Types of Volcanoes and Associated Risks
- Volcanoes are fascinating yet formidable natural features, each with unique characteristics and hazards.
- Understanding these differences is essential for assessing the risks they pose to human life and the environment.
- 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
- Shield volcanoes are characterized by their broad, gently sloping profiles, resembling a warrior's shield lying on the ground.
- They form from low-viscosity basaltic lava that can flow over long distances before solidifying.
Mauna Loain Hawaii is a classic example, covering an area of over 5,000 square kilometers.
Key Characteristics
- Low Viscosity Lava: The lava is hot and runny, allowing it to travel far from the vent.
- Gentle Slopes: Slopes are typically less than 10°, creating a wide base.
- Non-Explosive Eruptions: Eruptions are usually less violent due to the easy escape of gases.
Shield volcanoes are often found at hotspots (e.g., Hawaii) or along constructive (divergent) plate boundaries (e.g., Iceland).
Associated Hazards
- Lava Flows: Although slow-moving, lava can destroy infrastructure and agricultural land.
- Gas Emissions: Gases like sulfur dioxide can impact air quality and human health.
A common misconception is that shield volcanoes are safe because they are non-explosive. However, their lava flows can still cause significant damage over time.
Composite Volcanoes (Stratovolcanoes): Steep and Explosive
- 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).
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
- High Viscosity Lava: The lava is thicker and cooler, leading to steeper slopes.
- Steep Slopes: Slopes can reach up to 30° near the summit.
- Explosive Eruptions: Trapped gases in viscous magma lead to violent eruptions.
Think of a composite volcano like a layered cake, with alternating layers of lava and ash building up over time.
Associated Hazards
- Pyroclastic Flows: Fast-moving, hot clouds of gas and volcanic material that can devastate everything in their path.
- Ashfall: Ash can blanket large areas, disrupting transportation and damaging crops.
- Lahars: Volcanic mudflows triggered by rain mixing with ash deposits.
Many students confuse pyroclastic flows with lava flows. Remember, pyroclastic flows are much faster and deadlier.
Cinder Cones: Small but Steep
- Cinder cones are the smallest type of volcano, formed by explosive eruptions that eject pyroclastic material, which accumulates around the vent.
Parícutin in Mexico is a famous cinder cone that emerged in a farmer's field in 1943.
Key Characteristics
- Steep Slopes: Slopes range from 30° to 40°, depending on the size of the material.
- Small Size: Typically less than 300 meters high.
- Short-Lived Eruptions: Eruptions are usually brief but intense.
Cinder cones often form on the flanks of larger volcanoes or in volcanic fields.
Associated Hazards
- Lava Bombs: Large fragments of molten rock ejected during eruptions.
- Localised Ashfall: Ash deposits are usually confined to the immediate area.
Cinder cones are less destructive than shield or composite volcanoes but can still pose risks to nearby communities.
