Contents Summary 1 Introduction 1 Geological Considerations 1 Underneath Mount Mazama 2 Structural setting 2 Seismicity of Crater Lake and vicinity 2 Monogenetic volcanoes 3 Mount Mazama proper 4 The last 30,000 years 5 Products of the climactic magma chamber 5 Granitic rocks 6 Postcaldera volcanism 7 Nature of the crustal magmatic heat source 7 Estimate of magmatic heat 7 Structure of the caldera floor and its relation to hydrothermal systems 9 Gravity anomaly of the Crater Lake magma chamber 9 Conclusions from geological information 10 Hydrothermal system(s) of Mount Mazama 10 Surface thermal data 10 Subsurface thermal data 11 Water geochemistry 12 Stable isotopes 13 Chemical geothermometry 13 Helium isotopes 14 Comparison with Mount Hood 14 Geothermal resources 14 Acknowledgments 16 References 16 Tables 21 Figures 25