Introduction

Volcanic activity involves movements of fluids, magma, and vapor, inside the volcano and its feeding systems. These movements will cause external signals, eruption precursors, which may alert us to the proximity of a volcanic event. Such precursory signals can be detected by an adequate volcano monitoring program. Monitoring techniques include a range of geophysical and geochemical techniques, encompassing seismic, ground-deformation, gravity, and magnetic observations, gas monitoring, and remote sensing. Successful forecasting of volcanic events depends on the precision of the surveillance network in detecting any changes in the volcano's current behavior. To interpret the geochemical and geophysical precursors correctly, however, it is also important to understand the physics of the volcanic processes involved in volcanic eruptions. Detailed knowledge of the volcano, its internal structure and style, and potential triggering mechanisms of past eruptions must be combined with adequate monitoring if future volcanic eruptions are to be anticipated and their effects mitigated (Fig. 3.1). In summary, prediction of volcanic activity has the aims of determining when and where a future eruption will occur, and how it will proceed. However, it is also important to understand why the next eruption will occur.

Volcanic eruptions are caused mainly by processes occurring in magma chambers at depth. During the lifetime of the volcano the corresponding magma chamber may change in size and shape.