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Chapter Four

Chapter Four. Volcanism and Extrusive Rocks. VOLCANOES AND VOLCANISMS. VOLCANO: LANDFORM THAT RESULTS FROM THE ACCUMULATION OF LAVA AND ROCK PARTICLES AROUND AN OPENING (OR VENT) IN EARTH’S SURFACE VOLCANISM: SET OF GEOLOGICAL PROCESSES THAT MARK THE ASCENT OF MAGMA TO THE EARTH’S SURFACE

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Chapter Four

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  1. Chapter Four Volcanism and Extrusive Rocks

  2. VOLCANOES AND VOLCANISMS VOLCANO:LANDFORM THAT RESULTS FROM THE ACCUMULATION OF LAVA AND ROCK PARTICLES AROUND AN OPENING (OR VENT) IN EARTH’S SURFACE VOLCANISM:SET OF GEOLOGICAL PROCESSES THAT MARK THE ASCENT OF MAGMA TO THE EARTH’S SURFACE ~ 2000 YRS ------- 600 VOLCANOES ERUPTED 50 VOLCANOES/YEAR VOLCANOES ARE WINDOWS INTO THE EARTH WATER AND AIR ----- CAME DUE TO VOLCANIC ERUPTIONS HYDROSPHERE AND ATMOSPHERE

  3. Volcanism and Earth’s Systems • Atmosphere originally created from gases released by magmas • Hydrosphere produced by condensation of volcanic water vapor • Biosphere both positively and negatively influenced by volcanism • Lava flows and ash weather to produce fertile soils • Violent eruptions can destroy nearly all life in their paths • Large amounts of ash and volcanic gases in atmosphere can trigger rapid climate changes and contribute to mass extinctions

  4. Volcanic Eruptions • Lava is produced when magma reaches Earth’s surface • Explosive eruptions can produce rapidly cooled rock fragments called pyroclasts • Size range from dust (ash) to boulders (blocks and volcanic bombs) • Calm oozing of magma out of the ground produces lava flows • Lava flows and pyroclasts pile up to form volcanoes

  5. Classification of volcanoes VOLCANOES: ACTIVE EXTINCT ACTIVE CURRENTLY NOT ERUPTED FOR OR 10,000 YRS OR SO ERUPTED RECENTLY e.g., DORMANT VOLCANO: NOT ERUPTED RECENTLY (~1000 YRS OR SO) LIKELY WILL ERUPT IN THE FUTURE VOLCANOES PRESENT IN ALL CONTINENTS EXCEPT AUSTRALIA ALL MAJOR OCEAN BASINS INDONESIA HAS 76 ACTIVE VOLCANOES JAPAN HAS 60 & USA HAS 53 ACTIVE VOLCANOES

  6. Lava tube formation

  7. Causes for Volcanism • GAS IN VOLCANIC MAGMA (1-9% IN MOST CASES) • CAUSES FOR VOLCANISM: • COMPONENT GASES (WATER VAPOR, CO2) • RISING MAGMA RELEASES GASES • EXPLOSIVE PRESSURE BY GAS • MAGMA VELOCITY: • -RESISTANCE TO FLOW • -HEAT REDUCES VISCOSITY • -VISCOSITY INCREASES WITH SILICA CONTENT • -EXPLOSIVENESS INCREASES WITH VISCOSITY

  8. Types of Lava • Basaltic Lava- low viscosity and non-explosive • Pahoehoe (pay-HOY-hoy) and a a (AH-AH) • Andesitic Lava – more viscous than basaltic and erupts • Rhyolitic lava- highly viscous and erupts explosively • Pyroclastic materials • Tephra- volcanic dust, ash, cinders (lapilli) and volcanic bombs • Welded tuff (fussed tephra particles) • Pyroclastic flows or Nuees ardents (noo-Ay AR-dant) • Lahars (hot volcanic mudflows) • Water/air pollution, global climate change .

  9. Basaltic lava flow

  10. Basaltic pillow lavas

  11. Basaltic pillow lavas-contd.

  12. Basaltic pillow lavas-contd.

  13. Tephra Layers

  14. Pyroclastic Flows

  15. Lahars

  16. Gas emissions

  17. Extrusive Textures • Vesicular/Frothy - trapped gas bubbles • Vesicular basalt • Pumice • Fragmental - particles blasted apart by explosive eruptions • Dust and ash (<2 mm) • Cinders (2-64 mm) • Blocks and bombs (>64 mm)

  18. Volcanic Landforms • Vent - opening through which lava erupts • Crater - basin-like depression over the vent at the summit of the volcano • Caldera - volcanic depression much larger than the original crater, having a diameter of at least 1 km

  19. Volcanic Landforms Determined by the composition of Lava • Shield volcanoes…gentle slope, effusive eruption- non-explosive • Crater and Caldera • Volcanic Domes • Produce great amount of solid volcanic fragments • Composite (Stratovolcanoe) • Alternating layers of pyroclast and solid lava • Pyroclastic Cones (Cinder Cones) • Loose pyroclastic materials around a vent

  20. Types of Volcanoes • Shield volcanoes • Broad • Gently sloping • Composed of solidified lava flows • Cinder cones • Small • Steep-sided • Composed of a pile of loose cinders

  21. Lava-plateau formation

  22. The Gorda ridge

  23. Submarine volcano, infiltration

  24. Submarine volcano, eruption

  25. Volcanic domes, gas escape

  26. Volcanic domes, explosion

  27. Mount Mazama

  28. Mount Mazama-contd

  29. Mount Mazama-contd.

  30. Mount Mazama-contd.

  31. Mount Mazama-contd.

  32. PYROCLASTIC MATERIAL • TEPHRA: • CLASSIFIED BY SIZE • SMALL: VOLCANIC DUST & ASH- 1 MICRON • MEDIUM: CINDERS OR LAPILLI (>1-62 MICRON) • LARGE: VOLCANIC BOMBS (>64 MICRON) • RECORDERS IN LAKES • VOLCANIC MUDFLOWS-LAHAR • SECONDARY VOLCANIC EFFECTS: • ACIDIC PRECIPITATION • GLOBAL CLIMATIC CHANGES- • REDUCED SUNLIGHT • REDUCED TEMPERATURE

  33. PRODUCTS OF VOLCANISM • OCCURRENCE: • MAFIC LAVA (BASALT)-MOST COMMON • ANDESITIC LAVA – INTERMEDIATE • FELSIC LAVA – LEAST COMMON

  34. ERUPTIVE STYLES • ERUPTIVE STYLES: • EFFUSIVE ERUPTION: NONEXPLOSIVE, BASALTIC • CENTRAL VENT ERUPTIONS • PRODUCE SHIELD VOLCANOES • PRODUCE CALDERAS AND FLANK ERUPTIONS • FISSURE ERUPTIONS ON LAND PRODUCE LAVA PLATEAUS OR FLOOD BASALTS • SUBAQUEOUS ERUPTIONS (MAINLY OCEAN) • SUBGLACIAL ERUPTIONS (MAINLY UNDER GLACIERS)

  35. Volcanic Hazards • Life threatening situation- explosive ejection of pyroclastic materials • Water and air pollution • Forest fire • Global climate change • Mitigation • Avoid area… zoning • Structural/strategic defense- lava wall, warning system, evacuation plan, measuring and predicting

  36. Ash flow eruption

  37. . Extrusive Rocks and Gases • Scientific Investigation of Volcanism • Rocks, gases and events from observed eruptions compared to similar lavas elsewhere to infer past activity • Rock Composition • Rhyolite - high silica; light color • Basalt - low silica; dark color • Andesite - intermediate silica and color

  38. Chapter Summary - 4 Benefits of Volcanic Eruption/Environmental Effects Active, Dormant, and Extinct Volcano Pyroclastic Volcanoes Definition of Viscosity Relationship between Viscosity, Temperature, Si content in magma Basaltic, Andesitic, Rhyolitic Lava – Most volcanic eruptions occur in what type of lava Classification of Tephra Definition of Lahar Contents of Volcanoes Volcanoes, Pyroclastic occurrence Effective means of dealing with volcanic hazards

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