Lecture OutlinesLecture Outlines

Physical Geology, 14/ePhysical Geology, 14/e

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Plummer, Carlson & Plummer, Carlson & HammersleyHammersley

Volcanism and Extrusive RocksVolcanism and Extrusive RocksPhysical Geology 14/e, Chapter 4Physical Geology 14/e, Chapter 4

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Atmosphere – originally created from gases released from volcanic eruption

Hydrosphere – produced by condensation of volcanic water vapor

Biosphere both positively and negatively influenced by volcanism

• lava flows and ash weather to produce fertile soils

• violent eruptions can destroy nearly all life in their paths

• large amounts of ash and volcanic gases in atmosphere can trigger rapid climate changes and contribute to mass extinctions

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Volcanism & earth’s systemsVolcanism & earth’s systems

Lava – produced when magma reaches Earth’s surface

• explosive eruptions can produce rapidly cooled rock fragments called pyroclasts, size ranges from dust (ash) to boulders (blocks and volcanic bombs)

• calm oozing of magma out of the ground produces lava flows

• pyroclastics and lava flows form extrusive igneous rocks

• lava flows and pyroclasts pile up to form volcanoes

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Volcanic eruptionsVolcanic eruptions

Mythology, religion and volcanoes• Hawaii – Pele, Iceland – Loki

Growth of volcanic islands (Hawaii)Geothermal energy• natural steam harnessed as clean energy resource

Climatic effects• very large eruptions can result in measurable

global cooling resulting crop failures and famines

Volcanic catastrophes• Mt. St. Helens, Pompeii, Krakatoa, Tambora,

Crater Lake

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Living with volcanoesLiving with volcanoes

Violence of eruptions controlled by:• dissolved gases in the magma

• ease/difficulty of gases escaping to atmosphere

Viscosity- a fluid’s resistance to flow•higher silica contents produce higher viscosities•cooler lavas have higher viscosities•amount of dissolved gases, the more dissolved gases, the more fluid the lava

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Eruptive violence & physical Eruptive violence & physical characteristics of lavacharacteristics of lava

Scientific Investigation of Volcanism

• rocks, gases and events from observed eruptions compared to similar lavas elsewhere to infer the nature of past activity

Rock Composition• rhyolite - high silica; lightlight color

• basalt - low silica; darkdark color

• andesite - intermediate silica and color

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Extrusive rocks and gasesExtrusive rocks and gases

Vent - opening through which lava erupts

Crater - basin-like depression over the vent at the summit of the volcano

Caldera - volcanic depression much larger than the original crater, having a diameter of at least 1 km

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Volcanic landformsVolcanic landforms

Shield volcanoes• broad

• gently sloping

• composed of solidified lava flows

• flows often contain lava tubes

Cinder cones • small

• steeply sloping

• composed of a pile of loose cinders

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Types of volcanoesTypes of volcanoes

Composite volcanoes• aka stratovolcanoes• moderately to steeply sloping• constructed of alternating layers of

pyroclastic debris and solidified lava flows• composed primarily of intermediate

composition volcanic rocks (i.e., andesite)• most common type of volcano at

convergent plate boundaries

Volcanic domes• extremely high viscosity, degassed, felsic

lavas (often glassy, e.g., obsidian)

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Types of volcanoesTypes of volcanoes

Flood eruptions• very fluid (basalts)

• extremely large in volume

• create extensive lava plateaus

• eruption times correspond with largest mass extinction events

Submarine eruptions• nearly always basaltic

• mid-ocean ridge eruptions

• pillow basalts

Columbia river flood basalts

Pillow basalts

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Other eruption typesOther eruption types

Shield volcanoes• Venus, Mars, Io

Lava domes• Venus, Moon

Flood eruptions• very fluid (basalts)

• extremely large in volume

• extensive flat lava plains (Moon, Mars, Venus, Mercury?)

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Volcanism on other planetsVolcanism on other planets

End of Chapter 4End of Chapter 4

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