The document outlines different types of volcanic eruptions, volcanic materials produced during eruptions such as lava flows, pyroclastic debris, and volcanic gases. It describes the architecture of volcanoes including features like magma chambers, vents, craters, and calderas. The document also discusses different shapes and sizes of volcanoes, eruption styles linked to tectonic settings, and volcanic hazards posed to humans.

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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