Volcanoes are not randomly distributed over the Earth’s surface. Most are concentrated on the edges of continents, along island chains, or beneath the sea where they form long mountain ranges. More than half of the world’s active volcanoes above sea level encircle the Pacifi c Ocean (see Fig. 1). The concept of plate tectonics explains the locations of volcanoes and their relationship to other large-scale geologic features. The Earth’s surface is made up of a patchwork of about a dozen large plates and a number of smaller ones that move relative to one another at <1 cm to ~10 cm/yr (about the speed at which fingernails grow). These rigid plates, with average thickness of ~80 km, are separating, sliding past each other, or colliding on top of the Earth’s hot, viscous interior. Volcanoes tend to form where plates collide or spread apart (Fig. 2) but can also grow in the middle of a plate, like the Hawaiian volcanoes (Fig. 3). Volcanic events are probabilistic, that is, the time between eruptions, flank collapses, debris flows, or floods can be shorter or longer than the average, and the time intervals can be described by probability distributions (Nathenson, 2001). The simplest approach is to divide the number of known events by the total time encompassed to arrive at an average recurrence interval, but this approach has significant drawbacks. Some volcanoes are known to undergo episodic behavior, where multiple events occur closely spaced in time, and then no events occur for a time interval much longer than the average. Clustering of events is probably typical of all volcanoes to some degree, and underlies the intuition that it is more dangerous to approach a volcano that erupted recently, than it is to approach a volcano which has not erupted for a long time. Furthermore, the likelihood of an event is coupled in some way with the magnitude of the event, with smaller eruptions, debris flows, landslides, or floods taking place more frequently than larger ones. Global data illustrate that long periods of quiet commonly precede the more explosive and dangerous eruptions. For example, of the 16 largest explosive eruptions of the nineteenth and twentieth centuries, 12 were from volcanoes with no previously known historical eruptions (Simkin and Siebert, 1994). Geologic mapping and related field investigations, coupled with age dating, provide the information necessary for probability estimates that link event style, magnitude, and frequency.