Lecture Outlines Physical Geology, 15/e

Lecture Outlines

Physical Geology, 15/e

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Plummer, Carlson & Hammersley

Volcanism and Extrusive RocksPhysical Geology 15/e, Chapter 4

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Atmosphere – originally created from gases releasedfrom volcanic eruption

Hydrosphere – produced by condensation of volcanicwater vapor

Biosphere both positively and negatively influenced byvolcanism

• 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 cantrigger rapid climate changes and contribute to mass extinctions

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Volcanism & Earth’s systems

Volcanism – occurs when magmamakes its way to the Earth’s surface

Volcanoes – landforms formed by theextrusion of lava.

Lava – produced when magmareaches Earth’s surface

Explosive eruptions – producesrapidly cooled rock fragments calledpyroclasts, size ranges from dust (ash) toboulders (blocks and volcanic bombs)

Effusive eruptions - calm oozing ofmagma out of the ground produces lavaflows

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What are Volcanoes andWhy Should We Study Them

Creation of New Land•Lava flows build up volcanic islands like HawaiiGeothermal Energy•Underground heat generated by igneous activityEffect on Climate• Very large eruptions can result in measurable

global cooling resulting in crop failures andfamines

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What are Volcanoes andWhy Should We Study Them

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, themore fluid the lava

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Eruptive Violence andPhysical Characteristics of Lava

Effusive Eruptions

• Mafic Lava Flows – low viscosity andflows easily

• Pahoehoe, A’a, Lava Tube

• Flood Basalts – very low viscosity andflows very easily from erupting fissures

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The Eruptive Products of Volcanoes

Pahoehoe

A’a

Effusive Eruptions cont.

• Columnar Jointing – parallel mostly six-sided vertical columns

• Submarine Lava Flows – pillow structure formed as lava flowsinto water.

• Intermediate and Felsic Lava Flows – thicker viscous lavasthat flow short distances

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The Eruptive Products of Volcanoes

Explosive Eruptions

• Pyroclastic Materials

• Dust, ash, cinders, lapilli, blocks and bombs

• Pyroclastic Flows

• mixture of gas and pyroclastic debris that flows rapidlydown slope.

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The Eruptive Products of Volcanoes

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 loosecinders

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Types of Volcanoes

Composite Volcanoes• moderately to steeply sloping

• constructed of alternating layers ofpyroclastic debris and solidified lava flows

• composed primarily of intermediatecomposition volcanic rocks (i.e., andesite)

• most common type of volcano atconvergent plate boundaries

Distribution of Composite Volcanoes• Circum-Pacific belt• Mediterranean belt

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Types of Volcanoes

Lava Domes

• extremely high viscosity,degassed, felsic lavas (oftenglassy, e.g., obsidian)

Calderas – volcanic depression atleast 1 km in diameter

• Result from very violenteruptions

• Crater Lake in Oregon is anexample

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Types of Volcanoes

Volcanic Hazards

• Pyroclastic flows – account forthe largest number of deadlyevents - Pompeii

• Volcanic gases – carbon dioxide,Nyos Cameroon

• Volcanic mudflows (Lahars),Armero Colombia

• Indirect hazards such as famineand lightning

• Eruption times correspond withlargest mass extinction events

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Living with Volcanoes

Pompeii and Mt. Vesuvius

Monitoring Volcanoes

• Volcano status – Approximately 1500 potentially activevolcanoes worldwide

• Active – if currently or recently eruptive (Approximately500 in the world today)

• Dormant – if it hasn’t erupted in many thousands of yearsbut is expected to erupt in the future

• Extinct – haven’t erupted in many years and show nosigns of any future eruptions.

• Hawaiian Volcano Observatory

• Cascade Volcano Observatory

• Hazard mapping

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Living with Volcanoes

• Shield Volcanoes

• Venus, Mars, Io

• Mons Olympus

• 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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Extraterrestrial Volcanic Activity

Volcanic Activity at Divergent Boundaries

• Decompression Melting

• Effusive eruptions of basaltic magmas and pillow lavas

• Formation of most of the sea floor

• Mid-oceanic ridges, Iceland

Volcanic Activity at Convergent Boundaries

• Most large well-known volcanoes

• Explosive composite volcanoes

• Viscous andesitic lavas

Within-Plate Volcanic Activity

• Mantle Plumes (Hot Spots) – Hawaii, Yellowstone

• Basaltic magma/lava

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Plate Tectonics and Volcanism

End of Chapter 4

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