Geological Hazards

1. Geological Hazards Volcanoes, Earthquakes, and Tsunami Session 3

2. Journey to the Center of the Earth • Core • Mantle • Crust

3. Cross Section of Earth

4. Core • Inner Core: solid, iron, 13.5 g/cm³ • Outer Core: molten, iron, 10.7 g/cm³ • Water: 1.0 g/cm³ • Mercury: 13.0 g/cm³

5. Mantle • Iron oxides, magnesium, and silicates • Lower and Upper Mantle

6. Upper Mantle • Asthenosphere (plastic-like, hot tar) • Source of magma • Lithosphere (rigid) • Top of Lithosphere is Mohorovicic Discontinuity (Moho) • Seismic waves change due to density and material contrasts

7. Crust • Oceanic • Continental

8. Oceanic • Averages 3 miles in depth • Composed primarily of silica (49.8%) and alumina (16.7%) • Sima • Rock is basalt (non-explosive) • 3.0 gm/cm³

9. Continental • 19 miles interior, 31-37 miles under mountains • Silica (60.1%) and alumina (15.6%) • Rock is granite (continents formed of this material) • 2.7 gm/cm³ • Sial

10. Plate Tectonics • Lithosphere moving over the asthenosphere • Movement and interaction of plates • Tectonic cycle: magma from asthenosphere, seafloor spreading, subduction • Approximately 250 million years

11. Continental Drift • German climatologist Alfred Wegener proposed theory in early 1900s • Pangaea: supercontinent • Jigsaw puzzle

12. Pangaea

13. Changes in Continents Over Time

14. Subduction Zones

15. Pacific Ocean Plates

16. Plate Boundaries • Divergent • Convergent • Transform

17. Divergent Zones • Spreading Center • Pulls apart • Usually in oceanic ridge • Shallow earthquakes • Volcanic activity • Adds material from asthenosphere

18. Divergent Zones

19. Subduction Zones

20. Oceanic Trenches and Ridges

21. Convergent Zones • Plates moving in opposite directions • One plate subducts the other • Remove part of crust

22. Convergent Landforms • Mountain ranges • Volcanoes • Deep ocean trenches

23. Convergent Examples

24. Subduction Zones • “Recycling” of crustal material in the lithosphere • Convective process • Powerful earthquakes with strong compressive forces • Due to strong rocks under compression, store greater energy before rupturing

25. Subduction Animation • http://www.classzone.com/books/earth_science/terc/content/visualizations/es0902/es0902page01.cfm • http://oceanexplorer.noaa.gov/explorations/03fire/logs/subduction.html

26. Transform Boundaries • Plates slip past each other laterally • Vertical fractures called transform faults • Many boundaries near midocean ridges • California (San Andreas)

27. Transform Example

28. What is Vulcanism? • Refers to all phenomena connected with the origin and movement of molten rock

29. Magma vs. Lava • Magma: molten rock under the surface • Lava: molten rock above the surface

30. Extrusive • Magma expelled on the surface while still molten • Volcanism

31. Intrusive • Magma solidifies in the shallow crust near the surface

32. Plutonic • Magma solidifies deep inside the Earth

33. Volcano • Mountain formed by the accumulation of erupted lava and/or volcanic ash

34. Classifying Volcanoes • Appearance (size and slopes) • Magma composition • Volatile content

35. Types of Volcanism • Silicic: explosive • Basaltic: non-explosive

36. Explosive Eruption Styles • Phreatic: Violent steam-driven explosions • Phreatomagmatic: magma more than steam • Strombolian: rapidly expanding steam bubbles in magma forms a cinder cone • Vulcanian: ash-fall dominated • Peléan: high ash columns and ash flows • Plinian: silica-rich ash falls, large volume of magma potentially causes a collapse

37. Volcanic Explosivity Index

38. Silicic Volcanism • Source is subduction zones consists of basalt and silicate sediment (high silica content) • Requires large amount of heat to melt • Results in cooler magma temperatures (870˚ C) • Gases can’t dissolve due to lower temperatures, trapped in bubbles • Magma near surface, confining pressure results in explosive release

39. Basaltic Volcanism • Low silica content • Higher magma temperatures (1200˚ C) • Greater heat dissolves most of the gases • Lava is more fluid than explosive • Non-explosive, large quantities of lava (basaltic flood)

40. Temporal Patterns • Active: relatively recent or frequent activity • Dormant: quiet for some time but considered potentially active • Extinct: not known to erupt since discovery • Can go from extinct to dormant to active

41. Major Categories • Shield • Stratovolcano • Lava Domes • Cinder Cones

42. Volcanic Types

43. Shield Volcanoes • Categorized as Basaltic • Broad, gently sloping mountains • Structure are from layers of lava flows • “Quiet” eruptions of fluid lava

44. Shield Examples • Mauna Loa • Kilauea • Mount Etna

45. Stratovolcanoes • Composite or Andesitic • Medium to Large, medium-to-steep-sided, with a symmetrical cone • Moderate viscosity • Moderate to high volatile content

46. Stratovolcano Examples • Mount St. Helens • Mount Fuji • Mount Vesuvius

47. Lava Domes • Rhyolitic • Small to moderate size, high magma viscosity, steep flanks, and low to moderate volatile content • Usually erupts only once but can be replaced by another dome

48. Lava Dome Example • On Mount St. Helens • Mount Pelée

49. Cinder Cones • Small, steep-sided cones, low viscosity, and moderate volatile content • Rising basalt magma meets groundwater • Pyroclastics ejected from a central vent and occasional lava flows

50. Cinder Cone Examples • Haleakala Caldera • Paricutín