chapter 5 volcanoes and other igneous activity

Chapter 5 Chapter 5

Volcanoes and Volcanoes and Other Igneous Other Igneous

ActivityActivity

The nature of The nature of volcanic eruptionsvolcanic eruptions

• Characteristics of a magma determine Characteristics of a magma determine the “the “violenceviolence” or explosiveness of a ” or explosiveness of a volcanic eruptionvolcanic eruption

• Composition Composition • TemperatureTemperature• Dissolved gasesDissolved gases

• The above three factors actually The above three factors actually control the viscosity of a given magmacontrol the viscosity of a given magma


• ViscosityViscosity is a measure of a material’s resistance is a measure of a material’s resistance to flowto flow

• Factors affecting viscosityFactors affecting viscosity• Temperature - Hotter magmas are less Temperature - Hotter magmas are less

viscousviscous• Composition - Silica (SiOComposition - Silica (SiO22) content) content

– Higher silica content = higher viscosity Higher silica content = higher viscosity (e.g., felsic lava such as rhyolite)(e.g., felsic lava such as rhyolite)

– Lower silica content = lower viscosityLower silica content = lower viscosity (e.g., mafic lava such as basalt)(e.g., mafic lava such as basalt)


• Dissolved gasesDissolved gases– Gas content affects magma mobilityGas content affects magma mobility– Gases expand within a magma as it nears Gases expand within a magma as it nears

the Earth’s surface due to decreasing the Earth’s surface due to decreasing pressurepressure

– The violence of an eruption is related to The violence of an eruption is related to how easily gases escape from magmahow easily gases escape from magma

• In summaryIn summary– Basaltic lavas = mild eruptionsBasaltic lavas = mild eruptions– Rhyolitic or andesitic lavas = explosive Rhyolitic or andesitic lavas = explosive

eruptionseruptions

Materials extruded Materials extruded from a volcanofrom a volcano

• Lava flowsLava flows• Basaltic lavas exhibit fluidBasaltic lavas exhibit fluid behaviorbehavior• Types of basaltic flowsTypes of basaltic flows

– PahoehoePahoehoe lava (resembles a twisted or lava (resembles a twisted or ropey texture)ropey texture)

– AaAa lava (rough, jagged blocky texture) lava (rough, jagged blocky texture)

• Dissolved gasesDissolved gases• 1% - 6% by weight1% - 6% by weight• Mainly HMainly H22O and COO and CO22

A pahoehoe lava flowA pahoehoe lava flow

Aa lava flowAa lava flow

Materials extruded Materials extruded from a volcano from a volcano

• Pyroclastic materials – “fire fragments”• Types of pyroclastic debris

– Ash and dust - fine, glassy fragments– Pumice - porous rock from “frothy” lava– Cinders - pea-sized material– Lapilli - walnut-sized material – Particles larger than lapilli

» Blocks - hardened or cooled lava» Bombs - ejected as hot lava

• Eruption of Kilauea Volcano in Hawaii

A volcanic bombA volcanic bomb

Bomb is approximately 10 cm long

VolcanoesVolcanoes

• General featuresGeneral features• Opening at the summit of a volcanoOpening at the summit of a volcano

– CraterCrater - summit depression < 1 km diameter - summit depression < 1 km diameter– Caldera Caldera - summit depression > 1 km - summit depression > 1 km

diameter produced by collapse following a diameter produced by collapse following a massive eruptionmassive eruption

• VentVent – surface opening connected to the – surface opening connected to the magma chambermagma chamber

• FumaroleFumarole – emit only gases and smoke – emit only gases and smoke

VolcanoesVolcanoes

• Types of volcanoesTypes of volcanoes• Shield volcanoShield volcano

– Broad, slightly domed-shapedBroad, slightly domed-shaped– Generally cover large areasGenerally cover large areas– Produced by mild eruptions of large Produced by mild eruptions of large

volumes of basaltic lavavolumes of basaltic lava– Example = Mauna Loa on HawaiiExample = Mauna Loa on Hawaii

Anatomy of a shield volcanoAnatomy of a shield volcano

• JDR Life Goal #38. JDR Life Goal #38. Climb Mauna Kea (13,796 ft) shield Climb Mauna Kea (13,796 ft) shield volcano in Hawaii. Highest mountain on the planet volcano in Hawaii. Highest mountain on the planet from base to crest (almost 30,000 ft). Completed with from base to crest (almost 30,000 ft). Completed with my son Jonathan in May 2000. This shows profile of my son Jonathan in May 2000. This shows profile of Mauna Loa shield volcano, as seen from summit of Mauna Loa shield volcano, as seen from summit of Mauna Kea (benchmark at lower right). Mauna Kea (benchmark at lower right).

Profiles of volcanic Profiles of volcanic landformslandforms

VolcanoesVolcanoes

• Cinder conesCinder cones– Built from ejected Built from ejected

lava (mainly cinder-lava (mainly cinder-sized) fragmentssized) fragments

– Steep slope angleSteep slope angle– Small sizeSmall size– Frequently occur in Frequently occur in

groupsgroups

Cinder cone volcanoCinder cone volcano

The Paricutin volcano in Mexico erupted in a corn field in 1943, burying The Paricutin volcano in Mexico erupted in a corn field in 1943, burying the entire town. the entire town.

VolcanoesVolcanoes

• Composite coneComposite cone (stratovolcano) (stratovolcano)– Most are located adjacent to the Pacific Most are located adjacent to the Pacific

Ocean (e.g., Fujiyama, Mt. St. Helens)Ocean (e.g., Fujiyama, Mt. St. Helens)– Large, classic-shaped volcano (1000’s of ft. Large, classic-shaped volcano (1000’s of ft.

high and several miles wide at base)high and several miles wide at base)– Composed of interbedded lava flows and Composed of interbedded lava flows and

pyroclastic debrispyroclastic debris– Most violent type of activity (e.g., Mt. Most violent type of activity (e.g., Mt.

Vesuvius)Vesuvius)

Anatomy of a Anatomy of a composite volcanocomposite volcano

Seamounts over hot spotsSeamounts over hot spots

Volcanoes over hot spots have spawned several prominent chains of islands in the Pacific Ocean, as shown here.

These volcanoes become increasing younger preceding southerly, then southeasterly. Most of their mass lies unseen, below the current sea level.

The The Hawaiian Hawaiian IslandsIslands

As the Pacific Plate moves WNW at a rate of about 6 mm/yr, magma rises through the thin oceanic crust, causing volcanoes. The unrelenting erosion of waves tends to plane off the islands, unless protected by coral reefs

The most active area is currently the Kilauea Rift along the southeastern coast of the island of Hawaii. Here large slump blocks produce tensile scarps, which allows molten lava to flow up to the surface more easily, loading the head of the slumps. Loihi is the next island forming.

The Pali The Pali EscarpmentEscarpment

The Pali Escarpment across Molokai and Oahu is a gigantic landslide headscarp, formed when the northern side of those islands detached itself and slid into the ocean, likely on a layer of altered volcanic ash

Bathymetry of the detachmentsBathymetry of the detachments

Bathymetry surveys off the north coasts of Oahu and Molokai, showing enormous debris fields

Massive subaqueous landslide debris fields extending around the Hawaiian Islands, initially identified by J. G. Moore of the U.S. Geological Survey in the n1980s, which working on the 200 miles economic exclusions zone around the islands.

Cascade Cascade VolcanoesVolcanoes

• There are 13 potentially active volcanoes in the Cascade Range of the northwest-ern United States

Mt. St. Helens – prior Mt. St. Helens – prior to the 1980 eruptionto the 1980 eruption

Mt. St. Helens after Mt. St. Helens after the 1980 eruptionthe 1980 eruption

• Precursory stages of the Mt St Helens eruption

• A seismically-induced landslide reduced lateral and vertical A seismically-induced landslide reduced lateral and vertical confinement, triggering the May 18, 1980 eruption of Mt. St confinement, triggering the May 18, 1980 eruption of Mt. St HelensHelens

• Cross sections thru Mt St Helens showing deep rotational slide blocks which slid off the peak

• Atmospheric impacts of the May 18, 1980 Mt St Helens eruption

• The Mt St Helens eruption shot volcanic ash to an The Mt St Helens eruption shot volcanic ash to an altitude of 60,000 feet, into the stratospherealtitude of 60,000 feet, into the stratosphere

• Relative distribution of blown down trees and lahar debris flows in the Toutle River Valley on the north side of Mt St Helens

The St Helens blast flattened Douglas fir trees over an area of 400 The St Helens blast flattened Douglas fir trees over an area of 400 square kilometerssquare kilometers

• Lahars, debris flows, and debris chocking of rivers caused by the Mt. St. Helens eruption

VolcanoesVolcanoes

• NuNuéée ardentee ardente – A deadly pyroclastic – A deadly pyroclastic flowflow

» Fiery pyroclastic flow made of hot gases Fiery pyroclastic flow made of hot gases infused with ash and other debrisinfused with ash and other debris

» Also known as glowing avalanchesAlso known as glowing avalanches» Move down the slopes of a volcano at Move down the slopes of a volcano at

speeds up to 200 km per hourspeeds up to 200 km per hour

• LaharLahar – volcanic mudflow – volcanic mudflow» Mixture of volcanic debris and waterMixture of volcanic debris and water» Move down stream valleys and volcanic Move down stream valleys and volcanic

slopes, often with destructive resultsslopes, often with destructive results

A nueA nueé é ardente ardente

on on Mt. St. Mt. St. Helens Helens

after the after the May May 1980 1980

eruptioneruption

• Viscous lava flow passing through the village Viscous lava flow passing through the village of Goma in the Congo during the eruption of of Goma in the Congo during the eruption of Mt. Nyiragongo in January 2002 Mt. Nyiragongo in January 2002

Other volcanic landformsOther volcanic landforms

• CalderaCaldera• Steep-walled depressions at the summitSteep-walled depressions at the summit• Generally > 1 km in diameterGenerally > 1 km in diameter• Produced by collapseProduced by collapse

• Pyroclastic flowPyroclastic flow• Felsic and intermediate magmasFelsic and intermediate magmas• Consists of ash, pumice, and other debrisConsists of ash, pumice, and other debris• Material ejected at high velocitiesMaterial ejected at high velocities• Example = Yellowstone plateauExample = Yellowstone plateau

Formation of Formation of Crater Lake, OregonCrater Lake, Oregon

Other volcanic landformsOther volcanic landforms

• Fissure eruptionsFissure eruptions and and lava plateauslava plateaus• Fluid basaltic lava extruded from crustal Fluid basaltic lava extruded from crustal

fractures called fissuresfractures called fissures• Example = Columbia River PlateauExample = Columbia River Plateau

• Lava domesLava domes• Bulbous mass of congealed lavaBulbous mass of congealed lava• Associated with explosive eruptions of Associated with explosive eruptions of

gas-rich magma gas-rich magma

A lava domeA lava dome

Other volcanic landforms

• Volcanic pipes and necksVolcanic pipes and necks• PipesPipes - short conduits that connect a - short conduits that connect a

magma chamber to the surfacemagma chamber to the surface• Volcanic necksVolcanic necks (e.g., Ship Rock, New (e.g., Ship Rock, New

Mexico) - resistant vents left standing Mexico) - resistant vents left standing after erosion has removed the volcanic after erosion has removed the volcanic conecone

Formation of a Formation of a volcanic neckvolcanic neck

The most famous volcanic neck The most famous volcanic neck in the United States is in the United States is Shiprock, New MexicoShiprock, New Mexico

JDR Life Goal #54: Climb Shiprock, New Mexico, 1700’ climb, to elevation 7,178 ft, in Four Corners area . Completed in June 1973 (after climb, discovered that climbing had been outlawed in 1970).

Intrusive igneous activityIntrusive igneous activity

• Most magma is emplaced at depth in Most magma is emplaced at depth in the Earththe Earth

• Once cooled and solidified, is called a Once cooled and solidified, is called a plutonpluton

• Nature of Nature of plutonsplutons• Shape - tabular (sheetlike) vs. massiveShape - tabular (sheetlike) vs. massive• Orientation with respect to the host Orientation with respect to the host

(surrounding) rock(surrounding) rock– ConcordantConcordant vs. vs. discordantdiscordant

Intrusive igneous activityIntrusive igneous activity• Types of intrusive igneous featuresTypes of intrusive igneous features

• DikeDike – a tabular, discordant pluton– a tabular, discordant pluton• Sill Sill – a tabular, concordant pluton (e.g., – a tabular, concordant pluton (e.g.,

Palisades Sill in New York)Palisades Sill in New York)• LaccolithLaccolith

– Similar to a sillSimilar to a sill– Lens or mushroom-shaped massLens or mushroom-shaped mass– Arches overlying strata upwardArches overlying strata upward

Igneous Igneous StructuresStructures

• Relationships between volcanism and intrusive igneous activity. Cross cutting intrusions are called dikes, while those emplaced parallel to structure are called sills.

• Diabase dike cutting thru preCambrian age Hakatai Shale at Hance Diabase dike cutting thru preCambrian age Hakatai Shale at Hance Rapid in the Grand Canyon.Rapid in the Grand Canyon. JDR Life Goal# 40. JDR Life Goal# 40. Row a rubber raft down the Colorado River through the rapids of Grand Canyon from Glen Canyon Dam to Lake Mead . Completed in June-July 1978. Rowed it again in 1983, twice in 1984, and 1985.

A sill in the Salt River A sill in the Salt River Canyon, ArizonaCanyon, Arizona

Intrusive igneous activityIntrusive igneous activity

• Intrusive igneous features continuedIntrusive igneous features continued• BatholithBatholith

– Largest intrusive bodyLargest intrusive body– Surface exposure > 100+ kmSurface exposure > 100+ km2 2 (smaller (smaller

bodies are termed stocks)bodies are termed stocks)– Frequently form the cores of mountainsFrequently form the cores of mountains

ClassicClassic Igneous BatholithIgneous Batholith

• When batholiths are uplifted and exposed, they are usually resistant strata that form the roots of mountain ranges or eroded highlands.

Batholithsof western

NorthAmerica

Plate tectonics and Plate tectonics and igneous activityigneous activity

• Global distribution of igneous activity Global distribution of igneous activity is not randomis not random

• Most volcanoes are located within or Most volcanoes are located within or near ocean basinsnear ocean basins

• Basaltic rocksBasaltic rocks = oceanic and continental = oceanic and continental settingssettings

• Granitic rocksGranitic rocks = continental settings = continental settings

Distribution of some of the Distribution of some of the world’s major volcanoes world’s major volcanoes


• Igneous activity at plate marginsIgneous activity at plate margins• Spreading centersSpreading centers

– Greatest volume of volcanic rock is Greatest volume of volcanic rock is produced along the oceanic ridge systemproduced along the oceanic ridge system

– Mechanism of spreadingMechanism of spreading» Decompression melting of the mantle Decompression melting of the mantle

occurs as the lithosphere is pulled apartoccurs as the lithosphere is pulled apart» Large quantities of basaltic magma are Large quantities of basaltic magma are

producedproduced

Volatiles driven from Volatiles driven from subducting crust subducting crust lower the melting lower the melting temperature of these temperature of these rocks, and they rise rocks, and they rise by density contrast.by density contrast.


• Subduction zonesSubduction zones– Occur in conjunction with deep oceanic trenchesOccur in conjunction with deep oceanic trenches– Partially melting of descending plate and upper mantlePartially melting of descending plate and upper mantle– Rising magma can form eitherRising magma can form either

» An island arc if in the oceanAn island arc if in the ocean» A volcanic arc if on a continental marginA volcanic arc if on a continental margin

– Associated with the Pacific Ocean BasinAssociated with the Pacific Ocean Basin» Region around the margin is known as the Region around the margin is known as the “Ring of “Ring of

Fire”Fire” » Majority of world’s explosive volcanoesMajority of world’s explosive volcanoes

• A. Rising mantle plume; B. Rapid decompression melting producing flood basalts; and C. Rising plume tail produced by linear seafloor volcanic chain


• Intraplate volcanismIntraplate volcanism• Occurs within a tectonic plateOccurs within a tectonic plate• Associated with mantle plumesAssociated with mantle plumes• Localized volcanic regions in the Localized volcanic regions in the

overriding plate are called a overriding plate are called a hot spothot spot– Produces basaltic magma sources in Produces basaltic magma sources in

oceanic crust (e.g., Hawaii and Iceland)oceanic crust (e.g., Hawaii and Iceland)– Produces granitic magma sources in Produces granitic magma sources in

continental crust (e.g., Yellowstone Park)continental crust (e.g., Yellowstone Park)

• Global distribution of flood basalt provinces and associated hot spots in the Earth’s crust. Some of these formed in failed continental rifts, like Siberia and the Keweenawan Rift in the USA

Volcanoes and climateVolcanoes and climate

• The basic premiseThe basic premise• Explosive eruptions emit huge Explosive eruptions emit huge

quantities of gases (SOquantities of gases (SO22) and fine-) and fine-grained debris grained debris

• A portion of the incoming solar A portion of the incoming solar radiation is reflected and filtered outradiation is reflected and filtered out

• Past examples of volcanism affecting Past examples of volcanism affecting climateclimate

• Mount Tambora, Indonesia – 1815Mount Tambora, Indonesia – 1815• Krakatau, Indonesia – 1883Krakatau, Indonesia – 1883

Volcanoes and climateVolcanoes and climate

• Modern examplesModern examples• Mount St. Helens, Washington - 1980Mount St. Helens, Washington - 1980• El ChichEl Chichóón, Mexico - 1815n, Mexico - 1815• Mount Pinatubo, Phillippines - 1991Mount Pinatubo, Phillippines - 1991

Pinatubo Pinatubo Eruption June Eruption June

19911991• Map showing

areal distribution of pyroclastic flows of June 1991 and destructive lahars that ensued in September 1991, killing many more people than the eruption

• Sulfer dioxide emissions of large volcanic eruptions from 1979-91, in thousands and millions of tons

• Impact of SO2 emissions on global climate, caused by formation of H2SO4 aerosol, deflecting radiant energy from the Sun into the Stratosphere

chapter 5 volcanoes and other igneous activity

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