Baseline Hydrothermal Monitoring Data from Cascade Range Volcanoes

Abstract:

Since 2009 the U.S. Geological Survey has systematically monitored hydrothermal behavior at selected Cascade Range volcanoes in order to define baseline hydrothermal and geochemical conditions. Gas and water data have been collected at 25 monitoring sites on 10 of the highest-risk volcanoes in the Cascade Range. These sites include summit-fumarole groups and springs/streams that show clear evidence of magmatic influence in the form of high ³He/⁴He ratios and (or) large fluxes of magmatic CO₂ or heat. The monitoring data are intended to (1) be suitable for comparison with other continuous geophysical monitoring data and (2) provide baseline data that will be useful during future episodes of volcanic unrest. Site records consist mainly of hourly temperature and (or) hydrothermal solute flux data spanning multiple years. Many of the hydrothermal time series data show considerable variability during quiescent periods, including diurnal, seasonal, and inter-annual variability. Having established baseline conditions, we are reducing our monitoring frequency, and data are being archived and analyzed with a view to providing useful and succinct summaries of baseline behavior during quiescent periods. The hydrothermal response to recent (November 2014 to present) unrest at Lassen Volcanic Center demonstrates the utility of long-term background data, which has made it straightforward to isolate symptoms of unrest.