The crater lake and hydrothermal system of Mount Pinatubo, Philippines: evolution in the decade after eruption

Abstract

The June 1991 eruption of Mount Pinatubo, Philippines breached a significant, pre-eruptive magmatic-hydrothermal system consisting of a hot (>300 °C) core at two-phase conditions and surrounding, cooler (<260 °C) liquid outflows to the N and S. The eruption created a large, closed crater that accumulated hydrothermal upwellings, near-surface aquifer and meteoric inflows. A shallow lake formed by early September 1991, and showed a long-term increase in level of ~1 m/month until an artificial drainage was created in September 2001. Comparison of 1 the temporal trends in lake chemistry to pre-and post-eruptive springs distinguishes processes important in lake evolution. The lake was initially near-neutral pH and dominated by meteoric influx and Cl-SO 4 and Cl-HCO 3 hydrothermal waters, with peaks in SO 4 and Ca concentrations resulting from leaching of anhydrite and aerosol-laden tephra. Magmatic discharge, acidity (pH~2) and rock dissolution peaked in late 1992, during and immediately after eruption of a lava dome on the crater floor. Since cessation of dome growth, trends in lake pH (increase from 3 to 5.5), temperature (decline from 40 to 26 °C), and chemical and isotopic composition indicate that magmatic degassing and rock dissolution have declined significantly relative to the input of meteoric water and immature hydrothermal brine. Higher concentrations of Cl, Na, K, Li and B, and lower concentrations of Mg, Ca, Fe, SO 4 and F up to 1999 highlight the importance of a dilute hydrothermal contribution, as do stable-isotope and tritium compositions of the various fluids.However, samples taken since that time indicate further dilution and steeper trends of increasing pH and declining temperature. Present gas and brine compositions from crater fumaroles and hot springs indicate boiling of an immature Cl-SO 4 geothermal fluid of near-neutral pH at approximately 200 °C, rather than direct discharge from magma. It appears that remnants of the pre-eruptive hydrothermal system invaded the magma conduit shortly after the end of dome emplacement, blocking the direct degassing path. This, along with the large catchment area (~5 km 2 ) and the high precipitation rate of the area, led to a rapid transition from a small and hot acid lake to a large lake with near-ambient temperature and pH. This behavior contrasts with that of peak-activity lakes that have more sustained volcanic gas influx (e.g., Kawah Ijen, Indonesia; Poas and Rincón de la Vieja, Costa Rica).

Hengill geothermal volcanic complex (Iceland) characterized by integrated geophysical observations

“…The exploitation of geothermal energy is limited at present because drilling exploration wells is expensive and hydrothermal systems are often associated with volcanic and seismic activity (Gudmundsson and Arnórson, 2002;Stimac et al, 2003), which may put geothermal exploitation at risk (Ward, 1972;Tester et al, 2006;Majer et al, 2007). Remote characterization of reservoir properties is of significant economic importance for the geothermal industry (Ragnarsson, 2010).…”

Idiosyncrasies of Volcanic Sulfur Viscosity and the Triggering of Unheralded Volcanic Eruptions

“…In fact, the presence of crater lakes can be only a feature limited in time. Several volcanoes have shown transitions between periods of dome growth and periods with crater lakes (e.g., Pinatubo, Philippines Stimac et al, 2004;Soufrière, St.Vincent, Lesser Antilles;Fournier et al, 2011), and sulfur melting and remobilization has been observed at volcanoes with no records of historical eruptions, and no crater lakes (e.g., Lastarria, Chile, Naranjo, 1985). Accumulated sulfur can be subjected to "combustion" (Oppenheimer, 1996) or "volatilization" (Christenson, 2000), during different eruptive phases, contributing to an increase in the SO 2 released.…”

Recognizing and tracking volcanic hazards related to non-magmatic unrest: a review

“…1998; Stimac et al 2004;Rouwet et al 2004Rouwet et al , 2008Rouwet et al , 2014, and/or hydrothermal alteration (Getahun et al 1996;Gehring et al 1999;Salaün et al 2011). Volcano-hydrothermal systems seem to develop more frequently in closed-conduit dome complexes and calderas (e.g.…”