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Composite Volcanoes II. Morphology of Composite Volcanoes E. Degradation In a heuristic and phenomenological sense, the history of a composite cone can be thought of as the interplay of short-term eruptive phenomena that construct volcanoes periodically and the longer term erosive processes of degradation that conspire to bring them down (Table III). However, while this is a useful working hypothesis, it is important to recognize that the while the averaged rate of degradation on composite volcanoes suggests longer term equilibrium, in reality degradation is highly variable, consisting of a regional "background" rate modulated by more rapid erosion and mass-wasting events. The high degradation rates often accompany and closely follow eruptions and punctuate the equilibrium. For instance, observations after the 1980s eruptions of Mount St. Helens in the USA and the 1991 eruption of Mount Pinatubo in the Philippines showed that peaks of erosion follow closely on peaks of eruption as pyroclastic material is rapidly (months to years) removed from the cone. Catastrophic processes such as landsliding/avalanching further conspire to accelerate degradation. Long-term erosion depends on climate and the composition of the volcanic edifice. Given comparable rates of production and cone growth, volcanic cones are better preserved in arid/cool climates rather than humid equatorial climates. For instance, climatic differences between the hyperarid central Andes and the tropical northern Andes are highlighted by pristine morphologies of Pleistocene volcanoes in the former and degraded morphologies in the latter. In response to eruptive events that produced large volumes of unconsolidated material and greatly disturbed local topographic equilibria, at Mount St. Helens, 10-m-deep gullies were cut into the pyroclastic deposits within a few months of the eruption. At Mount Pinatubo, the 1991 pyroclastic flows had similar gullies within a matter of days. On the other hand, pyroclastic deposits at Katmai (Alaska), where the climate is somewhat cooler and drier, are only moderately eroded nearly 100 years after eruption. TABLE III Stages in the Erosional History of Composite Volcanoes
The composition of the edifice is an equally important control on degradation styles and rates. Clearly, unconsolidated pyroclastic material will be more readily eroded. Lava is less porous than scoria and will encourage runoff. Porosity and permeability will therefore be important in controlling if runoff or percolation dominates. Since extensive gullying is common on composite volcanoes, we can surmise that runoff dominates. Gullying is the most common indication of degradation of composite cones and occurs in several stages. One of the most significant stages of gullying involves the formation of planezes (Table III). These are the result of two master gullies intersecting in the upper reaches of a cone and isolating a triangular, flat-faced, facet. The biggest single obstacle to the growth of a volcano is gravity and the hackneyed cliche "what goes up must come down" is nowhere better exemplified than in the catastrophic debris avalanches (cf. "Debris Avalanches") that are now recognized as being part of the normal mode of activity of composite volcanoes. The mass transfer associated with these events is largely responsible for modifying the shapes of volcanoes to the steady-state concave-upward profile as the deposits from these events extend the talus apron of the lower flanks out to several tens of kilometers from the summit. As a volcano grows, its slopes become steeper and therefore gravitationally unstable. Further, asymmetry resulting from unequal distribution of mass on an edifice adds to the instability. Edifices built on slopes are inherently unstable. Gravitational collapse requires a trigger, and many events of this type probably involve earthquake-triggered flank failures (e.g., Bezymianny, 1956; Mount St. Helens, May 1980). The growing volcanic load may reactivate local basement faults that may penetrate up into the volcanic edifice (e.g., Socomopa, Chile, 7500 a). Finally, inflation of the cone due to a recharge event or degassing may trigger the collapse of an unstable flank (e.g., Tata Sabaya, Bolivia). The most spectacular examples of gravitational collapse involve the failure of 25-30% of a volcanic cone in a matter of minutes. The morphologic signatures of collapse are invariably a collapse amphitheater and hummocky topography on the avalanche deposit; the May 18, 1980, eruption of Mount St. Helens (see "Debris Avalanches") provides a typical example. Several other amazing examples have also been recognized around the world, but nowhere more so than in the central Andes.The studies in the central Andes highlight several key features. First, collapsed portions may vary in scale from relatively small (10-15%) portions of the edifice as at Irruputuncu on the Bolivia/Chile border to quite large (30%) portions of the edifice as at Tata Sabaya in Bolivia. Second, regardless of how extensively an edifice is eviscerated, the conical shape is reestablished very rapidly. This may explain why there are currently few clear examples of collapse-scarred volcanoes (Table II). At Parinacota in Chile, reconstruction since a massive collapse 13,000 years ago has virtually rebuilt the entire edifice (Fig. 6e; see also color insert). Only very careful examination of aerial photographs reveals the trace of the amphitheater. At Bezymianny in Kamchatka, a dome growing since the 1956 sector collapse has nearly filled the amphitheater (Fig. 6f). Third, the deposits from these collapses can form a significant portion of the talus apron of volcanoes (e.g., Volcan Misti, Peru). Gravitational collapse is therefore a significant mechanism of mass transfer on composite volcanoes. The reduction of steep slopes of the upper and middle flanks by collapse and deposition of these materials far from the flanks results in extensive low-angled aprons (commonly referred to as ring plains; vide infra) extending to several tens of kilometers out from the edifice. Subsequent "healing" of the cones results in reestablishment of steep middle and upper flanks, perpetuating the steady-state profile. Also important in determining the volcano morphology resulting from degradation are the effects of hydrothermal alteration. Mature conduits can provide a sufficient heat flux to drive groundwater through a hydrothermal circulation system, with the consequence that the rock volume affected by the system is altered and weakened, making it more vulnerable to erosion by both long-term, slow-mass-wasting, glacial or fluvial processes and catastrophic failure. In volcanoes with well-established hydrothermal systems, the vent region can occupy an enormous bowl of altered volcanic material (such as Mutnovsky volcano, Kamchatka) that may resemble—or on occasion coincide with—a sector collapse amphitheater. The occurrence of hydrothermal activity and the influence of consequent alteration can extend long after magmatic addition to the edifice has effectively ceased and exert considerable control on its morphological degradation.            Back to Contents | |||||||||||||||||||||||