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Volcanoes

Volcanoes. 12.10.07 / 12.11.07 . Volcanic eruptions . Factors that determine the violence of an eruption Composition of the magma Temperature of the magma Dissolved gases in the magma Viscosity of magma Viscosity is a measure of a material's resistance to flow . Volcanic eruptions .

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Volcanoes

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  1. Volcanoes 12.10.07 / 12.11.07

  2. Volcanic eruptions • Factors that determine the violence of an eruption • Composition of the magma • Temperature of the magma • Dissolved gases in the magma • Viscosity of magma • Viscosity is a measure of a material's resistance to flow

  3. Volcanic eruptions • Viscosity of magma • Factors affecting viscosity • Temperature (hotter magmas are less viscous) • Composition (silica content) • High silica – high viscosity • Low silica – more fluid • Dissolved gases (volatiles) • mostly H2O and CO2 • Gases expand near the surface

  4. Volcanic eruptions • Viscosity of magma • Factors affecting viscosity • Dissolved gases (volatiles) • Provide the force to extrude lava • Violence of an eruption is related to how easily gases escape from magma • Easy escape from fluid magma • Viscous magma produces a more violent eruption • Think of boiling water vs spaghetti sauce

  5. A low silica lava flow

  6. A high silica flow

  7. Pyroclastics • Pyroclastic material: fragments of rock formed during eruptions • Ash and dust: fine, glassy fragments • Lapilli: walnut sized rocks • Bombs: magma ejected into the air cools and forms rounded rocks • Blocks: ejected as large rocks

  8. Volcanoes • General features • Conduit, or pipe carries gas-rich magma to the surface • Vent, the surface opening (connected to the magma chamber via a conduit) • Crater • Steep-walled depression at the summit • Caldera (a summit depression greater than 1 km diameter)

  9. Volcanoes • Types of volcanoes • Shield volcano • Broad, slightly domed (like a shield) • Primarily made of fluid lava • Generally large size

  10. A shield volcano

  11. Volcanoes • Types of volcanoes • Cinder cone • Built from ejected lava fragments • Steep slope angle • Relatively small size

  12. Sunset Crater – a cinder cone near Flagstaff, Arizona

  13. Volcanoes • Types of volcanoes • Composite cone (or stratovolcano) • Most are adjacent to the Pacific Ocean (e.g., Mt. Rainier) • Large size • Interbedded lavas and pyroclastics • Most violent type of activity

  14. A composite volcano (stratovolcano)

  15. Mt. St. Helens – a typical composite volcano

  16. Mt. St. Helens after eruption

  17. A size comparison volcanoes

  18. Other volcanic landforms • Calderas • Steep walled depression at the summit • Formed by collapse • Nearly circular • Size exceeds one kilometer in diameter

  19. Crater Lake, OR caldera

  20. Crater Lake in Oregon

  21. Plate tectonics • Plate motions provide the mechanism by which mantle rocks melt to form magma • Convergent plate boundaries • Deep-ocean trenches are generated • Descending plate partially melts • Magma slowly rises upward • Rising magma can form • Volcanic island arcs in an ocean (Aleutian Islands) • Continental volcanic arcs (Andes Mountains)

  22. Convergent boundaries

  23. Plate tectonics • Plate motions provide the mechanism by which mantle rocks melt to form magma • Divergent plate boundaries • The greatest volume of volcanic rock is produced along the oceanic ridge system

  24. Divergent boundaries

  25. Plate tectonics • Plate motions provide the mechanism by which mantle rocks melt to form magma • Intraplate igneous activity • Activity within a rigid plate • Plumes of hot mantle material rise • Form localized volcanic regions called hot spots • One example is the Hawaiian Islands

  26. Hot Spots

  27. Locations of some of Earth’s major volcanoes r

  28. Continental and island volcanic arcs

  29. Review • Name and describe the three types of volcanoes. • Describe the different types of pyroclastics. • Explain the factors that affect the severity (violence) of an eruption. • Describe how continental and island arcs are formed.

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