Pyroclastic Flows: These are superheated, high-velocity clouds of ash, gas, and rock fragments. They move at speeds exceeding 100 km/h and are arguably the most lethal primary hazard due to their heat and speed.
Lahars: Volcanic mudflows created when eruptive material mixes with water from heavy rain or melting ice. They can travel long distances down river valleys, often occurring long after the initial eruption has ceased.
Tephra and Ash Fallout: Tephra refers to all solid material ejected into the air. While large 'bombs' fall near the vent, fine ash can be carried thousands of kilometers, disrupting aviation and causing respiratory issues.
Seismic Activity: Monitoring earthquakes caused by the movement of magma through the crust. An increase in frequency or intensity often serves as a primary indicator of an impending eruption.
Ground Deformation: The use of tiltmeters and GPS to detect the swelling or bulging of the volcano's surface. This 'inflation' suggests that a magma chamber is filling and exerting pressure on the overlying rock.
Gas Emissions: Changes in the quantity or composition of gases, such as an increase in sulfur dioxide (), can signal that magma is rising closer to the surface.
| Feature | Pyroclastic Flow | Lahar |
|---|---|---|
| Composition | Gas and rock fragments | Water and volcanic debris |
| Temperature | Extremely high (up to 1000°C) | Ambient to warm |
| Trigger | Column collapse or dome failure | Rainfall, snowmelt, or crater lake breach |
| Classification | Primary Hazard | Secondary Hazard |
Magma vs. Lava: Magma is molten rock located beneath the Earth's surface, while lava is the term used once that molten rock breaks through the crust and flows onto the surface.
Prevention vs. Mitigation: While eruptions cannot be prevented, mitigation involves reducing their impact through engineering (e.g., lava diversion) or land-use zoning.
Identify the Margin: Always link the hazard type to the tectonic setting. Destructive margins usually produce explosive stratovolcanoes, while constructive margins produce effusive shield volcanoes.
Temporal Awareness: When discussing impacts, distinguish between short-term responses (evacuation, aid) and long-term responses (hazard mapping, building codes).
Terminology Precision: Do not use 'smoke' to describe volcanic clouds; use ash or tephra. Unlike wood smoke, volcanic ash is composed of pulverized rock and glass, which is abrasive and heavy.
Check for Cascading Effects: Remember that a primary hazard like an eruption under a glacier will almost always lead to a secondary hazard like a jökulhlaup.
