Communities in lowlands near volcanoes are vulnerable to significant volcanic flow hazards in addition to those associated directly with eruptions. The largest such risk is from debris flows beginning as volcanic landslides, with the potential to travel over 100 kilometers. Stratovolcanic edifices commonly are hydrothermal aquifers composed of unstable, altered rock forming steep slopes at high altitudes, and the terrain surrounding them is commonly mantled by readily mobilized, weathered airfall and ashflow deposits. We propose that volcano hazard assessments integrate the potential for unanticipated debris flows with, at active volcanoes, the greater but more predictable potential of magmatically triggered flows. This proposal reinforces the already powerful arguments for minimizing populations in potential flow pathways below both active and selected inactive volcanoes. It also addresses the potential for volcano flank collapse to occur with instability early in a magmatic episode, as well as the "false-alarm lu.s. Geological Survey,

The 1982 eruption of El Chichón volcano ejected more than 1 km 3 of anhydrite-bearing trachyandesite pyroclastic material to form a new 1-km-wide and 300-m-deep crater and uncovered the upper 500 m of an active volcano-hydrothermal system. Instead of the weak boiling-point temperature fumaroles of the former lava dome, a vigorously boiling crater spring now discharges 1 20 kg/s of Cl-rich (F15 000 mg/kg) and sulphur-poor ( 1 200 mg/kg of SO 4 ), almost neutral (pH up to 6.7) water with an isotopic composition close to that of subduction-type magmatic water (dDp-15‰, d 18 Opc6.5‰). This spring, as well as numerous Clfree boiling springs discharging a mixture of meteoric water with fumarolic condensates, feed the crater lake, which, compared with values in 1983, is now much more diluted (F3000 mg/kg of Cl vs 24 030 mg/kg), less acidic (pHp2.6 vs 0.56) and contains much lower amounts of S ( 1 200 mg/kg of SO 4 , vs 3550 mg/kg) with d 34 Sp0.5-4.2‰ (c17‰ in 1983). Agua Caliente thermal waters, on the southeast slope of the volcano, have an outflow rate of approximately 100 kg/s of 71 7C Na-Ca-Cl water and are five times more concentrated than before the eruption (B. R. Molina, unpublished data). Relative N 2 , Ar and He gas concentrations suggest extensional tectonics for the El Chichón volcanic centre. The 3 He/ 4 He and 4 He/ 20 Ne ratios in gases from the crater fumaroles (7.3R a , 2560) and Agua Caliente hot springs (5.3R a , 44) indicate a strong magmatic contribution. However, relative concentrations of reactive species are typical of equilibrium in a two-phase boiling aquifer. Sulphur and C isotopic data indicate highly reducing conditions within the system, probably associated with the presence of buried vegetation resulting from the 1982 eruption. All Cl-rich waters at El Chichón have a common source. This water has the appearence of a "partially matured" magmatic fluid: condensed magmatic vapour neutralized by interaction with fresh volcaniclastic deposits and depleted in S due to anhydrite precipitation. Shallow ground waters emerging around the volcano from the thick cover of fresh pumice deposits (Red waters) are Ca-SO 4 -rich and have a negative oxygen isotopic shift, probably due to ongoing formation of clay at low temperatures.

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