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

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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

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• Mt. St. Helens, May 18, 1980. Washington State.• Pyroclastic flows killed ~60 people and wiped out the ecosystem

Volcanic Eruption Example

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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

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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

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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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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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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

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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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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

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Columnar Jointing

• Flows cool/contact with vertical fracture that are hexagonal in shape

• Columnar jointing- indicates basaltic lava flow

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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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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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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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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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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

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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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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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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

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Chapter 9

Volcanic Architecture• Volcanoes have characteristic features:

• A magma chamber• Fissures and vents• Craters• Calderas• Distinctive topo profile

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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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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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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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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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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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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

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Chapter 9

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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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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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

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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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Eruptive Style• Will it flow or blow? Two dominant styles

• Effusive eruptions > flow• Explosive eruptions > blow

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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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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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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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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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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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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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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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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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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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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

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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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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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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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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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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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Volcanic Hazards• Blast – rarely, explosions are ejected sideways.

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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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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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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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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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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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Mitigating Volcanic Hazards

• Danger assessment maps.• Delineate hazardous areas

• Pyroclastic flows• Lahars• Landslides

• Used for planning, zoning

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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

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Volcanoes and Climate• Volcanic aerosols & fine-debris block sunlight.