Geology lecture 10

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Geology lecture 10

  1. 1. Volcanic Eruptions Chapter 9
  2. 2. Outline• Volcanoes -Basics & an example• Eruptions and their products -3 major types, lava flows and composition -Basaltic flows: types, columnar jointing, pillows -Andesitic and rhyolitic flows -Pyroclastics, lahars, and volcanic gas• Architecture of a volcano -Magma chamber, vents, craters, calderas• Volcanoes -Shapes, sizes, types -Eruption styles and tectonic settings -Volcanic hazards Chapter 9 9 Chapter
  3. 3. Volcanics• What is a volcano? • An erupting vent through which moleon rock surfaces • A mountain built from magmatic eruptions• Volcanoes > result form tectonic activity• Volcanoes pose a number of hazards to humans • Mexico City • Seattle, US • Naples, Italy Chapter 9
  4. 4. Volcanic Eruption Example• Mt. St. Helens, May 18, 1980. Washington State. • Pyroclastic flows killed ~60 people and wiped out the ecosystem Chapter 9
  5. 5. Volcanic Eruption Example• Mt. St. Helens – Erupted May 18, 1980, 8:32 A.M. • Earthquake-triggered landslide released pressure • Vertical blast followed by stronger lateral blast • Ash fell in North Dakota Chapter 9
  6. 6. Outline• Volcanoes -Basics & an example• Eruptions and their products -3 major types, lava flows and composition -Basaltic flows: types, columnar jointing, pillows -Andesitic and rhyolitic flows -Pyroclastics, lahars, and volcanic gas• Architecture of a volcano -Magma chamber, vents, craters, calderas• Volcanoes -Shapes, sizes, types -Eruption styles and tectonic settings -Volcanic hazards Chapter 9 9 Chapter
  7. 7. Volcanic Eruptions• Unpredictable, dangerous. • Build and destroy mountains• Eruptions can… • Provide highly productive soils to feed civilization • Can extinguish a civilization in minutes• Eruptions affect climate. • Reduce average global temperature by 1-3 degrees C for a few years Chapter 9
  8. 8. Volcanic Materials• Eruption products take 3 forms: 1. Lava flows – molten rock that moves over ground 2. Pyroclastic debris – fragments blown out of a volcano 3. Volcanic gases – vapor and aerosols that exit a volcano Chapter 9
  9. 9. Lava Flows• Lava can be thin and runny or thick and sticky• Flow type depends on viscosity (due to composition)• Composition depends on silica (SiO2), Fe, and MG Chapter 9
  10. 10. Lava Composition1. Lavas with low silica/high Fe and MG are called… Mafic or basaltic2. Lavas with moderate silica, Fe and Mg are called.. Intermediate or andesitic3. Lavas with high silica/low Fe and MG are called Silicic, felsic, rhyolite Chapter 9
  11. 11. Basaltic Lava Flows• Mafic lava – very hot, low silica, low viscosity• Basalt flows are thin and fluid • Rapid flow (up to 100 km/hr) • Long distance flow (up to 100s km) Chapter 9
  12. 12. Pahoehoe• Pahoehoe (pa-hoy-hoy; Hawaiian word) – type of basalt • Forms when hot basalt skin cools • “ropy” texture Chapter 9
  13. 13. A’a’• A’a’ (ah-ah; also Hawaiian) – basalt that solidifies with a jagged, sharp, angular texture • A’a’ forms when hot flowing basalt cool and thickens • Lava crumbles – “blocky” fragments Chapter 9
  14. 14. Columnar Jointing• Flows cool/contact with vertical fracture that are hexagonal in shape• Columnar jointing- indicates basaltic lava flow Chapter 9
  15. 15. Andesitic Lava Flows• Higher SiO2 content makes andesitic lavas viscous. • They mound near vent, flow slowly• The outer crust fractures, creating rubble Chapter 9
  16. 16. Rhyolitic Lava Flows• Rhyolite; highest silica – most viscous.• Rhyolitic lava rarely flows• Plugs vent as a lava dome• Sometimes, lava domes later explode Chapter 9
  17. 17. Pyroclastic Debris• Material fragments ejected from a volcano.• Glass shards, fragmented lava in a range of sizes • Ash- powdery glass shards • Lapilli- pea-to-plum-sized materials • Blocks and bombs- apple-to-refrigerator sized • Blocks- pre-existing rock torn from the volcano • Bombs- streamlined fragments of ejected lava Chapter 9
  18. 18. Pyroclastic Debris• Tephra – pyroclastic debris deposits. • lapilli and bombs near the vent • Tuff- lithified ash with or without lapilli • Air-fall tuff- accumulations of ash that fell like snow • Welded tuff (ignimbrite)- tuff deposited while still hot • Pyroclastic material fuses while cooling Chapter 9
  19. 19. Pyroclastic Flows• Pyroclastic flows (or, nuée ardentes – french for incandescent cloud): • 200-450 degrees C avalanches of hot ash/lava fragments • Move up to ~300 km/hr; incinerate all in their path • Famous examples: esuvius, Mt. Pelé e Chapter 9
  20. 20. Lahars• Tephra is readily moved by water as debris flows.• Called lahars, these flows are destructive. • Move fast (up to 50 km/hr) • Consistency of wet cement • Hazard to people living in valleys near volcanoes • Triggered by eruption or later by heavy rain Chapter 9
  21. 21. Volcanic Gas• 1-10% of magma may be gas. • Water (H2O)- most abundant gas • Carbon dioxide (CO2)- second most abundant • Sulfur dioxide (SO2)- rotten egg smell• Magma composition controls gas content. • Felsic magmas are gas-rich; mafic magmas less so Chapter 9
  22. 22. Volcanic Gas• Expelled as magma rises (P drops).• Escape style controls eruption violence. • Low viscosity (basalt)- easy escape; effusive eruption • High viscosity (rhyolite)- difficult to escape; explosive eruption• Gas bubbles in rock are called vesticles Chapter 9
  23. 23. Outline• Volcanoes -Basics & an example• Eruptions and their products -3 major types, lava flows and composition -Basaltic flows: types, columnar jointing, pillows -Andesitic and rhyolitic flows -Pyroclastics, lahars, and volcanic gas• Architecture of a volcano -Magma chamber, vents, craters, calderas• Volcanoes -Shapes, sizes, types -Eruption styles and tectonic settings -Volcanic hazards Chapter 9 9 Chapter
  24. 24. Volcanic Architecture• Volcanoes have characteristic features: • A magma chamber • Fissures and vents • Craters • Calderas • Distinctive topo profile Chapter 9
  25. 25. Magma Chamber• Located in upper crust. • Open cavity or area of highly fractured rock • Contains a lot of magma • Some magma cools here to form intrusive rock Chapter 9
  26. 26. Fissures• Some magma rises via a conduit to the surface.• Magma may also erupt along a linear tear (fissure) • Fissure eruptions > “curtain of fire” Chapter 9
  27. 27. Vents• A lava outlet on a volcano• vents can form anywhere on the volcano • Summit vent- located at the top • Flank vent- located on the side Chapter 9
  28. 28. Craters• Crater – a bowl-shaped depression atop a volcano • Up to ~500 m across, ~200 m deep • Form as erupted lava piles up around the vent • Accentuated by summit collapse into conduit Chapter 9
  29. 29. Calderas• A gigantic volcanic depression. • Much larger than a crater • 1-10s km across• Magma chamber empties• Volcano collapses into empty chamber • Crater lake, Oregon • Yellowstone National Park Chapter 9
  30. 30. Crater Lake Caldera, Oregon Chapter 9
  31. 31. Outline• Volcanoes -Basics & an example• Eruptions and their products -3 major types, lava flows and composition -Basaltic flows: types, columnar jointing, pillows -Andesitic and rhyolitic flows -Pyroclastics, lahars, and volcanic gas• Architecture of a volcano -Magma chamber, vents, craters, calderas• Volcanoes -Shapes, sizes, types -Eruption styles and tectonic settings -Volcanic hazards Chapter 9 9 Chapter
  32. 32. Volcano shape and size• Magma type governs volcano shape & size.• Categories: • 1. shield volcanoes- largest • 2. cinder cones- smallest • 3. stratovolcanoes- intermediate Chapter 9
  33. 33. Volcano Types1. Shield volcanoes: broad, slightly domed (like inverted shield) lateral flow of low-viscosity basaltic lava low slopes and cover large areasExample: Mauna Loa Chapter 9
  34. 34. Volcano Types2. Cinder cone – Conical piles of tephra. smallest type build of ejected lapilli-sized fragments piled up at a vent slopes at angle of repose often symmetrical with a deep summit crater Chapter 9
  35. 35. Volcano Types3. Stratovolcanoes (composite volcanoes). large, cone-shaped alternating layers of lava and tephra often symmetric (can be odd shapes form landslides, etc.) examples: Mt. Fuji, Mt. rainier/St. Helens, Mt Vesuvius Chapter 9
  36. 36. Eruptive Style• Will it flow or blow? Two dominant styles • Effusive eruptions > flow • Explosive eruptions > blow Chapter 9
  37. 37. Effusive Eruptions• Lava flows. • Lava flows stream away from vents • Lava lakes can form around the vent • Lava fountains• Commonly basaltic, these eruptions create shield volcanoes Chapter 9
  38. 38. Explosive Eruptions• Produce pyroclastic debris & flows. • Caused by gas pressure in viscous magma • Pressure released suddenly • Create stratovolcanoes, sometimes calderas • Blanket landscape with tephra• Andesitic and rhyolitic compositions Chapter 9
  39. 39. Phreatomagmatic Eruptions• Less common style. • Magma interacts with water • Some can be cataclysmic • Magma chamber breaches and admits water • Water > produces stream, blows volcano apart • Examples: Santorini, Krakatau Chapter 9
  40. 40. Eruptive Style Controls• Viscosity – Controls the ease of lava flow. • Mafic- low viscosity lava flows away from vent • Felsic- high viscosity lava builds up at the vent• Gas Pressure – Greater P favors explosive style. • Mafic- low viscosity allows gas release • Felsic- high viscosity prevents gas release• Environment – Eruption location important. • Subaerial lava flowing on land cools slower than… • Submarine lava, which is quickly quenched Chapter 9
  41. 41. Tectonic Settings• Plate tectonics is a dominant control on volcanism.• Volcanic types are linked to tectonic settings: • Hot sports- where mantle plumes intrude lithosphere • Oceanic and continental hot spots and flood basalts • MORs- spreading axes • Convergent boundaries- subduction zones • Continental rifts- incipient ocean basins Chapter 9
  42. 42. Chapter 9
  43. 43. Oceanic Hot Spots• Plume under an oceanic plate. • Sea floor basalt erupts – forms growing mound • Eventually breaches sea level • Then basalt will not quench and can flow long distances • Lava builds upward/outward, island grows • Submarine slumps remove large masses of the volcano Chapter 9
  44. 44. Continental Hot Spots• Cuts a continental plate. • Often erupts both basaltic and rhyolitic material • Basaltic- from the mantle plume source • Rhyolitic- basalt mixed with the granitic crust passes through Chapter 9
  45. 45. Continental Hot Spots• Continental hot spots – Yellowstone. • Most recent eruption ~640 ka; created a 100 km caldera • 1000xs Mt. St. Helens • Deposited ignimbrites, ash made it to east coast • Magma beneath caldera still fuels geysers/hot springs Chapter 9
  46. 46. Continental Hot Spots• Flood basalts – massive lava eruption above a plume. • Thinned lithosphere erupts magma from fissure • Thick flows spread long distances • With time, a shrinking plume creates “normal” volcanoes Chapter 9
  47. 47. Iceland: unique location• Iceland is a hot spot at a MOR. • Lava has built the hot MOR above sea level • Island is being torn apart by plate notion • Volcanoes traces the MOR rift Chapter 9
  48. 48. Convergent Boundaries• Most volcanoes form at convergent boundaries. • Volatiles from subducting plate causes melting • Arc volcanoes develop on overriding plates • May cut through oceanic or continental crust • “Ring of fire” dominates the Pacific margin Chapter 9
  49. 49. Continental Rifts• Yield an array of volcano types reflecting… • Partial melting of the mantle (mafic magmas) • Partial melting of the crust (felsic melting) • Example: East African rift Chapter 9
  50. 50. Volcanic Hazards• Volcanic eruptions cause great human harm. • Eruptions have influenced human history • In past 2,000 years ~250,000 deaths• Many populated areas ring active volcanoesWhat to do? understanding volcanicBehavior is the best defense Chapter 9
  51. 51. Volcanic Hazards• Lava flows – threats mostly from basalt. • Lava may completely destroy immovable objects • Rare for lava flows to kill people > they move slowly • Usually enough to notice • Sometimes people watching flows are killed Chapter 9
  52. 52. Volcanic Hazards• Tephra – Ash & lapilli fall around the volcano. • Can bury landscapes, kill crops • Tephra is heavy; causes rood collapses • Tephra is gritty; abrades cars/airplane engines • Floods easily move tephra as lahars (mudflows) later Chapter 9
  53. 53. Volcanic Hazards• Pyroclastic flows – aka nuée ardente • Clouds of hot ash and gas that race downslope • 100s km/hr speeds • Deadly to anything in its path Chapter 9
  54. 54. Volcanic Hazards• Blast – rarely, explosions are ejected sideways. Chapter 9
  55. 55. Volcanic Hazards• Landslides – slope failure. • eruptions/earthquakes can trigger landslides • Earthquakes initiate failure of unstable slopes • Mt. St. Helens • Eruption followed a 3 km3 slope failure • Slide material traveled >20 km Chapter 9
  56. 56. Volcanic Hazards• Lahars – mudflows result when water moves ash. • It can carry destroy many things 9people, houses, bridges) • Nevada del Ruiz, Colombia, buried Armero +25,000 people Chapter 9
  57. 57. Volcanic Hazards• Gas – Volcanic gases can be poisonous (H2S, CO2). • Lake Nyos, Cameroon, 1986. • Magmatic CO2 built up in a crater lake • Lave overturned (burped), the CO2… • Moved down the valleys as heavier-than-air underflow • Killed 1,742 people, 6,000 cattle Chapter 9
  58. 58. Active vs. Extinct• Recurrence interval – average time between eruptions • Active – erupting, recently erupted or likely to erupt • Dormant – volcano not erupted in 100-1000s of years (but could still do so) • Extinct – no longer capable of erupting• Tectonic changes can shut off the magma “fuel”• Once extinct, erosion takes over Chapter 9
  59. 59. Predicting Eruptions• Warning signs precede many eruptions. • Earthquake activity- magma flow increases seismicity • Heat flow- magma causes volcanoes to “heat-up” • Volcano shape changes- magma causes expansion • Emission increases- changes in gas mix and volume• Still doesn’t allow accurate eruption prediction Chapter 9
  60. 60. Mitigating Volcanic Hazards• Danger assessment maps. • Delineate hazardous areas • Pyroclastic flows • Lahars • Landslides • Used for planning, zoning Chapter 9
  61. 61. Mitigating Volcanic Hazards• Evacuation – moving those at high risk saves lives • Mt. St. Helens- timey evacuation saved 100s • Sometimes eruptions don’t occur, large expenses• Diverting flows- lava can be diverted • Explosives • Seawater Heimaey, Iceland Chapter 9
  62. 62. Volcanoes and Climate• Volcanic aerosols & fine-debris block sunlight. Chapter 9

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