Volcanic Activity: Understanding Eruptions and Their Wide-Reaching Hazards

Volcanic eruptions represent one of the most geologically powerful processes on Earth, capable of reshaping landscapes, disrupting global climate, and affecting communities hundreds to thousands of miles from the eruption itself. The United States is home to numerous active volcanoes, making volcanic hazard awareness critically important for millions of Americans.

Volcanic Hazards: More Than Just Lava

Volcanic eruptions produce a wide array of hazards, each with distinct characteristics and geographic reach:

• Lava flows: Streams of molten rock that destroy everything in their path but typically move slowly enough (in most cases) for evacuation. Hawaiian lava flows, for example, rarely directly cause fatalities.
• Pyroclastic flows: Dense, superheated mixtures of volcanic gas, ash, and rock fragments that move at speeds of 100-700 mph. Pyroclastic flows are the most deadly direct volcanic hazard and can travel tens of kilometers from their source.
• Tephra fall (ash): Fine volcanic ash can blanket areas hundreds of miles downwind, collapsing roofs under its weight, contaminating water supplies, abrade aircraft engines, and cause respiratory illness. The 1980 eruption of Mount St. Helens deposited ash across 11 states.
• Lahars (volcanic mudflows): Mixtures of volcanic material and water that flow rapidly down river valleys, burying communities with concrete-like material. Lahars can occur during eruptions or long after when volcanic deposits are remobilized by rainfall.
• Volcanic gases: Sulfur dioxide, carbon dioxide, and hydrogen sulfide can be fatal at high concentrations and can cause acid rain across broad regions.

Active Volcanoes in the United States

The United States is home to approximately 169 potentially active volcanoes, most of them in Alaska, Hawaii, Washington, Oregon, and California. The USGS Volcano Hazards Program monitors these volcanoes using seismic networks, GPS deformation monitoring, gas measurements, and satellite remote sensing, issuing Aviation Color Codes and Volcano Alert Levels to communicate current activity and hazard status.

Preparedness for Volcanic Hazards

• Know your local volcano hazard: USGS volcano hazard assessments and eruption scenario planning documents are publicly available for all major U.S. volcanoes and describe the likely extent of various hazard zones.
• For ash fall: Protect respiratory health with N95 or P100 respirators, stay indoors with windows and doors sealed, protect water storage, and clear ash from roofs if accumulation exceeds 4 inches.
• For evacuation: Follow official evacuation orders without delay. Lahars and pyroclastic flows can move faster than a vehicle on a congested evacuation route.