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Lecture Outlines
Physical Geology, 14/e
Plummer, Carlson & Hammersley
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Volcanism and Extrusive Rocks
Physical Geology 14/e, Chapter 4
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Volcanism & earth’s systems
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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Volcanic eruptions
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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Living with volcanoes
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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Eruptive violence & physical
characteristics of lava
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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Extrusive rocks and gases
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; light color
• basalt - low silica; dark color
• andesite - intermediate silica and color
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Volcanic landforms
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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Types 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 loose
cinders
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Types 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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Other eruption types
Flood eruptions
•
•
•
•
very fluid (basalts)
extremely large in volume
create extensive lava plateaus
eruption times correspond with
largest mass extinction events
Columbia river flood basalts
Submarine eruptions
• nearly always basaltic
• mid-ocean ridge eruptions
• pillow basalts
Pillow basalts
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Volcanism on other planets
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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End of Chapter 4
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