Benjamin van Wyk de Vries*
Magmas et Volcans (UMR 6524), Observatoire du Physique du Globe, Université Blaise Pascal, Clermont-Ferrand, France
Department of Geography, Volcano Remote Sensing Group, University of Cambridge, Cambridge CB2 3EN, UK
School of Earth, Environmental and Physical Sciences, Universityof Portsmouth, Portsmouth PO1 3QL, UK
ABSTRACT Catastrophic sector collapse occurs when a volcano becomes structurally unable to support its own load. One process particularly capable of weakening the edifice is hydrothermal activity. It can produce high pore pressures and alter strong rock to clays. Alteration can extend progressively over long periods (>100 yr), allowing deformation todevelop slowly before collapse. An important finding is that structures produced by such deformation are recognizable and could permit collapse prediction. We present the case of Casita, Nicaragua, where hydrothermal activity has been weakening the edifice core, causing flank spreading, altering original constructional shape, and steepening flank slopes. One side is slumping outward, producing acrescentic scar with a basal bulge. We identify this feature as the site of a potential sector collapse, with conditions ripe for failure. Keywords: Casita, volcano, sector collapse, hydrothermal, gravitational spreading, Nicaragua.
INTRODUCTION Sector collapse is a major catastrophic event in the evolution of volcanoes, causing debris avalanches that have been responsible for several historicdisasters (e.g., Mayu-yama, Bandai-San, Japan, and Mount St. Helens, United States) and about 20 000 associated fatalities (Siebert, 1992). Until the present there have been no definite locations or predictions of future sector-collapse sites. There are many possible causes and triggers for collapse, such as seismic shock, fault movement, and dike emplacement (Siebert et al., 1987; Nakamura, 1977;McGuire et al., 1991; Vidal, 1998). However, there are two scenarios in which structures may appear well before failure and can therefore allow prediction. These are cryptodome intrusion, as at Mount St. Helens (Donnadieu and Merle, 1998; Moore and Albee, 1981), and slow, gravity-driven flank spreading promoted principally by hydrothermal weakening (van Wyk de Vries and Francis, 1997; Voight andElsworth, 1997). In the former case, clear signs of instability become apparent months before failure (Donnadieu and Merle, 1998). In contrast, slow spreading creates deformation that can develop over decades or centuries before collapse. In such cases, structural analysis can locate sites of potential failures (van Wyk de Vries and Francis, 1997). We present the case of Casita volcano, Nicaragua, anddescribe an actively deforming edifice containing a large gravity slide likely to collapse. THEORY OF VOLCANO FLANK SPREADING All volcanoes deform under their own weight to some degree (Shteynberg and Solov’yev, 1976; Dieterich, 1988). Simple cones should stabilize with time as material compacts and stresses relax (van Wyk de Vries and Borgia, 1996). Usually, however, magma intrusion andhydrothermal activity reduce internal strength (Voight and Elsworth, 1997). Hydrothermal activity also alters rock to weak clays and, with high pore pressures, decreases effective strength and viscosity (Lopez and Williams, 1993). Thus, stratovolcanoes with extensive hydrothermal activity are likely to become weak and deform. In such cases, the volcano shape will be altered. If the actual shape is comparedwith a known or inferred original construction, then the overall effect of deformation can be determined (van Wyk de Vries and
*E-mail: vanWyk de Vries, email@example.com; Kerle, firstname.lastname@example.org; Petley, email@example.com. Geology; February 2000; v. 28; no. 2; p. 167–170; 5 figures.
Borgia, 1996). Structures produced by the spreading can provide information about...