Overview
This lecture examines major volcanic hazards, their origins, and their impacts, using examples from composite and shield volcanoes, particularly in the Cascade Range.
Types of Volcanic Hazards
- Volcanic hazards threaten life, land, and infrastructure near volcanoes.
- Main hazards include tephra, lahars, pyroclastic flows, and lava flows.
- Most fatalities result from mudflows (lahars) or toxic gas clouds, not direct lava.
Composite vs. Shield Volcanoes
- Composite volcanoes form along convergent plate boundaries, such as the Cascade Range.
- Shield volcanoes, like those in Hawaii, produce abundant, fluid lava.
Cascade Range & Volcanic Activity
- Portland, Seattle, and Vancouver are near the active Cascade volcano chain.
- Cascade volcanoes have erupted roughly 1–2 times per century in the last 4000 years.
- The North American plate overrides the Juan de Fuca plate, forming these volcanoes.
Eruption Products & Hazards
- Tephra: Rock fragments, ash, and lava bombs ejected into the atmosphere; can travel long distances and cause transportation hazards.
- Ash clouds can disrupt air travel and cause engine failure in vehicles and aircraft.
- Tephra deposits affect economic activity and can be used to analyze past eruptions.
Lahars (Volcanic Mudflows)
- Lahars are mixtures of tephra and water from rain or melting ice, forming fast-moving cement-like rivers.
- They can carry large debris and flood valleys, destroying infrastructure and causing fatalities miles from the volcano.
- Historical examples include catastrophic lahars in Colombia and deposits near Mount Rainier.
Pyroclastic Flows
- Dense, hot clouds of toxic gases and tephra move rapidly down volcano slopes, destroying everything in their path.
- The 1902 disaster in Martinique killed nearly all of Saint-Pierre’s residents.
- Pyroclastic flow deposits can be thick and contain pumice and other volcanic materials.
Lava Flows
- Composite volcanoes produce viscous lava that forms domes and does not travel far.
- Shield volcanoes generate fluid lava that flows long distances, creating broad landforms.
- Lava flows usually move slowly but can destroy property and infrastructure.
Key Terms & Definitions
- Volcanic hazard — a volcanic process threatening life or property.
- Composite volcano — a volcano with steep sides, built from layers of lava and tephra.
- Shield volcano — a broad, gently sloping volcano formed by fluid lava flows.
- Tephra — rock fragments and particles ejected by a volcanic eruption.
- Lahar — a destructive mudflow of volcanic debris and water.
- Pyroclastic flow — a fast-moving, hot mixture of gas and volcanic materials.
Action Items / Next Steps
- Review the characteristics and impacts of each volcanic hazard discussed.
- Be prepared to explain which hazards are most deadly and why.
- Study local volcano risk maps for residents near active volcano regions.