Volcanism Quick Review

Definitions

• Volcano: opening on Earth’s surface releasing lava, ash, and gases
• Volcanism: all phenomena linked to eruption of molten rock through the crust/lithosphere

Parts of a Volcano

• Vent – central opening where magma exits
• Magma chamber – subterranean reservoir
• Fumaroles – gas/vapor vents
• Lava dome – mound of slow, viscous lava
• Pyroclastic flow – hot, fast mix of ash, gas, rock
• Lahar – mud/debris flow on volcanic slope

Philippine Context

• \approx 300 total volcanoes; 24 active
• Most active: Mayon; 3rd: Mt. Kanlaon (eruption 08/04/2025, 56 min)

Classification by Shape

• Shield – gentle slopes, low-viscosity basalt; non-explosive (Mauna Loa)
• Stratovolcano (Composite) – steep cone, viscous lava; explosive (Mayon)
• Cinder Cone – small, common; scoria, dark lava (Mt. Omuro)
• Caldera – large depression from collapse (Yellowstone)
• Lava Dome – steep mound of very viscous magma (Soufrière Hills)

Classification by Eruption Style

• Phreatic – steam-driven; water + ash only
• Phreatomagmatic – magma–water; tall column with ring cloud
• Vulcanian – short, explosive; cauliflower ash clouds
• Strombolian – moderate lava fountains
• Pelean – dome growth & collapse → nuee ardente
• Plinian – extremely explosive; umbrella ash columns
• Icelandic – basaltic fissure flows; lava plateaus
• Hawaiian – non-explosive “fire fountains”; fluid basalt rivers

Classification by Activity

• Active – recent or ongoing eruptions
• Dormant (Potentially Active) – no eruption in <10\,000 yr but expected (≈1350 globally)
• Extinct – no future activity anticipated

Zones of Volcanism

• Convergent: oceanic–oceanic → island arcs; oceanic–continental → volcanic arcs (Pacific Ring of Fire)
• Divergent (Ocean Ridge): decompression melting forms new ocean floor
• Hotspots: mantle plumes within plates (Hawaii chain)

Volcanic Hazards

• Gases: H2O, CO2, SO2, H2S → asphyxiation, respiratory/skin issues
• Pyroclastic materials: ash, lapilli, blocks, bombs
• Lava flows: Pahoehoe (smooth), A’a (rough), Pillow (submarine)
• Lahar: >60\% volcanic debris; dense, deadly
• Ashfall: <2 mm glassy particles; health & infrastructure damage

Benefits of Volcanism

• Fertile soils for agriculture
• Geothermal energy (clean, renewable)
• Mineral & sulfur resources
• Hot springs & tourism
• Atmospheric cooling via SO2 \rightarrow H2SO_4 aerosols
• New landforms & water release through mantle degassing