Significant rock strength reductions caused by hydrothermal alteration, similar to weathering, can produce slope instability. Hydrothermal alteration differs from weathering in one important aspect; it can be vertically continuous to great depth. Economic geologists have long recognized the relationship between alteration and mineralization, but only recently has this relationship been correlated with rock strength and applied to surface and underground excavations. Extrapolating the concepts of engineering slope design to the volcano environment shows potential for improving hazard assessment.

Many stratovolcanoes exhibit significant hydrothermal alteration. Alteration is time-dependent; thus, an initial "stable" volcano undergoes a progressive rock strength reduction, as a result of argillic alteration, which may result in slope instability. The failure mass can form a rock avalanche or lahar, depending on clay or water content. Travel distance for such failures is often measured in tens of kilometers, occasionally in excess of 100 kilometers. Many urban areas and transportation routes are constructed on debris deposits from these failures. The Holocene contains many of these events and the historical record is dotted with failure deposits throughout the world. Major slope instability can be independent of eruptive forces, particularly when the failure mass is small (< 0.1 cubic kilometers), consequently warning signs may not be readily apparent. On-going research considers failure volume and frequency, previous failure deposits, and the strength and location of weak and potentially unstable rock masses. Examples and analyses are presented from volcanoes in the Cascade Range (USA) and the Mexican Volcanic Belt. The types of field investigations performed at elevations up to 5,500 meters, collection of strength data at potential landslide sources, and volcano modeling and hazard appraisal will be discussed.