1. Volcanoes Lecture 4 http://www.soest.hawaii.edu/GG/HCV/kilauea.html Igneous Rock Summary.doc will be helpful for this chapter’s homework.

2. The Nature of Volcanic Eruptions The Nature of Volcanic Eruptions Factors determining the “violence” or explosiveness of a volcanic eruption Composition of the magma –Silica Content Temperature of the magma Dissolved gases in the magma Composition and Temperature control the viscosity (resistance to flow) of magma. Viscous magmas cannot release gasses coming out of solution, and explode lava as it freezes. http://www.soest.hawaii.edu/GG/HCV/kilauea.html

3. The Nature of Volcanic Eruptions The Nature of Volcanic Eruptions Water has very low viscosity, cold molasses high viscosity Factors affecting Viscosity Temperature - hotter magma is less viscous (more fluid). Basaltic (mafic) magmas (Olivine, Pyroxene, Ca-Feldspars) are hotter than Granitic (felsics) (Quartz, K- feldspars) Composition - Felsic lava (e.g. rhyolite) is most viscous due to high silica content - intermediate lavas (e.g. andesite) viscous. - mafic lava (basalt) has lower viscosity - more fluid-like due to lower silica content

4. The Nature of Volcanic Eruptions The Nature of Volcanic Eruptions Factors affecting explosiveness Dissolved Gases –Gases come out of solution and expand in a magma as it nears the Earth’s surface due to decreasing pressure –The violence of an eruption is related to how easily gases escape from magma – trapped gasses expand and shatter solidifying lavas, causing explosions http://vulcan.wr.usgs.gov/Volcanoes/MSH/Images/MSH04/framework.html

5. The Nature of Volcanic Eruptions The Nature of Volcanic Eruptions Summary Fluid basaltic lavas generally produce quiet eruptions (Hawaiian lava flows) Viscous lavas (rhyolite or andesite) produce more explosive eruptions (Yellowstone & Mt. St. Helens hot ash explosions)

6. Low-Viscosity Basaltic Lava Source: Phil Degginger/Earth Scenes

7. Viscous Andesitic Lava over crater floor Source: Eugene Iwatsubo/Cascade Volcano Observatory, USGS

8. Very Viscous Rhyolite Flow Source: Martin Miller Viscous, short path

9. Materials extruded from a Basaltic Volcano Materials extruded from a Basaltic Volcano Lava Flows Basaltic lavas are much more fluid Types of basaltic flows –Pahoehoe lava (- twisted or ropey texture) –Aa lava (rough, jagged blocky texture) Dissolved Gases 1-6% of a magma by weight Mainly H 2 O vapor and CO 2 and SO 2

10. A Pahoehoe lava flow A Pahoehoe lava flow

11. T ypical a’a’ flow T ypical a’a’ flow

12. Fluid basalt forms lava tubes Checking Bowens Reaction Series

13. Materials extruded from a volcano Pyroclastic materials – “Tephra” Propelled through the Air Types of pyroclastic debris Dust 0.001 mm and Ash < rice sized Cinders or Lapilli - pea to walnut-sized material Particles larger than lapilli Bombs - > 64 mm ejected as hot lava -Surtsey Is. Bombs the size of busses

14. A volcanic bomb A volcanic bomb Bomb is approximately 10 cm long

15. Tephra forms Tuff Source: Gerald & Buff Corsi/Visuals Unlimited, Inc.

16. Tephra layers fine away from source

17. Pumice Felsic magmas with high water content may bubble out of a vent as a froth of lava. Quickly solidifies into the glassy volcanic rock known as Pumice. http://volcanoes.usgs.gov/Products/Pglossary/pumice.html

18. Volcanoes General Features Opening at the summit of a volcano –Crater - steep-walled depression at the summit, generally less than 1 km diameter –Caldera - a summit depression typically greater than 1 km diameter, produced by collapse following a massive eruption. Vent – opening connected to the magma chamber via a pipe

19. Types of Volcanoes 1 Shield volcano - Largest –Broad, slightly domed-shaped –Composed primarily of basaltic lava –Generally cover large areas –Produced by mild eruptions of large volumes of lava –Mauna Loa on Hawaii is a good example

20. Shield Volcano (Hawaii's K’ilaueau Volcano) Source: Jeff Greenberg/Visuals Unlimited, Inc. Shield Volcanoes are often the largest in a chain of islands. They have basaltic lava, which is NOT very viscous, so it easily releases it’s gasses. Hence explosive pyroclastic eruptions are rare.

21. A size comparison of the three types of volcanoes A size comparison of the three types of volcanoes

22. A Shield Volcano

23. Olympus Mons Caldera 23 km high Mars

24. Types of Volcanoes - 2 Cinder cone - Smallest –Built from ejected lava fragments (mainly cinder-sized) –Steep slope angle –Rather small size –Frequently occur in groups

25. Cinder Cone

26. A Cinder Cone Fountain

27. Sunset Crater – a cinder cone near Flagstaff, Arizona

28. Types of Volcanoes - 3 Composite cone (Stratovolcano) –Most are located adjacent to the Pacific Ocean (e.g., Fujiyama, Mt. St. Helens) –Large, classic-shaped volcano (1000’s of ft. high & several miles wide at base) –Composed of interbedded lava flows and layers of pyroclastic debris

29. A composite volcano

30. Mt. St. Helens – a typical composite volcano (prior to eruption) Mt. St. Helens – a typical composite volcano (prior to eruption) Composite volcanoes typically have intermediate silica, andesitic magma. Gasses are trapped in the magma. When it erupts out onto the surface, low pressure causes dissolved gasses to come out of solution just as the lava is freezing. The lava explodes, Resulting in a nuee ardente.

31. Mt. St. Helens after 1980 eruption Mt. St. Helens after 1980 eruption

32. Pyroclastic Flows

33. St Helens Eruption Sequence How would Scientists Monitor this Process? Seismometers Tilt Meters

34. Composite Volcanoes –continued –Most violent type (e.g., Mt. Vesuvius, Mt. St. Helens, Mt. Pinatubo) –Often produce a nuée ardente Fiery pyroclastic flow made of hot gases infused with ash and other debris Move down the slopes of a volcano at speeds up to 200 km per hour Forms Welded Tuff http://volcanology.geol.ucsb.edu/pfs.htm

35. A nueé ardente on Mt. St. Helens

36. Location of Montserrat - 1997 Nuée Ardente Plus massive ashfall. Leaves of trees preserved. North American plate’s ocean crust Subducted under Caribbean Plate

37. Montserrat Eruption with Nuée Ardente, 1997 Montserrat Eruption with Nuée Ardente, 1997 Source: Kevin West/Liaison Agency

38. Lahars Pyroclastics on upper slopes may produce a lahar, which is a volcanic mudflow. Heat of volcanics melts ice.

39. Calderas Calderas

40. Caldera of Mt. Mazama now filled by Crater Lake

41. Mt Mazama Eruption and Caldera Collapse 4700 BC S Oregon Ngorongoro Crater in Tanzania similar 2 mya

42. Volcanism on a tectonic plate moving over a hot spot

43. Hey, the plate changed direction ! Flood Basalts Hot Spot currently forming Hawaii

44. Flood Basalts Flood Basalts Fluid basaltic lava extruded from crustal fractures called fissures e.g., Columbia River Plateau, Deccan Traps in India Cover huge areas Plumes from Mantle

45. Lava Plateau Formation

46. Flood Basalt erupted from fissures - Snake River Plain, southern Idaho Plume Activity

47. Volcanic landforms Volcanic landforms Lava Domes Bulbous mass of congealed lava Most are associated with explosive eruptions of silica-rich magma http://vulcan.wr.usgs.gov/Volcanoes/MSH/Images/MSH04/framework.html

48. St Helens Lava Dome Viscous magmas

49. Volcanic landforms Volcanic landforms Volcanic Pipes and Necks Pipes are short conduits that connect a magma chamber to the surface Volcanic necks (e.g., Devils Tower in Wyoming and Ship Rock in New Mexico) are resistant vents left standing after erosion has removed volcanic cone

51. Formation of a volcanic neck

52. Spanish Peaks and Radiating Dikes (southern CO)

53. Plutonic igneous activity Plutonic igneous activity Types of intrusive igneous features Dike – a sheetlike injection into a fracture Discordant - cuts across pre-existing Sill – a sheetlike injection into a bedding plane Concordant - lies parallel to bedding Laccolith – A mushroom-shaped concordant

54. Some intrusive igneous structures Some intrusive igneous structures

55. A sill in the Salt River Canyon, AZ Sill: Sediments above and below sill are baked. Lava Flow, just baked below.

56. Global distribution of magmatism is not randomGlobal distribution of magmatism is not random Most volcanoes are located within or near ocean basinsMost volcanoes are located within or near ocean basins Basalt common in both oceanic and continental settingsBasalt common in both oceanic and continental settings Granite is rare in oceans, mostly found in continentsGranite is rare in oceans, mostly found in continents This result is predicted by a crust melt model of Granite formationThis result is predicted by a crust melt model of Granite formation

57. Why No C-C collisions

58. Plate tectonics and igneous activity Plate tectonics and igneous activity Igneous activity along plate margins Mid-Ocean Ridges – Basaltic Pillow Lavas Great volumes of volcanic rock produced along oceanic ridges – New ocean floor –Mechanism of spreading or “rifting” Lithosphere pulls apart and thins Less pressure results in partial melting in mantle http://www.archipelago.nu/SKARGARD/ENGELSKA/ICELAND/surtsey.htm

59. http://volcanoes.usgs.gov/Products/Pglossary/ancientseq.html Basaltic Pillow Lavas

60. Plate tectonics and igneous activity Igneous activity along Subduction zones –Descending plate partially melts –Magma slowly moves upward –Rising magma can form either A Volcanic Island Arc if ocean-ocean plate collision (Aleutians, Japan, etc.) A Continental Volcanic Arc if ocean- continent plate collision (Sierra Nevada)

61. The Cascades, Washington State

63. Plate tectonics and magmatism Plate tectonics and magmatism Intraplate volcanism Associated with plumes of heat in mantle Form localized volcanic regions in the overriding plate called a hot spot –Produces basaltic magma sources in oceanic crust (Hawaii) –Produces granitic magma sources in continental crust (Yellowstone Park) –These differences are predicted by a Crust-Melting model of Granite generation

65. End of Volcanoes http://www.mt-fuji.co.jp/Photo/Photo.html