Model Answer

Global Distribution of Volcanoes

Volcanoes are primarily found in areas where tectonic plates interact. These regions are:

1. Convergent Boundaries: Volcanoes are common at convergent plate boundaries, where two plates collide. Oceanic plates often subduct beneath continental plates, creating explosive volcanoes at subduction zones.
   • Example: The Pacific Ring of Fire, where numerous volcanic arcs exist.
2. Divergent Boundaries: At divergent boundaries, where tectonic plates move away from each other, magma rises from the mantle to fill the gap, forming volcanoes.
   • Example: Mid-ocean ridges, where submarine volcanic activity is common.
3. Hotspots: These are areas where hot mantle plumes rise through the Earth’s crust, creating volcanic activity independent of plate boundaries.
   • Example: The Hawaiian Islands, formed as the Pacific Plate moves over a hotspot.

Factors Contributing to Explosiveness of Volcanoes

1. Magma Viscosity: More viscous magma, such as rhyolitic magma, leads to explosive eruptions because it traps gases, increasing internal pressure.
   • Example: Mount St. Helens, USA, has explosive eruptions due to its thick magma.
2. Gas Content: High concentrations of dissolved gases (water vapor, CO2, sulfur dioxide) in magma increase the pressure as the magma rises, making the eruption more violent.
   • Example: Mount Vesuvius, known for its catastrophic eruptions.
3. Ascent Rate of Magma: Rapid ascent leads to quicker decompression, causing violent eruptions.
   • Example: The 2015 Calbuco eruption in Chile.
4. Depth of Magma Chamber: Shallow magma chambers can result in more explosive eruptions due to greater pressure from overlying rocks.
   • Example: The explosive eruption of Mount Vesuvius in 79 AD.
5. Volcano Type: Stratovolcanoes (composite volcanoes) are more explosive due to alternating layers of lava and ash, while shield volcanoes have less explosive eruptions.
   • Example: Mount Fuji (stratovolcano) vs. Mauna Loa (shield volcano).
6. Magma Temperature: Higher magma temperatures lower viscosity, allowing gases to escape more easily and reducing explosiveness.
   • Example: Hawaiian volcanoes with basaltic magma.
7. External Water: Interaction of magma with water or ice can cause phreatic explosions due to the rapid conversion of water into steam.
   • Example: The 1980 eruption of Mount St. Helens.

Conclusion

The explosiveness of volcanoes depends on various factors, including the viscosity of magma, gas content, ascent rate, magma chamber depth, volcano type, and external environmental conditions.