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GEOL: CHAPTER 5

GEOL: CHAPTER 5. Volcanoes and Volcanism. 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. Lava Flows , cont. Pahoehoe Ropy surface like taffy Less viscous Aa

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GEOL: CHAPTER 5

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  1. GEOL: CHAPTER 5 Volcanoes and Volcanism

  2. 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

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

  4. 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

  5. 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

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

  7. 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

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

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

  10. 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

  11. 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

  12. 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

  13. 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

  14. Igneous Activity at Divergent Plate Boundaries • Mafic magma emplaced as vertical dikes and gabbro plutons in lower oceanic crust • Some erupts as lava flows and pillow basalts to form upper oceanic crust • Mid-Atlantic Ridge emerges and erupts in Iceland • East African Rift system

  15. Igneous Activity at Convergent Plate Boundaries • Many large active volcanoes in circum-Pacific Ring of Fire and Mediterranean • Composite cones near leading edges of overriding plates, oceanic or continental • Intermediate to felsic composition • Some lava domes • Mount St. Helens eruption, 1980, in the Cascade Range

  16. Intraplate Volcanism • Mantle plumes can create hot spots far from a divergent or convergent boundary • Mafic lava builds shield volcanoes • Hawaiian Islands built as the Pacific Plate moves over a hot spot

  17. Forecasting Eruptions • Only a few volcanoes are monitored • Tiltmeters: change in slope • Geodimeter: laser measures horizontal distances • Gas emissions • Groundwater and hot springs • Volcanic tremor • Eruptive history

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