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Understanding Volcanism: Eruptions, Rocks, and Living with Volcanoes

Explore the impact of volcanic eruptions on Earth's systems, the formation of extrusive rocks, and the relationship between mythology and volcanoes. Learn about volcanic landforms, eruption types, and scientific investigations of volcanism.

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Understanding Volcanism: Eruptions, Rocks, and Living with Volcanoes

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  1. Volcanism and Extrusive RocksPhysical Geology 13/e, Chapter 4 Tim Horner CSUS Geology Department

  2. 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

  3. 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 • Pyroclastics and lava flows form extrusive igneous rocks • Lava flows and pyroclasts pile up to form volcanoes

  4. Living with Volcanoes • Mythology, religion and volcanoes • Hawaii - Pele • Iceland - Loki • Growth of volcanic islands (Hawaii) • Geothermal energy • Natural steam harnessed as clean • energy resource • Climatic effects • Very large eruptions can result in measurable global cooling • Resulting crop failures and famines • Volcanic catastrophes • Mt. St. Helens, Pompeii, Krakatoa, • Tambora, Crater Lake

  5. Crater Lake, Oregon

  6. Eruptive Violence and Physical Characteristics of Lava • Violence of eruptions controlled by: • Amount ofDissolved gases in the magma • Water vapor, carbon dioxide, sulfur dioxide, etc. • The more dissolved gases, the more fluid the lava • Ease/difficulty of gases escaping to atmosphere • Viscosity- a fluid’s resistance to flow • Silica content • Higher silica contents produce higher viscosities • Lavatemperature • Cooler lavas have higher viscosities

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

  8. Extrusive Textures • Texture - appearance of a rock with respect to size, shape and arrangement of its grains • Texture terms: • Glassy - glass without mineral crystals • ex: Obsidian • Fine-grained - most crystals < 1 mm • ex: Basalt • Porphyritic - larger crystals in a matrix of much finer crystals or glass • ex: Andesite

  9. 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)

  10. 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

  11. Types of Volcanoes • Shield volcanoes • Broad • Gently sloping • Composed of solidified lava flows • Flows often contain lava tubes • Cinder cones • Small • Steeply sloping • Composed of a pile of loose cinders

  12. Types of Volcanoes • Composite volcanoes • Also called stratovolcanoes • Moderately to steeply sloping • Constructed of alternating layers of pyroclastic debris and solidified lava flows • Composed primarily of intermediate composition volcanic rocks (i.e., andesite) • Most common type of volcano at convergent plate boundaries (e.g., Pacific Ring of Fire) ex: Cascade volcanos • Volcanic domes • Extremely high viscosity, degassed, felsic lavas (often glassy, e.g., obsidian)

  13. Other Eruption Types • Flood eruptions • Very fluid (basalts) • Extremely large in volume • Create extensive lava plateaus • Eruption times correspond with largest mass extinction events • Submarine eruptions • Nearly always basaltic • Mid-ocean ridge eruptions • Pillow basalts Columbia river flood basalts Pillow basalts

  14. Volcanism on Other Planets • Shield volcanoes • Venus, Mars, Io • Lava domes • Venus, Moon • Flood eruptions • Very fluid (basalts) • Extremely large in volume • Extensive flat lava plains (Moon, Mars, Venus, Mercury?) Insert Fig. 22.18 here Insert Fig. 22.27A here Olympus Mons on Mars Volcanic eruption on Io Insert Fig. 22.14A here Lava domes on Venus Lava channel in volcanic plains on Moon

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