VolcanoesVolcanoes

and Other Igneous Activityand Other Igneous Activity

Types of VolcanoesTypes of Volcanoes

►Three basic typesThree basic types Shield VolcanoShield Volcano Cinder ConeCinder Cone Composite (Stratovolcano)Composite (Stratovolcano)

Shield VolcanoesShield Volcanoes

►CharacteristicsCharacteristics Broad, slightly domed-shapedBroad, slightly domed-shaped Composed primarily of basaltic lavaComposed primarily of basaltic lava Generally cover large areasGenerally cover large areas Produced by mild eruptions of large Produced by mild eruptions of large

volumes of lavavolumes of lava

ExamplesExamplesMauna LoaMauna Loa

Cinder ConesCinder Cones

► CharacteristicsCharacteristics Built from ejected lava (mainly cinder-Built from ejected lava (mainly cinder-

sized) fragmentssized) fragments Steep slope angle (Generally not greater Steep slope angle (Generally not greater

than 30 degrees)than 30 degrees) Rather small sizeRather small size Frequently occur in groupsFrequently occur in groups

ExampleExample

Sunset Crater

Composite VolcanoComposite Volcano►CharacteristicsCharacteristics

Most are located adjacent to the Pacific Ocean (e.g., Fujiyama, Mt. Most are located adjacent to the Pacific Ocean (e.g., Fujiyama, Mt. St. Helens)St. Helens)

Large, classic-shaped volcano (1000’s of ft. high & several miles Large, classic-shaped volcano (1000’s of ft. high & several miles wide at base)wide at base)

Composed of interbedded lava flows and layers of pyroclastic debrisComposed of interbedded lava flows and layers of pyroclastic debris► Most violent type of activity (e.g., Mt. Vesuvius)Most violent type of activity (e.g., Mt. Vesuvius)► Often produce a Often produce a nueé ardentenueé ardente► Fiery Fiery pyroclastic flowpyroclastic flow made of hot gases infused with ash and other made of hot gases infused with ash and other

debrisdebris Move down the slopes of a volcano at speeds up to 200 km per hourMove down the slopes of a volcano at speeds up to 200 km per hour

► May produce a May produce a laharlahar,, which is a volcanic mudflow which is a volcanic mudflow

ExamplesExamples►Mt. FujiMt. Fuji

Why so Different?Why so Different?

►ViscosityViscosity of Magma of Magma Viscosity is a measure of a fluids Viscosity is a measure of a fluids

resistance to flowresistance to flow►Water has a very low viscosityWater has a very low viscosity►Honey has a very high viscosityHoney has a very high viscosity

Viscosity of MagmaViscosity of Magma► Factors affecting viscosityFactors affecting viscosity

Temperature - Hotter magmas are less viscousTemperature - Hotter magmas are less viscous

Composition - Silica (SiO2) contentComposition - Silica (SiO2) content► Higher silica content = higher viscosity Higher silica content = higher viscosity ► Lower silica content = lower viscosity or more fluid-like Lower silica content = lower viscosity or more fluid-like

(Basaltic vs. Felsic magmas)(Basaltic vs. Felsic magmas)

Dissolved GasesDissolved Gases► One to six percent of a magma by weightOne to six percent of a magma by weight► Mostly water and CO2Mostly water and CO2

Gas content affects magma mobilityGas content affects magma mobility Gases expand within a magma as it nears the Earth’s Gases expand within a magma as it nears the Earth’s

surface due to decreasing pressuresurface due to decreasing pressure

So What???So What???

► Viscosity will determine not only how the Viscosity will determine not only how the lava behaves on the surface but also how lava behaves on the surface but also how violent the eruption isviolent the eruption is

**Fluid basaltic lavas generally produce quiet eruptions

**Highly viscous lavas (rhyolite or andesite) produce more explosive eruptions

Lava FlowsLava Flows

►Basaltic lavas are the most fluid and Basaltic lavas are the most fluid and probably the type we are all most probably the type we are all most familiarfamiliar

► Types of basaltic flowsTypes of basaltic flows PahoehoePahoehoe lava (resembles a twisted or ropey lava (resembles a twisted or ropey

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

ExamplesExamplesPahoehoe

ExamplesExamples

► AaAa

What comes out….What comes out….

► Types of pyroclastic debris:Types of pyroclastic debris: Ash and dust - fine, glassy fragments - fine, glassy fragments Pumice - porous rock from “frothy” lava - porous rock from “frothy” lava Lapilli - walnut-sized material - walnut-sized material Cinders - pea-sized material - pea-sized material

► Particles larger than lapilliParticles larger than lapilli Blocks - hardened or cooled lava - hardened or cooled lava Bombs - ejected as hot lava - ejected as hot lava

Bomb

Other Volcanic Landforms:Other Volcanic Landforms:► Fissure eruptionsFissure eruptions and lava plateaus and lava plateaus

Fluid basaltic lava extruded from crustal fractures called fissuresFluid basaltic lava extruded from crustal fractures called fissures e.g., Columbia River Plateaue.g., Columbia River Plateau

► Lava DomesLava Domes Bulbous mass of congealed lavaBulbous mass of congealed lava Most are associated with explosive eruptions of gas-rich magmaMost are associated with explosive eruptions of gas-rich magma

► Volcanic pipes and necksVolcanic pipes and necks Pipes are short conduits that connect a magma chamber to the surfacePipes are short conduits that connect a magma chamber to the surface Volcanic necks (e.g., Ship Rock, New Mexico) are resistant vents left Volcanic necks (e.g., Ship Rock, New Mexico) are resistant vents left

standing after erosion has removed the volcanic conestanding after erosion has removed the volcanic cone

Plutonic ActivityPlutonic Activity

Most magma is emplaced at depth in the EarthMost magma is emplaced at depth in the Earth► An underground igneous body, once cooled and An underground igneous body, once cooled and

solidified, is called a solidified, is called a plutonpluton► Classified by shape and orientation:Classified by shape and orientation:

Shape:Shape:►TabularTabular (sheetlike) (sheetlike)►MassiveMassive

Orientation with respect to the host (surrounding) rockOrientation with respect to the host (surrounding) rock►DiscordantDiscordant – cuts across sedimentary rock units– cuts across sedimentary rock units►ConcordantConcordant – parallel to sedimentary rock units – parallel to sedimentary rock units

Types of intrusive igneous Types of intrusive igneous featuresfeatures

► DikeDike – a tabular, discordant pluton – a tabular, discordant pluton► SillSill – a tabular, concordant pluton – a tabular, concordant pluton

(e.g., Palisades Sill in New York)(e.g., 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

► BatholithBatholith- - Largest intrusive bodyLargest intrusive body Surface exposure of 100+ square Surface exposure of 100+ square

kilometers (smaller bodies are termed kilometers (smaller bodies are termed stocks)stocks)

Frequently form the cores of Frequently form the cores of mountainsmountains

Emplacement of magmaEmplacement of magma

►Magma at depth is much less dense than the Magma at depth is much less dense than the

surrounding rocksurrounding rock►Increased temperature and pressure causes solid Increased temperature and pressure causes solid

rock to deform rock to deform plasticallyplastically►The more buoyant magma pushes aside the host The more buoyant magma pushes aside the host

rock and forcibly rises in the Earth as it deforms the rock and forcibly rises in the Earth as it deforms the “plastic” host rock“plastic” host rock

►At shallower depths, the host rock is cooler At shallower depths, the host rock is cooler and exhibits brittle deformationand exhibits brittle deformation

►Movement of magma here is accomplished by Movement of magma here is accomplished by fractures in the host rock and stopingfractures in the host rock and stoping

Global distribution of Global distribution of igneous activityigneous activity

►Not random!Not random! Most volcanoes are located within or near ocean Most volcanoes are located within or near ocean

basinsbasins►BasalticBasaltic rocks are common in both oceanic and rocks are common in both oceanic and

continental settings, whereas continental settings, whereas graniticgranitic rocks are rarely rocks are rarely found in the oceansfound in the oceans

Global distribution of Global distribution of igneous activityigneous activity

►Spreading centersSpreading centers The greatest volume of volcanic The greatest volume of volcanic

rock is produced along the oceanic rock is produced along the oceanic ridge systemridge system

Mechanism of spreadingMechanism of spreading►Lithosphere pulls apartLithosphere pulls apart►Less pressure on underlying rocks Less pressure on underlying rocks

results in partial melting of mantleresults in partial melting of mantle►Large quantities of basaltic magma Large quantities of basaltic magma

are producedare produced

Global distribution of Global distribution of igneous activityigneous activity

► Subduction zonesSubduction zones Occur in conjunction with deep oceanic trenchesOccur in conjunction with deep oceanic trenches

►Descending plate partially meltsDescending plate partially melts►Magma slowly moves upward Magma slowly moves upward ►Rising magma can form either an island arc if in the ocean or a Rising magma can form either an island arc if in the ocean or a

volcanic arc if on a continental marginvolcanic 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 FireRing of Fire””

Most of the world’s explosive volcanoes are found hereMost of the world’s explosive volcanoes are found here

Global distribution of Global distribution of igneous activityigneous activity

► Intraplate volcanismIntraplate volcanism Activity within a tectonic plateActivity within a tectonic plate Associated with plumes of heat in the mantleAssociated with plumes of heat in the mantle

►Form localized volcanic regions in the overriding plate called a Form localized volcanic regions in the overriding plate called a hot spothot spot

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

Produces granitic magma sources in continental crust (e.g., Produces granitic magma sources in continental crust (e.g., Yellowstone Park)Yellowstone Park)

Yeah and….Yeah and….

► Volcanoes and the ClimateVolcanoes and the Climate Explosive eruptions emit huge quantities of gases and fine-Explosive eruptions emit huge quantities of gases and fine-

grained debris into the atmosphere which filter out and reflect grained debris into the atmosphere which filter out and reflect a portion of the incoming solar radiationa portion of the incoming solar radiation

Examples of volcanism affecting climateExamples of volcanism affecting climate►Mount Tambora, Indonesia – 1815Mount Tambora, Indonesia – 1815

The year without a summerThe year without a summer

►Krakatau, Indonesia – 1883Krakatau, Indonesia – 1883►Mount Pinatubo, Philippines – 1991Mount Pinatubo, Philippines – 1991