Credit: B. VAN WYK DE VRIES

Volcanoes can be dangerous even when dormant. The centre of attention in this photograph lies not in the smoking cone of San Cristobal volcano in the background but in the less-obvious bulge on the nearer slope (circled). The bulge lies on the flanks of Casita, a dormant 1,400-m-high volcano in Nicaragua, some 100 km northwest of the capital, Managua.

Writing in Geology (29, 167–170; 2000), Benjamin van Wyk de Vries and colleagues describe their survey of Casita and experimental simulations of how such bulges grow and collapse. The cause of the deformation is hydrothermal activity which, over the course of decades or centuries, can weaken the core edifice of a dormant volcano by turning solid rock into much weaker clays. The resulting structures on the flanks are unstable, and earthquake activity or heavy rainfall can trigger an avalanche. Indeed, a secondary consequence of bulge formation on Casita was a comparatively small-scale landslide in 1998, which was triggered by Hurricane Mitch and caused widespread devastation.

The experiments of van Wyk de Vries et al. involved building 10-cm-high cones of sand and plaster to simulate solid rock, with silicone to simulate hydrothermally altered, deformable clays. If the silicone was offset from the central axis, the result was ‘slump-like’ structures on one side, and eventual collapse of those structures. The process of bulge formation took about 15 minutes. The authors estimate that the bulge on Casita has taken at least 500 years to develop; taking scaling into account, the experimental result is in broad agreement with that figure.

van Wyk de Vries et al. point out that monitoring of dormant volcanoes does not have high priority. But identifying and keeping tabs on unstable structures such as that on Casita would pay dividends in hazard assessment, especially when large populations are potentially under threat.