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Chapter 9
Volcanic EruptionsVolcanic Eruptions
Chapter 9
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
Chapter 9
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
• Mt. St. Helens, May 18, 1980. Washington State.• Pyroclastic flows killed ~60 people and wiped out the ecosystem
Volcanic Eruption Example
Chapter 9
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
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
Chapter 9
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
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
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
Lava Composition1. 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
Chapter 9
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
Pahoehoe
• Pahoehoe (pa-hoy-hoy; Hawaiian word) – type of basalt• Forms when hot basalt skin cools• “ropy” texture
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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
Columnar Jointing
• Flows cool/contact with vertical fracture that are hexagonal in shape
• Columnar jointing- indicates basaltic lava flow
Chapter 9
Andesitic Lava Flows• Higher SiO2 content makes andesitic lavas viscous.
• They mound near vent, flow slowly
• The outer crust fractures, creating rubble
Chapter 9
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
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
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
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. Pelée
Chapter 9
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
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
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
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
Chapter 9
Volcanic Architecture• Volcanoes have characteristic features:
• A magma chamber• Fissures and vents• Craters• Calderas• Distinctive topo profile
Chapter 9
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
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
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
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
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
Crater Lake Caldera, OregonCrater Lake Caldera, Oregon
Chapter 9
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
Chapter 9
Volcano shape and size• Magma type governs volcano shape & size.• Categories:
• 1. shield volcanoes- largest• 2. cinder cones- smallest• 3. stratovolcanoes- intermediate
Chapter 9
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
Chapter 9
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
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
Chapter 9
Eruptive Style• Will it flow or blow? Two dominant styles
• Effusive eruptions > flow• Explosive eruptions > blow
Chapter 9
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
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
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
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
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
Chapter 9
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
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
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
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
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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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
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
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
Chapter 9
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
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
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
Volcanic Hazards• Blast – rarely, explosions are ejected sideways.
Chapter 9
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
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
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
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
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
Mitigating Volcanic Hazards
• Danger assessment maps.• Delineate hazardous areas
• Pyroclastic flows• Lahars• Landslides
• Used for planning, zoning
Chapter 9
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
Volcanoes and Climate• Volcanic aerosols & fine-debris block sunlight.