Types of Volcanoes and Their Features

1. Chapter 5 Volcanoes

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 Volcanic activity is directly related to plate tectonics Most volcanoes are near plate boundaries At plate boundaries, magma is created Magma is molten rock Lava is magma on the Earth’s surface Volcanoes form around a vent Some active plate boundaries Subduction zones Mid-ocean ridges Continental rift zones

5. Figure 5.3

6. How Magma Forms Most magmas come from the asthenosphere Weak, but not liquid, layer of rock Three ways that silicate rocks melt Decompression melting Pressure exerted on hot rock is decreased Happens at divergent plate boundaries, continental rifts, and hot spots. Addition of volatiles Chemical compounds that lower the melting temperature of the rock Addition of heat As magmas rise, they release heat to overlying rocks

7. Figure 5.4

8. Magma Properties Described by silica content and amount of dissolved gasses Three types of magma based on silica content (low to high) Basaltic, andesitic, and rhyolitic Silica content affects viscosity Energy needed to make a liquid flow High silica content, high viscosity Viscosity affects the flow of lava and therefore the shape of resulting volcano

9. Magma Properties, cont. Volatile content determines how explosive the eruption will be High concentration of dissolved volatiles will explode violently Volatile content increases with increasing silica content In explosive eruptions, tephra or pyroclastic debris form All materials that are ejected from the volcano Ash, lapilli, and bombs Gasses break magma into small pieces to form ash fragments, or into frothy foam that later solidifies as pumice Pyroclastic deposit is an accumulation of tephra

10. Figure 5.5

11. Table 5.1 Figure 5.7

12. Volcano Types: Shield Volcanoes Largest volcanoes in the world Common in Hawaiian Islands, Iceland, and Indian Ocean Associated with basaltic magma Low viscosity, low volatile content Gently flowing lava with non-explosive eruptions Built almost entirely of lava flows Thin lava flows build up volcano with gentle slopes Resembles a warrior’s shield Can form lava tubes underground

13. Figure 5.6

14. Figure 5.8

15. Volcano Types: Composite Volcanoes Magma is more viscous with a higher volatile content A variety of lavas from basaltic to a combination of andesitic and rhyolitic Mixture of explosive activity and lava flows Produce a combination of lava flows and pyroclastic deposits Have a cone shape, also called stratovolcanoes Explosions are more violent and dangerous Example: Mount St. Helens, Mount Rainer, Mount Fuji

16. Figure 5.9

17. Volcano Types: Volcanic Domes Made from highly viscous magma Exhibit highly explosive eruptions Small domes often form within the crater after an eruption Example: Lassen Peak and Mono Craters

18. Volcano Types: Cinder Cone Volcanoes Small volcanoes built from an accumulation of tephra Small pieces of black or red lava Formed when lava meets groundwater Common on larger volcanoes, normal faults, or along cracks and fissures Also called scoria cones Example: Parícutin, Mexico

19. Figure 5.11

20. Volcanic Features Craters Depressions formed by explosion or collapse of volcano top A few kilometers (a mile) in diameter Calderas Very large craters formed from violent collapse of a cone May be 20 or more kilometers (12 or more miles) in diameter and contain vents and hot springs Vents Any opening for lava and debris May be circular or thin fissures Can produce flood basalts

21. Volcanic Features, cont. 1 Hot Springs Hot rocks heat groundwater that is discharged at surface Geysers Groundwater boils, erupting steam at surface Figure 5.14b

22. Volcanic Features, cont. 2 Caldera Eruptions Very rare, extremely violent eruptions Produce huge amounts of ash and forms calderas Sometimes referred to as supervolcanos by media Most recent North American caldera eruptions, 640 mya at Yellowstone National Park and 700 mya at Long Valley, California

23. Figure 5.15

24. Figure 5.19

25. Volcano Origins Mid-Ocean Ridges and Continental Rifts Basaltic magma directly from asthenosphere Three-quarters of all lava extruded on Earth If on land, shield volcanoes form Example: Iceland at Mid-Atlantic Ridge Subduction Zones Composite cones form here Most common on the Pacific Rim Andesitic magma with intermediate silica content Account for > 80 percent of eruptions in historic times Example: Cascade Mountains

26. Figure 5.21

27. Volcano Origins Hot spots beneath oceans Basaltic magma forms shield volcanoes Hot spot remains stationary building volcanic islands on the seafloor Example: Hawaiian Island chain Hot spots beneath continents Rhyolitic magma from mixes of rising magma and continental crust Caldera-forming eruptions Example: Yellowstone National Park

28. Geographic Regions Ring of Fire Pacific Ocean Subduction Zones Hot spots Hawai’i and Yellowstone Park Mid-ocean ridges Iceland Rift valleys East Africa

29. Figure 5.22

30. Effects of Volcanoes 50 to 60 volcanoes erupt each year In the United States 2 to 3 volcanoes erupt, mostly in Alaska Most eruptions are in sparsely populated regions 500 million people live close to volcanoes Japan, Mexico, Philippines, and Indonesia and several U.S. cities are vulnerable Primary Effects Lava flows, ash fall, pyroclastic flows and release of volcanic gases Secondary Effects debris flows, mudflows, landslides or debris avalanches, floods, fires, and both global cooling and global warming

31. Table 5.2

32. Figure 5.23

33. Effects of Volcanoes: Lava Flows Results when magma reaches the surface through crater or from a vent Three types: basaltic, andesitic, rhyolitic Basaltic is the most abundant Basaltic lavas are the most rapid at 15–35 km/h (10–30 mph) Pahoehoe lavas are smooth and ropey AA are blocky flows Move slow enough for people to get out of the way

34. Figure 5.24 Figure 5.25

35. Effects of Volcanoes: Pyroclastic Activity Explosive activity in which tephra is blown into atmosphere Volcanic ash eruptions Fine-grained rock and volcanic glass shatters, and gas is blown into the air Settle as an ash fall Lateral blast Rock fragments are blown horizontally from volcano at high speeds

36. Effects of Volcanoes: Pyroclastic Flow Avalanches of hot ash, rock, volcanic glass fragments, and gas that move rapidly down the sides of the volcano AKA: ash flows, hot avalanches, or nuée ardentes Incinerate everything in their paths Responsible for more deaths than any other hazard Flow at 160 km/hr (100 mph) with temperatures > 100°C

37. Figure 5.1

38. Ash Fall Vegetation may be destroyed Surface water may be contaminated by sediment Fine particles clog the gills of fish and kill other aquatic life. Ash accumulation on roofs may cause structural damage Irritation of the respiratory system and eyes Engines of jet aircraft may “flame out”

39. Figure 5.28

40. Effects of Volcanoes: Poisonous Gases Emitted gases: water vapor, carbon dioxide (CO2), carbon monoxide (CO), sulfur dioxide (SO2) and hydrogen sulfide (H2S) CO2 and water account for 90 percent of emissions Toxic concentrations of gases rarely reach populated areas Carbon dioxide gas is odorless and heavy. It can accumulate suffocating people. Sulfur dioxide Can produce acid rain Chemicals can contaminate soil and plants Can cause air pollution known as vog (volcanic smog)

41. Effects of Volcanoes: Debris and Mud Flows AKA: Lahars Loose volcanic ash becomes saturated with water, becomes unstable, and moves down volcano Can occur in the absence of an eruption Debris flows Glaciers and ice are melted by volcano and mix with sediment and rock Similar to wet concrete Mud flows Finer than debris flows Populous areas of Pacific Northwest are built on old mudflows Not unlikely for new flows to occur

42. Figure 5.32 Figure 5.33

43. Effects of Volcanoes: Landslides Secondary effects of volcanoes May be triggered outside of an eruption May affect areas far from their source Can cause tsunamis

44. Mount St. Helens 1980–2010 Scene of volcanic explosion in recent history Well studied example of Cascade volcanic eruption Figure 5.34a

45. Figure 5.34b

46. Figure 5.34c

47. Mount St. Helens—Timeline 120 years of dormancy March 1980 – seismic activity and small explosions May 1 – bulge begins to grow on northern flank at rate of 1.5 m (5 ft.) per day May 18, 8:32 A.M. – 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)

48. Mount 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 to 55 km/hr (18 to 34 mph)

49. Figure 5.35

50. Figure 5.36a Figure 5.36b