Natural Hazards, 2e

1. Natural Hazards, 2e Volcanoes Chapter 4

2. Learning Objectives Know the different types of volcanoes and their associated features Understand the relationship of volcanoes to plate tectonics Know what geographic regions are at risk from volcanoes Know the effects of volcanoes and how they are linked to other natural disasters

3. Learning Objectives, cont. Recognize the potential benefits of volcanic eruptions Understand how we can minimize the volcanic hazard Know what adjustments we can make to avoid death and damage from volcanoes

4. Introduction Most volcanoes are near plate boundaries. Plate boundaries are where the magma is. Magma is molten rock. Lava is magma on the earth’s surface. Some active plate boundaries: Subduction zones Mid-ocean ridges Continental rift zones

5. Magma Described by silica content and amount of dissolved gasses. Silica content affects viscosity. Energy needed to make a liquid flow High silica content, high viscosity Gas content determines how explosive the eruption will be. High gas content, greater the explosion

6. Tephra Gasses will cause lava and other debris to be expelled from the volcano. Also called pyroclastic materials. Range in size from dust-sized materials, gravel-sized lapilli, to large block-sized bombs.

7. Magma Types Basaltic Low silica content, low viscosity Andesitic Intermediate silica content, intermediate viscosity Rhyolitic High silica content, high viscosity

8. Volcano Types

9. Shield Volcanoes Largest volcanoes in the world Built almost entirely of lava flows Resemble a warrior’s shield Associated with basaltic magma Low viscosity, low gas content Gentle flowing lava with nonexplosive eruptions Can form lava tubes underground

10. Shield Volcanoes, cont. Found in Hawaiian Islands, Iceland, and around Indian Ocean

11. Composite Volcanoes Associated with variety of magmas, basaltic to lavas between andesitic and rhyolitic Higher viscosity and gas content Built from a combination of lava flows and pyroclastic deposits Have a cone shape, also called stratovolcanoes Explosions more violent and dangerous

12. Composite Volcanoes Ex.: Mt. St. Helens, Mt. Rainer, Mt. Fuji

13. Volcanic Domes Made from highly viscous rhyolite magma Exhibit highly explosive eruptions Ex.: Lassen Peak and Mono Craters

14. Cinder Cone Volcanoes Small volcanoes Built entirely from tephra Small pieces of black or red lava Common on larger volcanoes, normal faults, or along cracks and fissures Ex.: Paricutin, Mexico

15. Cinder Cone Volcanoes, cont.

16. Volcanic Features Craters Depressions formed by explosion or collapse of volcano top Calderas Very large craters formed from violent explosions Vents Any opening for lava and debris Can produce flood basalts

17. Volcanic Features, cont. Hot springs Hot rocks heat groundwater discharged at surface Geysers Groundwater boils, erupting steam at surface

18. Volcanic Features, cont. Caldera eruptions Very large, very violent eruptions Produce calderas Very rare Most recent North American caldera eruptions 640 mya at Yellowstone National Park and 700 mya at Long Valley, California

19. Caldera Eruptions

20. Volcanic Activity and Plate Tectonics

21. Volcano Origins Mid-ocean ridges Basaltic magma from asthenosphere Shield volcanoes Ex.: Iceland at Mid-Atlantic Ridge Subduction zones Andesitic magma from melting tectonic plate Composite volcanoes Ex.: Cascade Mountains

22. Volcano Origins Hot spots beneath oceans Basaltic magma Shield volcanoes Ex.: Big Island of Hawaii Hot spots beneath continents Rhyolitic magma from mixes of rising magma and continental crust Caldera eruptions Ex.: Yellowstone National Park

23. Geographic Regions Ring of fire Pacific Ocean subduction zones Hot spots Hawaii and Yellowstone Park Mid-ocean ridges Iceland Rift valleys East Africa

25. Effects of Volcanoes 50–60 volcanoes erupt each year. In U.S. 2–3 volcanoes 500 million people live close to volcanoes. Japan, Mexico, Philippines, and Indonesia Several U.S. cities vulnerable

28. Lava Flows Rhyolitic, andesitic, and basaltic lavas Basaltic lavas flow most abundantly: Pahoehoe – 1 m/hr A’A’-1-3 m/day

29. Pyroclastic Activity Tephra is blown into atmosphere. Ash fall Ash is blown high into air and falls onto areas. Lateral blast Rock fragments are blown horizontally from volcano. Pyroclastic flow Avalanches of hot rock, ash, glass fragments.

30. Ash Fall Vegetation destroyed Contaminates surface water Damage to buildings Health hazards Aircraft engine failure

31. Pyroclastic Flow Responsible for more deaths than any other hazard Flow at 160 km/hr (100 mph) Temperatures >1000C

32. Poisonous Gases Carbon dioxide (CO2) Odorless, heavy gas that can displace breathable air Sulfur dioxide Odorous gas that causes acid rain and can contaminate rock and soil

33. Debris & Mud Flows Also known as lahars Volcanic activity melts ice, snow, or glaciers on a volcano. Water mixes with ash, other tephra Mixture becomes unstable and flows down volcano Populous areas of Pacific Northwest are built on old mudflows. Not unlikely for new flows to occur.

35. Landslides Secondary effects of volcanoes Can cause tsunamis

36. Mt. St. Helens Scene of volcanic explosion in recent history Well-studied example of Cascade volcanic eruption

37. Mt. St. Helens – Before

38. Mt. St. Helens – After

39. Mt. St. Helens – Timeline 120 years of dormancy March 1980 – seismic activity & small explosions May 1 – bulge begins to grow on northern flank at rate of 1.5m (5 ft) per day May 18, 8:32 am – M 5.1 earthquake triggers landslide/debris avalanche of the bulge area Seconds later, lateral blast from bulge area at rate of 480 km/hr (300 mph)

40. Bulge & Avalanche

41. Lateral Blast & Vertical Eruption

42. Mt. St. Helens – Timeline, cont. One hour after blast: vertical cloud of ash extends to stratosphere. 9 hours of ash falls to cover areas of Washington, northern Idaho, western and center Montana. Pyroclastic flows begin at this time down the northern slope. Mudflows begin at speeds of 29-55 km/hr (18-34 mph).

43. Debris Avalanche and Ash Cloud

44. Mt. St. Helens – Summary 57 people were killed Flooding destroyed >100 homes 800 feet of timber flattened Damage >$1 billion September 23, 2004, Mt. St. Helens reawakens Lava dome begins to form on crater floor Continues to form today

46. Links to other Hazards Earthquakes Landslides Fire Hot lava ignites plants and structures. Climate Change CO2 (and other gasses) from eruption alters climate.

47. Benefits of Volcanoes Volcanic Soils Good for coffee, maize, pineapples, sugar cane, and grapes Geothermal power Can create energy for nearby urban areas Mineral Resources Gold, silver, etc. and nonmetallic rocks Used for soap, building stone, aggregate for roads, railroads, etc.

48. Benefits of Volcanoes cont. Recreation Health spas and hot springs Hiking, snow sports, and education Kilauea National Park Creation of New Land Hawaiian Islands

49. Forecasting a Volcanic Eruption Seismic activity Shallow earthquakes and swarms can precede eruption. May not provide enough time for evacuation. Thermal, magnetic, and hydrologic monitoring Accumulation of hot magma changes temperatures, magnetic properties, and temperature position of groundwater.

50. Forecasting a Volcanic Eruption Land surface monitoring Monitoring growth of bulges or domes. Kilauea tilts and swells. Monitoring volcanic gas emissions Changes in CO2 amounts correlate with volcanic processes. Geologic history Mapping of volcanic rocks and deposits give idea of types of effects to be expected.

51. Volcanic Alert or Warning

52. Attempts to Control Lava Flows Hydraulic chilling Water used to chill and control the lava flow Iceland Wall construction Walls used to redirect lava flow

53. End Volcanoes Chapter 4