In Cascade Range vicinity, the number of people at immediate risk during eruptions is greater than at any other volcanic area within the United States. The 2010 census notes that more than 10 million people live in Washington and Oregon alone, and populations are increasing in areas at risk for volcanic hazards. Additionally, aviation air space between the Canadian border and Mount Shasta accommodates almost 2,000 flights daily. The next eruption near a Cascade volcano could upset the lives of hundreds of thousands of people and disrupt many others.
When Cascade volcanoes erupt, a common sequence of events unfolds. Explosive eruptions of tephra are followed by effusion of lava flows. High-speed avalanches of hot rock and gas (pyroclastic flows) can accompany both of those events—columns of tephra collapse, or the fronts of lava flows crumble, both of which can send pyroclastic flows down the volcano's flanks. Finally, when the erupted material mixes with river water or melts snow and ice, volcanic mudflows (lahars) sweep down valleys and can devastate areas more than 50 miles downstream. Rivers can continue to carry volcanic sediment downstream and force flooding for decades to hundreds of years. The most significant threats are from volcanic ash and from the slurry of mud and debris within lahars.
Even in the absence of eruption, the flanks of Cascade volcanoes can collapse, which result in landslides and debris avalanches that can destroy areas downslope from the collapse location.
Volcanoes pose multiple types of hazards and the initiation and duration of eruptions is relatively uncertain. Therefore, authorities and populations at risk, both close to and far from the volcano, must be knowledgeable about volcanic hazards so that they can be flexible and prepared in their response.To learn more about general volcano hazards, visit the Volcano Hazards Program website.
Hazard maps illustrate potential for ground-based volcanic impacts—lava flows, hot rocks, volcanic gases, and more far-reaching hazards (primarily lahars) in valleys that drain the volcano. When eruptions are imminent, the USGS and its partners will analyze wind conditions and post maps that indicate areas most likely to receive ash fall. Use the "Find a Volcano" drop-down menu at the top of this website to learn about the volcanoes that could affect your community.