1. Volcanoes and volcanism

2. Goals To examine the relationship between magma composition, the kinds of volcanoes and volcanic processes that occur, and plate-tectonic setting.

3. Magma viscosity Viscosity: Measure of resistance to flow. The greater the viscosity, the harder it is to get a material to flow. Honey more viscous Olive oil less viscous

4. Magma viscosity Viscosity of magma is controlled by two things: temperature and composition –Higher T = lower viscosity –Higher silica content = higher viscosity. Felsic magmas don’t like to flow.

5. Magma gas content All magmas contain 1–9% dissolved gases— mostly CO 2 and H 2 O. These gases behave like the carbonation in a soft-drink or a beer.

6. Eruptive materials: What comes out of volcanoes Lava flows : Liquid or semi-liquid magma flowing on Earth’s surface Pyroclastic materials : Material blasted out of volcano during an explosive eruption

7. “Ropy” pahoehoe-type lavas: Lowest viscosity, always mafic. “Blocky” a’ a’-type lavas: Low viscosity, colder mafic lavas and hot intermediate lavas Lava flows

8. Pyroclastic materials Tephra: Magma blasted into the air to form very hot (500° C) solid particles Volcanic dust: Tephra particles less than 1/1000 mm in diameter. Volcanic ash: Tephra particles between 1/1000 mm to 2mm in diameter. Volcanic bombs: Tephra chunks >64mm in diameter

9. Volcanic dust can stay suspended in the upper atmosphere for years Large eruptions can alter global climate Really awesome sunsets

10. Volcanic ash falls like snow for 100’s of miles down-wind from eruption

11. Pyroclastic materials Pyroclastic flows a.k.a. Nuee Ardente: Billowing clouds of super-heated gas and ash that flow down the slope of the volcano

13. Pyroclastic flows travel at more than 50 mph

14. Pyroclastic materials Volcanic mudflows a.k.a. Lahars: Eruption melts snow, glacial ice, and/or frozen soil Mixture of water and debris flow rapidly down the slope of a volcano

15. Types of Volcanoes—Dictated by magma type Most volcanoes have two basic elements: A volcanic cone and a central crater Crater

16. Mafic magmas Shield volcanoes: Enormous, gently sloping volcanic mounds Form over oceanic hotspots: localized zone of hot mantle upwelling Largest topographic features on Earth Shield volcano—Big Island of Hawaii

17. Hot Spot Volcano Tracks

18. Hawaiian Islands are hot-spot volcanoes on the Pacific plate Which way is the pacific plate moving? Hawaii ? ?

19. Mafic magmas Fissure eruptions: Low-viscosity magma flows up along km-long cracks in the crust Often form in continental rifts Form Flood basalts: Lava spreads out over large areas Small fissure eruption

20. 15-m.y. old flood basalts in the western United States

21. Mafic magmas Cinder cones: Small volcanic cones produced by fountain eruptions Often found on the flanks of larger volcanoes or in continental rifts Fountain eruption Cinder cone in Arizona

22. Intermediate magmas Composite volcanoes: Contain alternating layers of pyroclastic deposits and lava flows Explosive eruptions producing ash clouds and pyroclastic flows Found almost exclusively in volcanic arcs related to subduction zones Mt. Vesuvius, ItalyMt. Fuji, Japan

24. Felsic magmas Volcanic domes: Mounds formed when lava is too viscous to flow away from the vent Lava Dome

25. Felsic magmas Caldera eruptions: Large magma chamber collapses, causing enormous, explosive eruptions Form over continental hotspots and in continental rifts Form Very large, steep-walled depressions called calderas

26. Steps in the formation of a caldera

27. Yellowstone National Park is a giant caldera Caldera

28. The Yellowstone volcanic system has had 3 catastrophic eruptions in the past 2.1 m.y. 2.1 m.y. eruption released 600 cubic miles of tephra 640 k.y. eruption released 240 cubic miles of tephra 1.3 m.y. eruption released 67 cubic miles of tephra

29. Tectonic Settings and Volcanic Activity