Report of the Sec. of the Interior under Sec. 7 of Public Law 100-443 on the Presence or Absence of Significant Thermal Features Within Crater Lake National Park, 1992
III. Conclusions Regarding Significance of the Hydrothermal Features of Crater Lake
B. Hydrothermal Research in Crater Lake
by U. S. Geological Survey
Review of studies concerning the presence of
thermal water inflows into Crater Lake
U.S. Geological Survey
The purpose of this section is to review available research that is relevant to the question of existence and significance of inflow of thermal water into Crater Lake. The pertinent research topics are:
* the formation of Crater Lake
* hydrologic and chemical balances for the lake as a well-mixed body of water
* thermal and chemical characteristics of springs on the flanks of Mount Mazama
* distributions of dissolved constituents as a function of depth in Crater Lake
* submersible observations in the deep part of the lake
The chemical balance of the lake shows that the inflow of warm, slightly saline fluid is important to explaining the chemical balance of Crater Lake. The data for springs on the flanks of Mount Mazama show what the temperatures and chemistry of non-thermal springs are in order to provide a basis for understanding the anomalies found in the lake. The distributions of dissolved constituents with depth show how the inflow of warm, slightly saline water affects the characteristics of water in the bottom part of the lake. The section on submersible observations describes features such as pools and bacterial mats on the bottom that result from the inflow of warm, slightly saline water and the samples that were obtained from these features.
This review is a summary of a large body of research performed by investigators primarily from Oregon State University, the National Park Service, the U.S. Geological Survey, and other institutions. Details of the various investigations can be found in the original references. Figures have been reproduced from the original studies with minor modifications to labels.
FORMATION OF CRATER LAKE
Mount Mazarna is the name for the volcanic mountain in which Crater Lake caldera formed (Bacon and Lanphere, 1990). The oldest lavas of Mount Mazama are approximately 400,000 years old, and continuing volcanic eruptions built the mountain to a summit elevation of approximately 3600 m. At 6850 years before present, a violent eruption destroyed the top of Mount Mazama and created the Crater Lake caldera. The total volume of magma erupted was approximately 50 km3 . After the formation of the caldera, eruptions took place on its floor to form the central platform, Merriam Cone, and Wizard Island (Figure 1). The lake reached nearly its current level before the end of these eruptions. At about 4,000 years before present, a small dome was extruded on the east flank of Wizard Island (Figure 1). Whether these post-caldera eruptions are from the magma chamber related to the climactic eruption or represent a new influx of magma is uncertain. In addition to the lava flows mentioned above, the floor of Crater Lake is comprised of debris from the caldera walls and relatively flat-lying sediments in the deep basins (Barber and Nelson, 1990).