A geologic map of a volcano delineates the type, location, and age of rock deposits found on the ground around a volcano. Geologists conduct fieldwork to locate and sample units of rocks then preform laboratory analyses to determine the age of deposits. The rock deposits are evidence of the range of hazardous phenomena that a particular volcano is capable of producing. The ages indicate the frequency of volcanic activity and length of pauses (recurrence intervals) between events. Because the past behavior of a volcano is a good guide to its expected future behavior, geologic maps are the foundation of long-term hazard assessments.