ALTERATION, DEGRADATION, AND SLOPE FAILURE OF VOLCANOES

Slopes of volcanoes are commonly altered, particularly near summit and other vent areas where fumarolic vapors discharge. Such alteration typically decreases the competence of the rock, and thus affects their susceptibility to gradual and catastrophic degradation. Formation of clay minerals is a major factor in decreasing slope stability, and where clays are incorporated in a lahar, they lengthen the distance of travel (e.g., the 120 km-long Osceola mudflow).

Alteration of volcanic rocks occurs by several interrelated mechanisms. 1) Condensation of magmatic vapor containing HCl and SO2 creates a liquid with pH ~1 to 2 (or lower if evaporation occurs in a crater lake). Groundwater (or summit ice) is required as a condenser. Dissociation of HCl and H2SO4 forms progressively more reactive solutions as the temperature decreases below ~300C, causing upward flaring zones of alteration. Such alteration is typically zoned outward from a structural conduit, and may mushroom into permeable lithologies where present. Alteration grades from a core of residual silica (which recrystallizes to resistant quartz) outward to advanced argillic assemblages (alunite, kaolinite, dickite, pyrophyllite, etc). The outer margin consists of clay-dominated assemblages. 2) Steam-heated waters form by vadose-zone condensation of vapor containing H2S; oxidation creates sulfuric acid water with pH 2 to 3. Alunite and kaolinite plus clay minerals form at ~100C above and along the groundwater table, in some cases multiple and perched. 3) Both environments of vapor condensation may lead to deposition of native S near fumarolic discharges. If buried, groundwater dissolution of S (or oxidation of sulfide minerals) creates highly reactive sulfuric acid solutions, even after hydrothermal activity ceases.

Alteration of a volcanic edifice contributes to an increase in the degradation rate, and in extreme cases, may facilitate catastrophic slope failure, particularly along permeable, altered horizons. Many epithermal and porphyry ore deposits hosted by volcanic rocks preserve evidence of syn-hydrothermal degradation of as much as 1 km over ~1 m.y. Evidence includes telescoping of shallowly formed alteration on deeper assemblages. Sector collapse may trigger exsolution of magmatic fluid and, hence, contribute to ore deposition at both porphyry and epithermal levels (e.g., Lihir Island, PNG).