruptions.

Two of the volcanoes on the lake floor also appear to have erupted entirely under water (i.e., all activity occured while these features were submerged). This information helps date their formation. Underwater landslides can be dated in a similar manner. The geologic history was illustrated through a series of GIS perspective views that showed the young caldera approximately 7,700 years ago, new volcanism underway just 80 years later, the end of volcanism approximately 4,900 years ago, and the later landslides that carried sediment into the lake.

Conclusion

Using GIS to visualize and analyze the depths of Crater Lake was extraordinarily successful in revealing its geology, geomorphology, and geologic history. GIS brought together information generated by a group of scientists with diverse backgrounds. Virtual flights around the various images of the underwater environment created using GIS have led to many new and exciting discoveries about one of the earth's most spectacular lakes.

For more information, contact

David W. Ramsey, Geologist/GIS Specialist
USGS Volcano Hazards Team
345 Middlefield Road-MS910
Menlo Park, California 94025
E-mail: dramsey@usgs.gov

Continuing Work

GIS visualization of the geology of Crater Lake continues. Charles R. Bacon is compiling the "Geologic Map of Mount Mazama and Crater Lake Caldera, Oregon." When completed, this unique map and digital database will provide continuous geologic coverage of Crater Lake and the former Mount Mazama—both above and below the water line. GIS visualization is also being employed in public lectures about the lake and in posters, fact sheets, and other outreach presentations.

References

2000 Multibeam Sonar Survey of Crater Lake, Oregon; Data, GIS, Images, and Movies at geopubs.wr.usgs.gov/dds/dds-72

Crater Lake Revealed at geopubs.wr.usgs.gov/i-map/i2790

About the Authors

David W. Ramsey is a geologist and GIS specialist for the USGS Volcano Hazards team in Menlo Park, California. He earned a bachelor's degree in geology from Mount Union College and a master's degree in geology from Bowling Green State University.

Joel E. Robinson is a geologist and GIS specialist for the USGS Volcano Hazards team in Menlo Park, California. He earned both a bachelor's and a master's degree in geology from Miami University.

Peter Dartnell is the project leader for the Pacific Seafloor Mapping Project of the USGS Coastal and Marine Geology team in Menlo Park, California. He earned a bachelor's degree in oceanography from Humboldt State University and a master's degree in environmental studies resource management from San Francisco State University.

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