Ongoing (1996-present) volcanic unrest near South Sister, Oregon, is accompanied by a striking set of hydrothermal anomalies, including elevated temperatures, elevated major ion concentrations, and 3 He/ 4 He ratios as large as 8.6 R A in slightly thermal springs. These observations prompted the US Geological Survey to begin a systematic hydrothermal-monitoring effort encompassing 25 sites and 10 of the highest-risk volcanoes in the Cascade volcanic arc, from Mount Baker near the Canadian border to Lassen Peak in northern California. A concerted effort was made to develop hourly, multiyear records of temperature and/or hydrothermal solute flux, suitable for retrospective comparison with other continuous geophysical monitoring data. Targets included summit fumarole groups and springs/streams that show clear evidence of magmatic influence in the form of high 3 He/ 4 He ratios and/or anomalous fluxes of magmatic CO 2 or heat. As of 2009-2012, summit fumarole temperatures in the Cascade Range were generally near or below the local pure water boiling point; the maximum observed superheat was <2.5°C at Mount Baker. Variability in ground temperature records from the summit fumarole sites is temperature-dependent, with the hottest sites tending to show less variability. Seasonal variability in the hydrothermal solute flux from magmatically influenced springs varied from essentially undetectable to a factor of 5-10. This range of observed behavior owes mainly to the local climate regime, with strongly snowmelt-influenced springs and streams exhibiting more variability. As of the end of the 2012 field season, there had been 87 occurrences of local seismic energy densities approximately ≥ 0.001 J/m 3 during periods of hourly record. Hydrothermal responses to these small seismic stimuli were generally undetectable or ambiguous. Evaluation of multiyear to multidecadal trends indicates that whereas the hydrothermal system at Mount St. Helens is still fast-evolving in response to the 1980-present eruptive cycle, there is no clear evidence of ongoing long-term trends in hydrothermal activity at other Cascade Range volcanoes that have been active or restless during the past century (Baker, South Sister, and Lassen). Experience gained during the Cascade Range hydrothermal-monitoring experiment informs ongoing efforts to capture entire unrest cycles at more active but generally less accessible volcanoes such as those in the Aleutian arc.

A few square kilometers of the Bishop Tuff in eastern California (USA) have evenly spaced columns that are more resistant to erosion than the surrounding tuff owing to the precipitation of mordenite, a low-temperature (100-130 °C) zeolite. We hypothesize that the columns are a result of instabilities at the liquid water and steam interface as cold water seeped into the still-cooling Bishop Tuff. We use two methods to quantitatively assess this hypothesis. First, scaling shows which hydrodynamic instabilities exist in the system. Second, to account for the effects of multiphase flow, latent heat, and the finite amplitude and temporal evolution of these instabilities we use two-dimensional numerical models of liquid water infiltrating hot tuff. These tests highlight several features of boiling hydrothermal systems. (1)The geometry of at least some convection appears to be broadly captured by linear stability theory that neglects reactive transport, heterogeneity of the host rock, and the finite amplitude of instabilities. (2) Slopes >10% set the wavelength of convection, meaning that these columns formed somewhere with relatively gentle topography. (3) For permeabilities of >10 −13 m 2 , the wavelength of the instability changes through time, slowing infiltration, while for permeabilities <10 −15 m 2 , cooling is dominated by conduction. The spacing and stability of columns increase with higher vertical permeability and decrease with higher horizontal permeability. These columns are a rare window into hydrothermal processes that may be widespread.

Several moons in the outer solar system host liquid water oceans. A key next step in assessing the habitability of these ocean worlds is to determine whether life’s elemental and energy requirements are also met. Phosphorus is required by all known life and is often limited to biological productivity in Earth’s oceans. This raises the possibility that its availability may limit the abundance or productivity of Earth-like life on ocean worlds. To address this potential problem, here we calculate the equilibrium dissolved phosphate concentrations associated with the reaction of water and rocks—a key driver of ocean chemical evolution—across a broad range of compositional inputs and reaction conditions. Equilibrium dissolved phosphate concentrations range from 10−11 to 10−1 mol/kg across the full range of carbonaceous chondrite compositions and reaction conditions considered, but are generally > 10−5 mol/kg for most plausible scenarios. Relative to the phosphate requirements and uptake kinetics of microorganisms in Earth’s oceans, such concentrations would be sufficient to support initially rapid cell growth and construction of global ocean cell populations larger than those observed in Earth’s deep oceans.

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