2. Volcanoes and Plate Tectonics

3. Factors which determine the nature of volcanic eruptions: The magma’s composition Temperature Amount of dissolved gases These factors affect the magma’s viscosity (thickness) and therefore its ability to flow.

4. Magmas formed from basaltic rock have a lower percentage of silica (mafic) than magmas formed from granitic rock (felsic). The higher the silica content, the more viscous (thick) the magma is so the more slowly the magma flows.

5. Not in notes.

7. Dissolved gases increase the fluidity of magma (decrease its viscosity). As magma rises, confining pressures drop. This allows dissolved gases to be released.

8. At temperatures of 1000°C and near-surface pressures, the gases expand to occupy hundreds of times their original volume. Very fluid magmas allow the gas to escape easily thereby carrying lava hundreds of metres into the air.

9. Not in notes.

10. Highly viscous magma is resistant to the upward migration of bubbles. Pressure builds until magma is explosively ejected thousands of metres into the air.

11. The explosion results in reduced pressure in the uppermost portion of the magma body. As a result, more gas is released.

12. Pressure builds again. A series of volcanic explosions occurs until the magma chamber is emptied.

13. Basaltic Lava Flows Very fluid As fast as 3 000 m / h on steep slopes (rare) 10 to 300 m / h more common May move 150 km before congealing

14. Two types: Pahoehoe flows – wrinkled skin Aa flows– cool and thick; lava rubble (Escaping gases create jagged edges.)

15. Not in notes.

17. Gases Account for 1 – 5% of total magma weight. This means thousands of tons of gases per day are released!

18. Gas Composition –70% water vapor –15% carbon dioxide –5% nitrogen –5% sulphur –Small percentages of chlorine, hydrogen, and argon

19. Gases Propel magma from a volcano Create narrow, round vents that connect the magma chamber with the surface by sandblasting overhead rock and walls. The vents allow magma to rise.

20. Granitic Lavas: Are highly viscous. Move very slowly. Have intense pressure that causes particles to be blown from the vent in a violent explosion. These particles are called pyroclastic materials.

21. Pyroclastic Materials All called tephra. Range in size from dust to large volcanic bombs or blocks. Dust is scattered large distances. Blocks and bombs fall on slopes of the volcano.

22. Ash: dust-sized particles Cinders: pea-sized particles Lapilli: walnut-sized particles Blocks: hardened lava Bombs: semi-molten rock

23. Not in notes.

24. Types of Volcanoes Shield Volcano Cinder Cone Composite Volcano (Stratovolcano)

25. Shield Volcanoes Are built primarily from basaltic lava flows. They contain very little pyroclastic material. They have very gentle slopes.

26. Not in notes.

27. Example of a Shield Volcano Kilauea – Hawaiian Islands Not in notes.

28. Cinder Cones Are built from ejected lava fragments. They have steep slopes. They are small and Frequently occur in groups

29. Not in notes.

32. Paricutin – Mexico City Cinder cone formed in 1943 in hole in farmer’s field Overnight grew to be 40 m high Five days later it was over 100 m high Within two years it was over 400 m high Major lava flow in 1944 buried village under 10 m of rock Not in notes.

33. Destruction from Paricutin Not in notes.

35. Composite Cone Form from alternating layers of very viscous lava and pyroclastic material. They are very steep and quite high.

36. Not in notes.

37. Composite Example – Mt. Fuji Not in notes.

38. Mt. Fuji – Japan Not in notes.

39. Mount St. Helens prior to 1980 Not in notes.

40. Mt. St. Helens – 1980 Eruption Not in notes.

41. Mt. St. Helens – after 1980 eruption Not in notes.

42. March 8, 2005

44. March 15, 2005

45. April 6, 2005

46. Pyroclastic Flow (Nuee ardente) Also known as a glowing avalanche Is very dense lava that is supported by expanding gases. Gases allow lava to move in near frictionless conditions. Can move at speeds of up to 150 km / h !!!

47. Not in notes. Mount Pinatubo – June 1991

49. Mount Pinatubo – 1999: still active

50. Mount Ngauruhoe New Zealand – 1974 Not in notes.

51. Mount Ngauruhoe New Zealand –today

52. Mt. Pelee – Martinique 1902 Not in notes.

54. Only 2 people on land survived – a shoemaker and a prisoner protected by his dungeon. Thousands of kegs of rum exploded and burned as a result of the intense heat. This caused ships in dry dock to catch on fire.

55. Closer to home.

56. Volcanic Features Lava Dome –Produced when volcanoes slowly extrude highly viscous material out a vent –They can act like plugs –They sometimes form on flanks

57. Mt. St. Helens Not in notes.

59. Calderas Are unusually large craters. They are thought to form when the summit of a volcano collapses because of insufficient support from a partially emptied magma chamber.

60. Caldera Not in notes.

62. Crater Lake and Wizard Island in Oregon Not in notes.

63. Crater

64. Plutons –Are the igneous rock intrusions that form when magma cools inside other rocks. They include: –Dikes Are sheets of igneous rock that cut across rock layers as magma intrudes vertical cracks

65. Sills –Sheets of igneous rock that have intruded between layers of existing rock Laccoliths –When the intruding igneous rock is viscous, instead of a sill forming, a domelike structure forms as the magma pushes the existing rock upward

66. Batholiths –Are very large igneous rock intrusions that become visible as covering rock layers are eroded. They form the cores of many mountain ranges. –Magma can rise from batholiths to form other features such as dikes, vents, sills and laccoliths Stock –Batholiths that are smaller than 100 square kilometres

67. Volcanic Neck Is formed from an eroded volcano leaving just its hardened vent behind

68. Shiprock, New Mexico

69. Label your diagram: