Volcanoes form when magma rises from below the Earth's surface and erupts through openings called vents. As magma builds up at the vent, it forms the volcano structure above ground. The main parts of a volcano include the vent, conduit that carries magma from the magma chamber below, and the cone or mountain shape built from erupted material. Volcanic eruptions can be explosive or quiet depending on the magma composition and amount of trapped gases.

2. Volcano : an opening in the earth’s surface through which lava, hot gases, and rock fragments erupt Introduction to volcanoes

3. Magma 50-100 miles below the earth’s surface slowly begins to rise to the surface As the magma rises it melts gaps in the surrounding rock As more magma rises a large reservoir forms as close as 2 miles below the surface (magma chamber) How do they form?

4. Pressure from the surrounding rock causes the magma to blast or melt a conduit (channel) to the surface where magma erupts onto the surface through a vent (opening) How do they form?

5. The magma, now called lava, builds up at the vent forming a volcano How do they form?

6. Often the volcano sides will be higher than the vent forming a depression called a crater How do they Form?

7. Crater :

8. Caldera: an unusually large crater or the remains when the cone collapses into its own magma chamber

9. Anatomy of a Volcano Cone : the above ground structure built from lava and/or tephra

10. Conduit : the path that magma takes from the magma chamber to the vent

11. Magma Chamber : the reservoir located under the volcano where magma collects and becomes the supply of magma/lava to build the volcano

12. Lava: molten, liquid rock on the surface of the earth

13. Parasitic Cone: a smaller secondary volcano built on the side of or near the main volcano, but sharing the same conduit to the magma chamber

14. Fumarole: a secondary vent that emits only gases

15. Fissure : a long fissure ( crack ) from which lava flows

16. Vent : opening of the volcano, through which lava, ash and gases flow

17. What comes out of volcanoes? Lava Tephra Gases

18. Lava— 3 kinds:

19. Pahoehoe lava: Hot, thin, fast flowing harden with a relatively smooth surface Often has a ropy or wrinkled appearance

20. Pahoehoe lava:

21. Aa lava: Cooler, thicker , slow moving Hardens with a rough, jagged, sharp edge surface

22. Pillow Lava: Lava suddenly cooled by water shows sack-like segments (stuffed pillows)

23. Tephra Basically, rock fragments Also known as pyroclastic rock fragments. There are many different possible sizes, from very small (volcanic ash or dust to much larger rocks (called volcanic bombs )

24. Ash & dust

25. Volcanic Bombs This dacite breadcrust bomb (about 15 cm in diameter) was erupted from the lava dome at Mount St. Helens, Washington.

26. Lahar (mudflow): mixture of ash, eroded land , and water flowing down river valleys

27. Lahar (mudflow):

28. GASES water vapor, carbon dioxide , nitrogen, sulfur dioxide , hydrogen sulfide, chlorine

29. Where do Volcanoes Occur?

30. Where do Volcanoes Occur? location Examples Divergent Boundaries : where plates move APART Iceland

31. Where do Volcanoes Occur? location Examples convergent Boundaries : where plates come together Cascade range volcanoes, Andes

32. Cascade Volcanoes

33. Where do Volcanoes Occur? location Examples Hot Spots : plates riding over an especially hot place in the mantle Hawaii, Yellowstone, Iceland

34. Hot Spots

35. Two factors determine the type of eruption : Amount of water vapor & other gases in the magma The chemical composition of the magma Types of Volcanic Eruptions

36. Trapped gases under high pressure will violently explode when the magma reaches the lower pressure of the surface. Explosive Eruptions

37. Has granitic magma is very thick and plugs the vent causing the pressure to build until it blows violently out the vent Explosive Eruptions Mt. St. Helens

38. The high water content of the magma produces more water vapor which when mixed in granitic magma produces explosive eruptions Explosive Eruptions Mont serrat

39. Low pressure gas Quiet Eruptions

40. Has basaltic magma (is more fluid and will flow instead of explode) Quiet Eruptions

41. … and has low water content Examples : Hawaii Quiet Eruptions

42. Cinder cones Small base, steep-sided, loosely consolidated Up to 1000 feet tall Life span of a few years Commonly built from gravel size lava rock fragments called cinders Violent eruptions, dangerous when close---High pressure gas bubbles causes thick lava to explode into the air, lava begins to cool as it rises and falls becoming very sticky When lava hits the ground it sticks rather than flows This builds a steep cone with a small base

44. Cinder Cones : Cinder Cones

45. Shield volcanoes Large base, gentle slope , lava rock layers A few miles high Life span of a million years or more The lava is hot, thin, very fluid often basaltic Example: Hawaiian Islands, Iceland

46. Shield volcano on Mars; Taken from space Take a look at these examples: http://www.volcano.si.edu/world/tpgallery.cfm?category=Shield%20Volcanoes The Mauna Loa volcano in Hawaii—the largest volcano on Earth—has the broad expanse characteristic of shield volcanoes. It spreads across half the island of Hawaii.

47. Shield Volcanoes Mauna Kea Shield volcanoes

48. Composite or strato Large mountain volcano often snow capped , a few miles high Life span of million years or more

49. Composite or strato Have alternating eruptions of tephra (air-borne) and lava . The tephra adds height to the volcano and the lava cements the tephra together and adds to the base . Found mostly in subduction zones and have violent eruptions.

50. STrato/Composite Llaima Volcano, Chile.

51. Composite (strato) Volcanoes examples: Mt. Rainier Mt. Fuji Mt. Kilimanjaro

52. Composite/strato volcanoes

53. Active (awake): Has erupted within recent time and can erupt again at any time . Pre-eruption activities : Increase in earthquake activity under the cone increase in temperature of cone, melting of ice/snow in the crater swelling of the cone steam eruptions minor ash eruptions Volcano Activity Levels (Stages)

54. Mt St. Helens

55. Dormant (sleeping): No eruption within recent times, but there is record of past eruptions Can become active and erupt again after a “wake up” period Example : Mt. Rainier

56. Extinct: No eruption within recorded history Not expected to ever erupt again Example: Mount Mazama (Crater Lake)

57. Crater Lake

58. Mount Rainier The most dangerous volcano in the US The danger is mostly from lahars traveling down river valleys at a speed of 25mph and destroying everything in its path 100,000 people live on the solidified mudflows of previous eruptions

59. Mount Rainier The mountain is dangerously unstable , a tall, steep heap of loose rock held together by the force of gravity and a cubic mile of glacier ice that could be melted or shaken loose Lahar flows average every 500 years and have gone as far as the Puget Sound lowlands Mount Rainier has erupted 4 times in the last 4000 years with the last eruption 200 years ago