Volcanic eruptions pose unique and localized dangers, making the question of universal evacuation complex. Not all individuals near a volcano must evacuate, as the decision is highly conditional and localized. Evacuation planning depends on the volcano’s current activity, its geological structure, and the proximity of human settlements to specific hazard paths. The determination of who leaves and who stays is based on a rigorous scientific assessment that translates into specific governmental mandates.
The Authority Behind Evacuation Decisions
The decision to order an evacuation involves both scientific monitoring and civil authority. Geological survey organizations, such as the United States Geological Survey (USGS), monitor unrest indicators like ground deformation, seismic activity, and gas emissions. These scientific bodies communicate the volcano’s status using standardized Volcano Alert Levels and Aviation Color Codes.
The ground-based alert levels range from “Normal” (Green) for typical background activity, to “Advisory” (Yellow) for elevated unrest, and finally to “Warning” (Red) when a highly hazardous eruption is imminent or occurring. The Aviation Color Code is a parallel system addressing the threat of volcanic ash to aircraft. These alert levels serve as a public warning system, but they are distinct from a formal evacuation order.
The authority to issue a mandatory evacuation order rests solely with local, state, or federal emergency management agencies. These civil defense bodies use the scientific data to determine which specific areas are at risk. A formal evacuation order is a legal mandate issued only when an imminent threat to life and safety is identified.
Risk Zoning and Hazard Specificity
Evacuation orders are geographically limited because volcanic risk is not uniform across the landscape. Scientists create detailed hazard maps that differentiate between primary and secondary danger zones based on the volcano’s history and topography. Primary hazard zones require immediate and mandatory evacuation, as the threat comes from ballistic projectiles and fast-moving, high-temperature flows near the vent.
These immediate threats include pyroclastic flows, which are mixtures of hot gas and rock fragments traveling at high speeds down the flanks of stratovolcanoes. Lava flows pose a different risk; low-viscosity basaltic lava, common in shield volcanoes, can move quickly down steep slopes. The size and shape of the evacuation zone are determined by the particular hazards expected from the volcano type.
Secondary hazard zones extend farther from the vent and are often subject to voluntary or conditional evacuation orders. These zones face threats like lahars, which are destructive mudflows racing down river valleys, or heavy ashfall, which can contaminate water supplies and cause respiratory issues. The distinction between mandatory and voluntary zones allows officials to focus resources and minimize disruption.
Actionable Steps During Alert Levels
Public safety requires preparedness and timely action once an alert level is raised or an evacuation order is given. Individuals living within designated hazard zones should maintain a “Go-Bag” containing essential medications, critical documents, and supplies sufficient for at least 72 hours. Family communication plans, including predetermined meeting places outside the risk zone, should be established in advance.
If a mandatory evacuation order is issued, leave the area promptly and follow the official, designated evacuation routes. If time permits, secure homes by shutting off gas, electricity, and water to prevent secondary hazards like fires or flooding. Pets should be taken along, as shelters often cannot accommodate animals.
Managing Ashfall
For those advised to shelter-in-place outside the mandatory zone, the main concern is volcanic ashfall. To manage ash, close and seal all windows and exterior doors, and turn off HVAC systems and fans to prevent drawing ash indoors. If outside, wearing a certified face mask, such as an N95, and protecting the eyes with goggles can prevent irritation. Avoid driving in heavy ashfall, as the ash can stall vehicles and severely reduce visibility.