VolcanoesVolcanoesEarth’s Interior in Action!Earth’s Interior in Action!

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• Volcanism:Volcanism:all phenomena associated with the origin and all phenomena associated with the origin and

movement of molten rockmovement of molten rock

⇐A volcanologist geared up for work

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What Constitutes AnWhat Constitutes An“Active” Volcano?“Active” Volcano?

• Not fully agreed upon among scientists• Intervals between eruptions may be thousands

of years• On land, close to 1500 have erupted in the past

10,000 years– Millions, if you count undersea eruptions

• Over a thousand active magma systems have been identified on land

• An accurate count of the world’s volcanoes remains elusive

Chaiten Volcano(southern Chile)

May 3, 2008—Chaiten volcano erupts May 2nd and thousands flee; ash from the volcano reaches Esquel, Argentina; last recorded eruption: over 9000 years ago (7420 BC ± 75 yrs)

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What’s that stuff on the ground?What’s that stuff on the ground?

• Lava– Magma that makes it to the

surface

• Pyroclastic material– When lava erupts into the air

and cools quickly, it can form material the size of ash up to large rocks

– Any of this solidified lava “spew” is called pyroclastics (pyro = fire; clast = rock)

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EruptionsEruptions

• Whether a volcanic eruption is explosive or mild depends on the type of magma chemistry that is involved.

– Felsic = explosive

– Mafic = mild

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Structures Associated With Structures Associated With Extrusive VolcanismExtrusive Volcanism

• Volcanic peaks

• Caldara

• Volcanic neck

• Flood basalt

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Volcanic PeaksVolcanic Peaks• The type of peak that forms depends on

the type of magma– Felsic

• Forms composite (strato) volcanoes– Magmas don’t flow far– Tall, steep-sided cones

– Mafic• Forms shield volcanoes

– Magmas flow for long distances– Broad, gently-sloping cones

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Composite VolcanoComposite Volcano(also called a Stratovolcano)(also called a Stratovolcano)

• Felsic magma, tall, steep-sided cone formed of alternating layers of lava flows and pyroclastic material

Mount Mayon

Philippines, Island of Luzon, province of Albay, Bicol region, 15 km NW of Legazpi City

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Shield VolcanoShield Volcano

• Mafic magma (basaltic), gentle slope (looks like a warrior’s shield laid on the ground)

Mauna Loa Shield Volcano

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Other Types of Volcanic PeaksOther Types of Volcanic Peaks

• Lava Dome—Formed from felsic magmas, like a bulge on the surface– Common where previous felsic eruptions have

occurred– Material not well fused together (easily eroded)

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Lava domeLava domeNew lava dome forming inside Mount St. Helens

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Other Types of Volcanic PeaksOther Types of Volcanic Peaks

• Cinder Cone– Usually less than 150 ft. high– Associated with flood basalts and shield

volcanoes– Frothy, (usually) mafic magma under high

pressure at a narrow vent– Tephra rains around the vent, forming small,

rounded volcanoes made of ash and cinders

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Cinder conesCinder cones

Wizard Island—Crater Lake, OR

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CalderaCaldera

• Collapsed and/or exploded volcanic peak, producing a crater

Long Valley Caldera in California

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Volcanic NeckVolcanic Neck((Shiprock, NM)Shiprock, NM)

• Solidified magma within the “neck” of a volcano• Surrounding material may later be eroded away,

leaving a tall, standing structure of igneous rock

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Flood BasaltsFlood Basalts(also called Fissure Eruptions)(also called Fissure Eruptions)

• No dome or cone-shaped structure• Magma flows out of breaks in the crust, usually

in high volume, in layers that may be hundreds of feet deep and thousands of miles wide– Deccan Plateau in India

• 200,000mi2

– Columbia Plateau, crossing WA, OR, and ID borders• 50,000mi2

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Flood BasaltFlood BasaltColumbia Gorge, along the Columbia River in the Cascade Mountains of Washington and Oregon

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Hazards Associated withHazards Associated withExtrusive Volcanism Extrusive Volcanism

• Volcanic blast• Earthquakes• Avalanches and

Debris Flows• Mudflows and

Lahars

• Eruption column and ashfall

• Pyroclastic flows• Lava flows• Volcanic gases

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Volcanic BlastVolcanic Blast(also called Blowdown)(also called Blowdown)

From Mount St. Helens:• A 600ºF blast of rocks, ash, and gases swept

across the land at 670 miles an hour• The force of the blast

stripped trees from hillsides 6 mi. away

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Earthquakes and Debris flowsEarthquakes and Debris flows

• The movement of magma up through the crust creates earthquakes

• The Mount St. Helens eruption began with a magnitude 5.1 earthquake. The entire north flank of the mountain broke loose in three separate blocks, which slid down the mountain at 100 mph.

• An avalanche of rock, ice, snow, and soil, like this one, is called a debris avalanche

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Debris FlowDebris Flow

• Rocks and mounds of debris filled river valleys for 14 miles, as much as 600 feet deep in some places, damming streams and forming new lakes

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Mudflows and LaharsMudflows and Lahars

• Superheated ash and magmas melt and mix with snow and ice, then speed down the volcano’s flanks– Nevado del Ruiz,

Colombia, 1985– Melted ice and snow mixed

with volcanic ash and mud, sending a 130 foot (40 m) high mud-flow down the Lagunilla River

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Mudflows—Mt. St. HelensMudflows—Mt. St. Helens• …sloshing from side-to-side as it rushes through

forests and clearcuts, ripping trees, houses, and bridges from the ground, devastating downstream environments and communities

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Eruption Column and AshfallEruption Column and Ashfall

• Can reach the stratosphere, where it is transported long distances

• Can block insolation, altering weather and climate

• Ash and gases mix with water in atmosphere, producing acid precipitation

• May produce enough ash to bury the landscape and kill residents

• Can cause electrical failure in jet engines

Pyroclastic Flows (Nueé Ardant)

• Swift, destructive cloud of hot ash and gases that flows rapidly downhill and burns all in its path

– Ash weighs down the gases, which would otherwise rise into the atmosphere

– Nueé Ardant—French for “glowing avalanche”• Mont Pelée in Martinique

Mt. Mayon, Island of Luzon, Phillipeans, 1984

Mt. Pelée, Island of Martinique, CaribbeanMt. Pelée, Island of Martinique, CaribbeanMay 8,1902—The town of St. Pierre was obliterated May 8,1902—The town of St. Pierre was obliterated

by a nuée ardante; over 28,000 lost their livesby a nuée ardante; over 28,000 lost their lives

St. Pierre today….

Mt. Pelée, Island of Martinique, CaribbeanMt. Pelée, Island of Martinique, CaribbeanMay 8,1902—The town of St. Pierre was obliterated May 8,1902—The town of St. Pierre was obliterated

by a nuée ardante; over 28,000 lost their livesby a nuée ardante; over 28,000 lost their lives

St. Pierre today….

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Pyroclastic FlowPyroclastic Flow

From Mount St. Helens:• Within a few hours of

the lateral blast, hot mixtures of volcanic gas, pumice, and ash swept down the north flank of the volcano at speeds up to 100 miles an hour and temperatures of over 1200ºF

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Soufriere Hills Pyroclastic FlowSoufriere Hills Pyroclastic Flow

The Soufrière Hills Volcano, Montserrat, West Indies, began erupting on July 18, 1995.

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Soufriere Hills Pyroclastic FlowSoufriere Hills Pyroclastic Flow

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Lava FlowsLava Flows

• Can travel long distances, burning and burying everything

• Lava does not need to actually touch an object to set it on fire

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Volcanic GasesVolcanic Gases

• Volcanic gases, some of which are colorless and odorless like CO2, can cause suffocation, killing plants, animals, and humans alike

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Volcanic GasesVolcanic Gases

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Volcanic GasesVolcanic Gases

• Such an incident happened at Mammoth Mountain in early 2006, where CO2 emanating from faults at the edge of Long Valley Caldera killed one member of the local ski patrol

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Volcanic GasesVolcanic Gases• Ash and gases (such as SO2—sulfur dioxide) can mix

with cloud droplets near the ground to form “vog” (a volcanic fog that causes health problems)

• Hydrogen from lava combines with chlorine in sea water to form hydrochloric acid, which becomes airborne as steam, forming corrosive lava haze, or “laze”

Hawaiian vog

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Structures Associated WithStructures Associated WithIntrusive VolcanismIntrusive Volcanism

• Pluton--a massive body of intrusive igneous rock which solidifies deeply within the crust

• Batholith• Laccolith• Stock

• Dike• Sill• Vein

Intrusive Igneous Structuresand Their Formation

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Structures Associated WithStructures Associated WithIntrusive VolcanismIntrusive Volcanism

• Batholith— >40 mi2 (100 km2) in diameter, amorphous, forms deep in the crust– granite often forms batholiths

• Sierra Nevada Mountains

• Stock—A few mi2 in diameter, amorphous, forms deep in the crust, may be an offshoot of a batholith

• Laccolith—Similar to a stock, but intrudes just beneath the surface, warping surface rocks and forming a hill; may form the base of small mountain chains– The Black Hills, South Dakota– Devil’s Tower, WY

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Batholiths inBatholiths inWestern North AmericaWestern North America

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Laccolith:Laccolith:Devil’s Tower, WYDevil’s Tower, WY

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DikeDike

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SillSill

Salisbury Craig, Edinburgh, Scotland

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SillSill

The Purcell Sill, Glacier National Park, MT

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VeinsVeins