California is host to all every type of volcano. The specific hazards to people and property depend on which volcano erupts, the style (effusive or explosive), the volume of lava, the location of the vent, the eruption duration, and local hydrologic (water) conditions. The severity of the hazard generally decreases with distance from the volcano vent.
Threat rankings issued by the U.S. Geological Survey (USGS) for California identify fifteen volcanic centers of Low-, Moderate-, High-, or Very High Threat. The USGS California Volcano Observatory (CalVO) is responsible for assessing the potential hazards associated with these "watch list" volcanoes and providing civil authorities, as well as the public, with timely and accurate information to mitigate, plan, and respond effectively to volcanic activity.
At least seven California volcanoes—Medicine Lake Volcano, Mount Shasta, Lassen Volcanic Center, Clear Lake Volcanic Field, Long Valley Volcanic Region, Coso Volcanic Field, and Salton Buttes - have partially molten rock (magma) deep within their roots, and research on past eruptions indicates they will erupt again in the future. The types of volcanic hazards that can unfold depend upon the specific volcano and the type of eruption.
Explosive volcanic eruptions blast lava fragments (tephra) and gas into the air with tremendous force from a volcanic vent. The finest particles, called volcanic ash, billow upward, forming an eruption column that can attain stratospheric heights in minutes. Simultaneously, searing volcanic gas laden with ash and coarse chunks of lava may sweep down the flanks of the volcano as a pyroclastic flow, and ballistics, chunks of solid rock or partially molten lava, may come crashing down around the vent. Ash in the eruption cloud, carried by the prevailing winds, may remain suspended for hundreds of miles before settling to the ground as ash fall.
During less energetic effusive eruptions, hot, fluid lava may issue from the volcano as lava flows that can cover many miles in a single day. Alternatively, a sluggish plug of cooler, partially solidified lava may push up through a crack during an effusive eruption, creating a lava dome. A growing lava dome may become so steep that it collapses, explosively releasing pyroclastic flows potentially as hazardous as those produced during explosive eruptions.
During and after an explosive or effusive eruption volcanic hazards are still a concern. Unstable ground, noxious gas emissions, and intense heat from fumaroles (steam vents) and hot springs are dangerous. Loose volcanic debris on the flanks of the volcano can be mobilized by heavy rainfall or melting snow and ice, forming powerful floods of mud and rock (lahars) resembling rivers of wet concrete. These can rush down valleys and stream channels, destroying roads and bridges and carrying away entire buildings.
Geologists produce volcanic hazard zone maps and assessments to show the types and locations of hazards that may occur during a future eruption. The USGS has published such maps for some, but not all, of California's Moderate to Very High Threat volcanoes. It is important to remember that hazard zone maps are dynamic—as geologic research progresses, existing maps are updated with additional detail and new maps are created.