Document Type


Date of Degree Completion

Summer 2012

Degree Name

Master of Science (MS)


Geological Sciences

Committee Chair

Wendy Bohrson

Second Committee Member

Carey Gazis

Third Committee Member

Chris Mattinson


Mount Mazama is the volcanic edifice that cataclysmically erupted ~503 km of relatively homogeneous rhyodacite lava ~ 7,700 years ago, forming the caldera known as Crater Lake. Within a few hundred years, andesitic eruptions built three distinct volcanic edifices on the floor of Crater Lake; ~ 3000 years later, rhyodacite eruptions formed a dome (Bacon et al., 2002). How magmatic systems evolve following a shallow, relatively large silicic eruption is the focus of this study. In situ geochemical analysis coupled with high-resolution textural images of plagioclase crystals in the four postcaldera volcanic edifices were used to identify distinct crystal populations and identify magmatic processes involved in their formation. Five distinct crystal populations were identified within the postcaldera rocks: LSr antecrysts and Bytownite antecrysts originated in the climactic magma chamber; granitoid antecrysts were part of the magma chamber wall rock; and andesite phenocrysts and basalt phenocrysts represent new magma or melt ± crystals in the postcaldera magma system.

The postcaldera magma system of Mount Mazama’s climactic eruption is an example of how large silicic magma chambers evolve following a large silicic eruption. The results show that assimilation, and fractional crystallization, magma mixing with new mafic magma occurred in the postcaldera magma chamber. These results establish that a large silicic eruption may be followed by imputs of new magma. Combining in situ chemical data with high resolution textural imaging is an effective way to identify distinct crystal populations and discern magmatic processes in a large silicic magma chamber and may be applicable to other magma systems.

A_Thin_Section_Photos.pdf (168540 kB)
Appendix A: Thin Section Photos

B_NDIC_and_BSE_images.pdf (40126 kB)
Appendix B: NDIC and BSE Images

D_EMP_data.xls (117 kB)
Appendix D: EMP Data

E_Combined_Data.xls (403 kB)
Appendix E: Combined Data

071409_Bohrson_CD-185-5_plag_calc.xls (299 kB)
Appendix C: part 1

071409_Bohrson_MC1172_calc.xls (314 kB)
Appendix C: part 2

071409_Bohrson_stds_calc.xls (190 kB)
Appendix C: part 3

071509_Bohrson_80c_516_plag_calc.xls (356 kB)
Appendix C: part 4

071509_Bohrson_81C_673_plag_calc.xls (331 kB)
Appendix C: part 5

071509_Bohrson_185_plag_calc.xls (180 kB)
Appendix C: part 6

071509_Bohrson_CD175_2_plag_calc.xls (408 kB)
Appendix C: part 7