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GEOL: CHAPTER 5

GEOL: CHAPTER 5. Volcanoes and Volcanism. Learning Outcomes. LO1: Understand volcanism and volcanoes LO2: Identify the types of volcanoes LO3: Identify other volcanic landforms LO4: Identify the distribution of volcanoes. Learning Outcomes, cont.

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GEOL: CHAPTER 5

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  1. GEOL: CHAPTER 5 Volcanoes and Volcanism

  2. Learning Outcomes • LO1: Understand volcanism and volcanoes • LO2: Identify the types of volcanoes • LO3: Identify other volcanic landforms • LO4: Identify the distribution of volcanoes

  3. Learning Outcomes, cont. • LO5: Understand the relationship between plate tectonics, volcanoes, and plutons • LO6: Understand volcanic hazards, volcano monitoring, and forecasting eruptions

  4. Intro to Volcanoes • Created much of Earth's atmosphere and hydrosphere • Create oceanic crust • Create volcanic islands • Profound effects of major eruptions • Tambora, 1815 • Krakatoa, 1883 • Pinatubo, 1991

  5. Intro to Volcanoes, cont. • Volcanism: process by which magma rises and issues onto Earth’s crust • 550 active volcanoes: erupted in historic time • Dormant volcanoes: haven’t erupted in historic time, but could in the future • Extinct/inactive volcanoes • Volcanoes on Io, Triton

  6. Volcanic Gases • 50%-80% water vapor • Carbon dioxide • Nitrogen • Sulfur dioxide and hydrogen sulfide • Fumaroles • Can cause explosive eruptions • Can cause fatalities

  7. Lava Flows • Usually move slowly • Lava tube: • Margins and upper surface solidify • Liquid lava flows rapidly through tube for long distances • Leaves cave with skylights

  8. Lava Flows, cont. • Pahoehoe • Ropy surface like taffy • Less viscous • Aa • Jagged and angular • More viscous

  9. Lava Flows, cont. • Pressure ridges: • Gas escaping from a flow hurls lava globs which form spatter cones • Columnar joints: • Stationary lava flow cools and contracts • Joints open: polygonal columns

  10. Columnar joints in a basalt lava flow at Devil’s Postpile National Monument in California. The rubble in the foreground is collapsed columns. Surface view of the columns from (b). The straight lines and polish resulted from abrasion by a glacier that moved over this surface. As lava cools and contracts, three-pronged cracks form that grow and intersect to form four- to seven-sided columns, most of which are six-sided. Stepped Art Fig. 5-5, p. 89

  11. Lava Flows, cont. • Pillow lava: • Nonexplosive eruptions of mafic lava • Upper part of oceanic crust • Lava chills beneath water

  12. Pyroclastic Materials • Ejected materials • Volcanic ash: <2 mm • Ash fall from atmosphere • Ash flow close to surface • Lapilli: 2 mm to 64 mm • Bombs and blocks: > 64 mm • Bombs: streamlined • Blocks: angular

  13. Volcanoes • Hill or mountain that forms around a vent where lava, pyroclastic materials, and gases erupt • Craters: less than 1 km diameter

  14. Calderas • Huge, from voluminous eruptions • Magma chamber drains • Summit collapses into partially vacated magma chamber • Crater Lake in Oregon

  15. Shield Volcanoes • Low, rounded profiles • Slopes from 2-10 degrees • Composed of mafic, low-viscosity flows • Basalt lava flows • Kilauea volcano and Mauna Loa on Hawaii

  16. Cinder Cones • Small and steep-sided • Pyroclastic materials accumulate around vent • Slope angles to 33 degrees • Often form on flanks of larger volcanoes, or in calderas: final stage of basaltic volcanism

  17. Composite Cones • Also called stratovolcanoes • Pyroclastic layers and lava flows of intermediate composition • Lahars: volcanic mudflow, rain or eruption • Steep-sided near top • Typical in continental and island volcanic arcs at convergent boundaries • Violent and dangerous eruptions

  18. Lava Domes • Also called volcanic domes and plug domes • Viscous felsic or intermediate magmas • Slow eruptions caused by great pressure below • Violent, destructive eruptions • Nuee ardente: fast-moving cloud of pyroclastic materials and gases

  19. Basalt Plateaus • Fissure eruptions from long cracks • Low-viscosity basalt • Covers vast areas, often with repeated eruptions • Columbia River basalt plateau • Currently occur in Iceland

  20. Pyroclastic Sheet Deposits • Vast areas covered by felsic volcanic rocks, a few to several hundred meters thick • Erupted from long fissures • Some welded tuff • May occur during the origins of calderas

  21. Volcano Distribution • Most in well-defined zones or belts • Ring of Fire: circum-Pacific belt • Central and South America volcanoes • Philippines, Indonesia, Japan, New Zealand • Aleutian Islands in Alaska • Cascade Range, northern California to southern British Columbia; Mount St. Helens 1980 eruption killed 57 people • Mediterranean

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