4.1 Types of volcanoes and eruption styles

Volcanoes come in various shapes and sizes, each with its own unique eruption style. Shield volcanoes ooze fluid lava, creating gentle slopes. Composite volcanoes alternate between lava and ash, forming steep cones. Cinder cones are small but mighty, built from ejected material.

Eruption styles range from gentle effusive flows to explosive blasts and steam-driven phreatic eruptions. What determines a volcano's personality? It's all about the magma's composition, gas content, and viscosity. These factors influence whether a volcano will quietly leak lava or violently explode.

Types of Volcanoes

Distinguish between the main types of volcanoes, including shield, composite, and cinder cones, based on their characteristics and eruptive styles

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• Shield volcanoes have broad, gently sloping flanks formed by fluid, low-viscosity basaltic lava
  • Produce effusive eruptions with low explosivity (Mauna Loa and Kilauea in Hawaii)
• Composite volcanoes (stratovolcanoes) have steep, conical shapes formed by alternating layers of lava flows, volcanic ash, and pyroclastic material
  • Created by intermediate to felsic magma with higher viscosity
  • Generate explosive eruptions with occasional lava flows (Mount St. Helens, Mount Fuji, and Mount Vesuvius)
• Cinder cones are small, steep-sided conical hills built from accumulation of ejected tephra and pyroclastic material
  • Formed by mafic to intermediate magma through Strombolian eruptions with moderate explosivity (Parícutin in Mexico and Sunset Crater in Arizona)

Describe the various eruption styles, such as effusive, explosive, and phreatic, and their associated volcanic products

• Effusive eruptions are characterized by the outpouring of fluid, low-viscosity lava associated with basaltic magma
  • Produce volcanic products such as lava flows, lava tubes, and lava plateaus
• Explosive eruptions involve the violent fragmentation and ejection of magma and rock, typically associated with intermediate to felsic magma
  • Generate volcanic products including pyroclastic material (ash, lapilli, and bombs), pyroclastic flows, and lahars
• Phreatic eruptions occur when water interacts with magma or hot rocks, without the eruption of fresh magma
  • Produce steam, ash, and rock fragments as volcanic products

Factors Influencing Eruption Styles

Explain the factors that influence the type of volcanic eruption, including magma composition, gas content, and magma viscosity

• Magma composition determines eruption style and viscosity:
  1. Mafic magma (basaltic) has low silica content, low viscosity, and produces effusive eruptions
  2. Felsic magma (rhyolitic) has high silica content, high viscosity, and generates explosive eruptions
  3. Intermediate magma (andesitic) has moderate silica content and viscosity, resulting in a mix of effusive and explosive eruptions
• Gas content influences explosivity:
  • Higher gas content leads to more explosive eruptions
  • Lower gas content results in less explosive or effusive eruptions
• Magma viscosity affects gas escape and eruption style:
  • Low viscosity magma allows gases to escape easily, leading to effusive eruptions
  • High viscosity magma traps gases, causing pressure buildup and explosive eruptions

Distribution of volcanoes worldwide

• Divergent plate boundaries (mid-ocean ridges) are characterized by basaltic magma and shield volcanoes (Iceland and the East Pacific Rise)
• Convergent plate boundaries (subduction zones) feature andesitic to rhyolitic magma and composite volcanoes (the Pacific Ring of Fire, including the Andes and the Cascades)
• Intraplate hotspots produce basaltic magma and shield volcanoes (Hawaiian Islands and Yellowstone)