D.1.2 Volcanic Characteristics and Hazards Notes

Types of Volcanoes and Associated Risks

1. Volcanoes are fascinating yet formidable natural features, each with unique characteristics and hazards.
2. Understanding these differences is essential for assessing the risks they pose to human life and the environment.
3. Volcanic activity is located along tectonic plate boundaries (near subduction zones, along oceanic ridges, in proximity of rift valleys) and in hotspots (intraplate volcanoes).

Shield Volcanoes: Broad and Gentle Giants

1. Shield volcanoes are characterized by their broad, gently sloping profiles, resembling a warrior's shield lying on the ground.
2. They form from low-viscosity basaltic lava that can flow over long distances before solidifying.

Key Characteristics

1. Low Viscosity Lava: The lava is hot and runny, allowing it to travel far from the vent.
2. Gentle Slopes: Slopes are typically less than 10°, creating a wide base.
3. Non-Explosive Eruptions: Eruptions are usually less violent due to the easy escape of gases.

Associated Hazards

1. Lava Flows: Although slow-moving, lava can destroy infrastructure and agricultural land.
2. Gas Emissions: Gases like sulfur dioxide can impact air quality and human health.

Composite Volcanoes (Stratovolcanoes): Steep and Explosive

1. Composite volcanoes, also known as stratovolcanoes, are steep-sided,characterized by andesitic lava of viscous nature (high degree of friction), and formed by alternating layers of lava and pyroclastic material (ash, rocks, and volcanic bombs).

Example

Mount St. Helens in the USA and Mount Fuji in Japan are well-known examples as stratovolcanoes are associated with destructive (convergent) plate margins.

Key Characteristics

1. High Viscosity Lava: The lava is thicker and cooler, leading to steeper slopes.
2. Steep Slopes: Slopes can reach up to 30° near the summit.
3. Explosive Eruptions: Trapped gases in viscous magma lead to violent eruptions.

Associated Hazards

1. Pyroclastic Flows: Fast-moving, hot clouds of gas and volcanic material that can devastate everything in their path.
2. Ashfall: Ash can blanket large areas, disrupting transportation and damaging crops.
3. Lahars: Volcanic mudflows triggered by rain mixing with ash deposits.

Cinder Cones: Small but Steep

1. Cinder cones are the smallest type of volcano, formed by explosive eruptions that eject pyroclastic material, which accumulates around the vent.

Key Characteristics

1. Steep Slopes: Slopes range from 30° to 40°, depending on the size of the material.
2. Small Size: Typically less than 300 meters high.
3. Short-Lived Eruptions: Eruptions are usually brief but intense.

Associated Hazards

1. Lava Bombs: Large fragments of molten rock ejected during eruptions.
2. Localised Ashfall: Ash deposits are usually confined to the immediate area.

Volcanic Hazards: Primary and Secondary

1. Volcanic hazards are classified into primary and secondary hazards, each with unique impacts.

Primary Hazards: Directly from the Eruption

Lava Flows

1. Characteristics: Molten rock that flows down the volcanos slopes.
2. Impact: Destroys everything in its path but is usually slow enough for people to evacuate.

Ashfall

1. Characteristics: Fine particles of volcanic ash ejected into the atmosphere.
2. Impact: Disrupts air travel, damages crops, and poses respiratory health risks.

Example

The 2010 eruption of Eyjafjallajökull in Iceland grounded flights across Europe for several days.

Gas Emissions

1. Characteristics: Volcanic gases like sulfur dioxide, carbon dioxide, and hydrogen sulfide.
2. Impact: Can cause acid rain, air pollution, and even suffocation in high concentrations.

Secondary Hazards: Indirect Consequences

Pyroclastic Flows

1. Characteristics: Fast-moving clouds of hot gas and volcanic material.
2. Impact: Can travel at speeds of up to 700 km/h, destroying everything in their path.

Example

The 1980 eruption of Mount St. Helens produced pyroclastic flows that flattened forests and killed 57 people.

Lahars

1. Characteristics: Volcanic mudflows formed by mixing ash with water.
2. Impact: Can bury entire communities under meters of mud and debris.

Landslides

1. Volcanic eruptions can destabilize slopes in several ways, leading to landslides.
2. Explosive eruptions, the rapid release of gases, or the intrusion of magma can weaken the surrounding rock.
3. Additionally, volcanic activity often melts snow and ice, saturating the ground and further reducing slope stability.
4. Key factors contributing to volcanic landslides include:

• Structural Weakening: The buildup of magma and gases within a volcano creates internal pressure, fracturing the rock and making it more prone to collapse.
• Explosive Eruptions: The force of an eruption can dislodge large sections of the volcano, sending debris tumbling down the slope.
• Water Saturation: Melting snow, ice, or heavy rainfall associated with volcanic activity can saturate the ground, reducing friction and triggering landslides.

1. Characteristics: Large-scale slope failures triggered by volcanic activity or earthquakes.
2. Impact: Can cause significant damage to infrastructure and trigger tsunamis if they enter bodies of water.

Why Understanding Volcanoes Matters

1. Risk Mitigation: Identifying the type of volcano and its associated hazards helps in developing effective evacuation plans and early warning systems.
2. Land Use Planning: Understanding volcanic hazards informs decisions about where to build infrastructure and how to protect vulnerable communities.
3. Global Impact: Volcanic eruptions can have far-reaching effects, such as disrupting air travel or altering global climate patterns.