INVESTIGATING THE MECHANISM BEHIND PERSISTENT DEGASSING AND THERMAL UNREST AT ARC VOLCANOES: AN EXAMPLE FROM MOUNT BAKER, WASHINGTON

Volcanic activity is often characterized in terms of cycles, with periods of quiescence, unrest, and eruption. Some composite volcanoes, however, exhibit persistent low-level unrest without eruption, complicating interpretations of activity and assessment of future eruptive hazards. Additional investigations, including geophysical studies, are needed to explore the processes that drive such unrest.

Mount Baker, Washington, offers an excellent chance to explore the activity associated with persistent degassing and thermal unrest. In March 1975, a ~10-fold increase in thermal activity and gas emission occurred at Sherman Crater, just south of the summit of the volcano. The area of snow-free, heated ground increased by a factor of 3, heat-flow estimates suggested an increase from 2 to 30 megawatts, and almost half of the glacier that filled Sherman Crater melted. Fumarole temperatures increased by 40 degrees C, and, in the months that followed, gaseous sulfur (probably H₂S) was emitted at rates of over 100 tonnes/day and gravity measurements (corrected for changes in seasonal ice and snow loads) showed a decrease of 0.33 milligals. Despite this activity, no anomalous seismicity or deformation was measured in the late 1980s and early 1980s. Passive degassing and elevated fumarole temperatures have persisted since 1975 at Mount Baker, and CO₂ was being emitted at a rate of over 100 tonnes/day in 2001.

At least four hypotheses can explain the 1975-onward thermal unrest and degassing at Mount Baker: (1) the current CO₂ flux indicates a constant resupply of magma at depth; (2) the 1975 activity resulted from a stalled intrusion, with no subsequent magmatic activity; (3) emissions since 1975 tap a reservoir of magmatic volatiles that continues to release gas; (4) in 1975 a path opened between a convecting deep magma source and the surface. Each of these processes would be manifested by different patterns of deformation and gravity change, and must also account for the observed lack of seismicity and deformation in 1975. By repeating geodetic surveys first conducted at Mount Baker in the 1970s and 1980s, and continuing to measure gases and fumarole temperatures, it may be possible to distinguish between these competing models and shed light on the processes that drive persistent low-level unrest at arc volcanoes.