1. Volcanic Eruptions
Chapter 9

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
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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
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4. Volcanic Eruption Example
• Mt. St. Helens, May 18, 1980. Washington State.
• Pyroclastic flows killed ~60 people and wiped out the ecosystem
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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
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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
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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
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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
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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
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10. Lava Composition
1. Lavas with low silica/high Fe and MG are called…
Mafic or basaltic
2. Lavas with moderate silica, Fe and Mg are called..
Intermediate or andesitic
3. Lavas with high silica/low Fe and MG are called
Silicic, felsic, rhyolite
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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)
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12. Pahoehoe
• Pahoehoe (pa-hoy-hoy; Hawaiian word) – type of basalt
• Forms when hot basalt skin cools
• “ropy” texture
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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
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14. Columnar Jointing
• Flows cool/contact with vertical fracture that are
hexagonal in shape
• Columnar jointing- indicates basaltic lava flow
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15. Andesitic Lava Flows
• Higher SiO2 content makes andesitic lavas viscous.
• They mound near vent, flow slowly
• The outer crust fractures, creating rubble
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16. Rhyolitic Lava Flows
• Rhyolite; highest silica – most viscous.
• Rhyolitic lava rarely flows
• Plugs vent as a lava dome
• Sometimes, lava domes later explode
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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
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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
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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
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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
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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
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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
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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
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24. Volcanic Architecture
• Volcanoes have characteristic features:
• A magma chamber
• Fissures and vents
• Craters
• Calderas
• Distinctive topo profile
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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
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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”
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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
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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
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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
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30. Crater Lake Caldera, Oregon
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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
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32. Volcano shape and size
• Magma type governs volcano shape & size.
• Categories:
• 1. shield volcanoes- largest
• 2. cinder cones- smallest
• 3. stratovolcanoes- intermediate
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33. Volcano Types
1. Shield volcanoes:
broad, slightly domed (like inverted shield)
lateral flow of low-viscosity basaltic lava
low slopes and cover large areas
Example: Mauna Loa
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34. Volcano Types
2. 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
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35. Volcano Types
3. 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
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36. Eruptive Style
• Will it flow or blow? Two dominant styles
• Effusive eruptions > flow
• Explosive eruptions > blow
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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
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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
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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
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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
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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
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42. Chapter 9

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
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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
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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
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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
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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
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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
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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
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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 volcanoes
What to do?
understanding volcanic
Behavior is the best defense
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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
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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
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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
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54. Volcanic Hazards
• Blast – rarely, explosions are ejected sideways.
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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
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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
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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
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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
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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
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60. Mitigating Volcanic Hazards
• Danger assessment maps.
• Delineate hazardous areas
• Pyroclastic flows
• Lahars
• Landslides
• Used for planning, zoning
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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. Volcanoes and Climate
• Volcanic aerosols & fine-debris block sunlight.
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