Volcanic Hazards
Résumé
The complexity of understanding volcanic risk is partly due to the fact that it is the result of different hazards, some of which are directly linked to the eruptive activity, such as, gas, lava flows, pyroclastic flows and ash fallout, and others which are directly or indirectly induced by these hazards, such as, debris avalanches, tsunamis, mudflows or lahars. A wide range of seismic phenomena is associated with volcanic activity. The main sources of seismic signals are: magma transfers; hydrothermal activity; and volcano-tectonic phenomena. Understanding the hazard associated with volcanic gases means first of all understanding the physico-chemistry of the outgassing process. Volcanic explosions produce and eject tephra of various sizes and gases. Tephra are classified into three main classes: bombs, lapilli and ash. Lahars are part of a continuum of water-rich flows, but sediment concentration, particle size distribution and density help distinguish the following two categories: hyperconcentrated flows and debris flows.