The Crater Lake and Hydrothermal System of Mount Pinatubo, Philippines: Evolution in the Decade After Eruption

We document a decade of change in the lake and hydrothermal system of Mount Pinatubo, Philippines following the climactic eruption of 1991. A shallow lake formed by early Sept. 1991 and has shown a long-term trend of growth even during dry periods ( ~1 m/month). It was initially dominated by meteoric influx and the residuum of the pre-eruptive hydrothermal system, but quickly became more acid (pH ~2-3) as magmatic discharges continued to condense into the rapidly accumulating water. Acidity and rock dissolution peaked in late 1992, during and immediately after eruption of a lava dome on the crater floor (July-Oct. 1992). The pH of the lake remained ~3.0 to 3.3 from late 1992 to early 1999, whereas temperature showed a slow decline from ~40 to 30^oC. Samples taken since cessation of dome growth suggest that magmatic degassing and rock dissolution have declined significantly relative to magmatic-hydrothermal brine and meteoric input. This is indicated by trends toward higher Cl, Na, K, Li and B and lower Mg, Ca, Fe, SO4 and F with time. Samples taken in 2001 indicate dilution, increase in pH (5.5), and decline in temperature ( ~27^oC) resulting from a growing contribution of meteoric water. The rapid transition from a small and hot acid lake to moderately large and warm near-neutral pH lake is related to the rapid cessation of direct magmatic degassing, large size of the catchment, and large volume of hydrothermal input compared to lakes hosted by more restive andesitic volcanoes. The pre-existing hydrothermal system was invaded by an increased flux of magmatic gas and eventually by magma itself. As the impermeable caprock of the system was progressively fractured, extensive boiling and steam loss occurred. A dry conduit system that conducted volcanic gases and magma to the surface was maintained locally until late 1992. As the magmatic flux waned and the magmatic system plugged itself, a liquid hydrothermal system encroached on this direct pathway. Current hydrothermal input is neutral-Cl fluid that equilibrated with rocks at ~200^oC and has similarities to geothermal fluids in wells drilled below the same area prior to eruption. This indicates that either very rapid fluid/rock interaction has occurred, or hydrothermal outflow that previously flowed south from the volcano now vents through the ruptured cap of the system. Considering evidence for declining magmatic input, edifice failure and due to rising lake-level and consequent lahars pose the greatest present risk to local populations. Two low areas on the crater rim, the Maraunot Notch and the O'Donnell River headwall are a few meters above lake level (late Aug. 2001), but the water level will probably top these points sometime in late 2001 or 2002. Several meters of ash deposits from the 1991 eruptions can be easily eroded from these areas if overtopping occurs. The most likely failure point is along structurally controlled zones of weakness and hydrothermal alteration that control the Maraunot Notch.