volcanoes and other igneous...
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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