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Volcanoes and Other Igneous Activity

Building Earth’s Surface, Part 3

Science 330 Summer 2005

Mount Saint Helens before May 18th 1980…

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Mount Saint Helens after…

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The nature of volcanic eruptions

Characteristics of a magma determine the “violence” or explosiveness of a volcanic eruption

Composition

TemperatureDissolved gases

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

The nature of volcanic eruptions

Viscosity is a measure of a material’s resistance to flowFactors affecting viscosity

Temperature - Hotter magmas are less viscousComposition - Silica (SiO2) content

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

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

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The nature of volcanic eruptions

Dissolved gasesGas content affects magma mobilityGases expand within a magma as it nears the Earth’s surface due to decreasing pressureThe violence of an eruption is related to how easily gases escape from magma

In summaryBasaltic lavas = mild eruptionsRhyolitic or andesitic lavas = explosive eruptions

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Materials extruded from a volcano

Lava flowsBasaltic lavas exhibit fluid behavior

Types of basaltic flowsPahoehoe lava (resembles a twisted or ropey texture)Aa lava (rough, jagged blocky texture)

Dissolved gases1% - 6% by weight

Mainly H2O and CO2

A pahoehoe lava flow

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Aa lava flow

Materials extruded from a volcano

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

Ash and dust - fine, glassy fragmentsPumice - porous rock from “frothy” lavaCinders - pea-sized materialLapilli - walnut-sized material Particles larger than lapilli

Blocks - hardened or cooled lavaBombs - ejected as hot lava

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Bomb is approximately 10 cm long

A volcanic bomb

Volcanoes

General featuresOpening at the summit of a volcano

Crater - summit depression < 1 km diameterCaldera - summit depression > 1 km diameter produced by collapse following a massive eruption

Vent – surface opening connected to the magma chamber

Fumarole – emit only gases and smoke

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Volcanoes

Types of volcanoesShield volcano

Broad, slightly domed-shapedGenerally cover large areasProduced by mild eruptions of large volumes of basaltic lavaExample = Mauna Loa on Hawaii

Anatomy of a shield volcano

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Volcanoes

Cinder coneBuilt from ejected lava (mainly cinder-sized) fragmentsSteep slope angleSmall sizeFrequently occur in groups

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Cinder cone volcano

Volcanoes

Composite cone (stratovolcano)Most are located adjacent to the Pacific Ocean (e.g., Fujiyama, Mt. St. Helens)Large, classic-shaped volcano (1000’s of ft. high and several miles wide at base)Composed of interbedded lava flows and pyroclastic debrisMost violent type of activity (e.g., Mt. Vesuvius)

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Anatomy of a composite volcano

Mt. St. Helens – prior to the 1980 eruption

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Mt. St. Helens after the 1980 eruption

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Profiles of volcanic landforms

Volcanoes

Nuée ardente – A deadly pyroclastic flowFiery pyroclastic flow made of hot gases infused with ash and other debrisAlso known as glowing avalanchesMove down the slopes of a volcano at speeds up to 200 km per hour

Lahar – volcanic mudflowMixture of volcanic debris and waterMove down stream valleys and volcanic slopes, often with destructive results

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A nueé ardente on Mt. St. Helens

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Other volcanic landforms

CalderaSteep-walled depressions at the summitGenerally > 1 km in diameterProduced by collapse

Pyroclastic flowFelsic and intermediate magmasConsists of ash, pumice, and other debrisMaterial ejected at high velocitiesExample = Yellowstone plateau

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Formation of Crater Lake, Oregon

Other volcanic landforms

Fissure eruptions and lava plateausFluid basaltic lava extruded from crustal fractures called fissuresExample = Columbia River Plateau

Lava domesBulbous mass of congealed lavaAssociated with explosive eruptions of gas-rich magma

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A lava dome

Other volcanic landforms

Volcanic pipes and necksPipes - short conduits that connect a magma chamber to the surfaceVolcanic necks (e.g., Ship Rock, New Mexico) - resistant vents left standing after erosion has removed the volcanic cone

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Formation of a volcanic neck

Shiprock, New Mexico

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Intrusive igneous activity

Most magma is emplaced at depth in the Earth

Once cooled and solidified, is called a pluton

Nature of plutonsShape - tabular (sheetlike) vs. massive

Orientation with respect to the host (surrounding) rock

Concordant vs. discordant

Intrusive igneous activity

Types of intrusive igneous featuresDike – a tabular, discordant plutonSill – a tabular, concordant pluton (e.g., Palisades Sill in New York)Laccolith

Similar to a sillLens or mushroom-shaped massArches overlying strata upward

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Igneous structures

A sill in the Salt River Canyon, Arizona

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Intrusive igneous activity

Intrusive igneous features continuedBatholith

Largest intrusive bodySurface exposure > 100+ km2 (smaller bodies are termed stocks)Frequently form the cores of mountains

Batholithsof western

NorthAmerica

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Plate tectonics and igneous activity

Global distribution of igneous activity is not random

Most volcanoes are located within or near ocean basins

Basaltic rocks = oceanic and continental settings

Granitic rocks = continental settings

Distribution of some of the world’s major volcanoes

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Plate tectonics and igneous activity

Igneous activity at plate marginsSpreading centers

Greatest volume of volcanic rock is produced along the oceanic ridge systemMechanism of spreading

Decompression melting of the mantle occurs as the lithosphere is pulled apartLarge quantities of basaltic magma are produced

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Plate tectonics and igneous activity

Subduction zonesOccur in conjunction with deep oceanic trenchesPartially melting of descending plate and upper mantleRising magma can form either

An island arc if in the oceanA volcanic arc if on a continental margin

Associated with the Pacific Ocean BasinRegion around the margin is known as the “Ring of Fire”Majority of world’s explosive volcanoes

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Plate tectonics and igneous activity

Intraplate volcanismOccurs within a tectonic plate

Associated with mantle plumesLocalized volcanic regions in the overriding plate are called a hot spot

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

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Volcanoes and climate

The basic premiseExplosive eruptions emit huge quantities of gases and fine-grained debris A portion of the incoming solar radiation is reflected and filtered out

Past examples of volcanism affecting climate

Mount Tambora, Indonesia – 1815Krakatau, Indonesia – 1883

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Volcanoes and climate

Modern examplesMount St. Helens, Washington - 1980

El Chichón, Mexico - 1815Mount Pinatubo, Phillippines - 1991

End of Chapter