Chapter 5 Volcanoes and Other Igneous Activity
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Transcript of Chapter 5 Volcanoes and Other Igneous Activity
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Chapter 5 Volcanoes and Other Igneous
Activity
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The nature of volcanic eruptions
Viscosity is a measure of a material’s resistance to flow
Factors affecting viscosity• Temperature - Hotter magmas are less viscous• Composition - 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 gases• Gases separate from a magma as it nears the
Earth’s surface due to decreasing pressure• The violence of an eruption is related to how
easily gases escape from magma
• In summary• Basaltic lavas: expanding gasses escape easily
producing mild eruptions• Rhyolitic or andesitic lavas: expanding gasses
cannot escape producing explosive eruptions
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Materials extruded from a volcano
Lava flows• Basaltic lavas exhibit fluid behavior• Types of basaltic flows
• Pahoehoe lava (resembles a twisted or ropey texture)
• Aa lava (rough, jagged blocky texture)
Dissolved gases• 1% - 6% by weight• Mainly H2O and CO2
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Table 5.1
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A pahoehoe lava flow
Figure 5.5 A
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Aa lava flow
Figure 5.5 B
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Figure 5.6B
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Figure 5.6A
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Volcanoes
General features• Opening at the summit of a volcano
• Crater - summit depression < 1 km diameter
• Caldera - 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 volcanoes• Shield volcano
• Broad, slightly domed-shaped
• Huge
• Produced by mild eruptions of large volumes of basaltic lava
• Example = Mauna Loa on Hawaii
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Anatomy of a shield volcano
Figure 5.10
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Volcanoes
• Cinder cone• Built from ejected lava (mainly cinder-sized)
fragments
• Steep slope angle
• Small
• Frequently occur in groups
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Cinder cone volcano
Figure 5.14
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Sunset Crater
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SP Crater
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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 debris
• Most violent type of activity (e.g., Mt. Vesuvius)
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Anatomy of a composite volcano
Figure 5.9
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Mt. St. Helens – prior to the 1980 eruption
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Mt. St. Helens after the 1980 eruption
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Figure 5.1Right
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Figure 5.2
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Figure 5.26A
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A lava dome
Figure 5.26
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A nueé ardente on Mt. St. Helens
Figure 5.20
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Volcanoes
• Nuée ardente – A deadly pyroclastic flow• Fiery pyroclastic flow made of hot gases
infused with ash and other debris• Also known as glowing avalanches• Move down the slopes of a volcano at speeds
up to 200 km per hour
• Lahar – volcanic mudflow• Mixture of volcanic debris and water• Move down stream valleys and volcanic
slopes, often with destructive results
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Materials extruded from a volcano
Pyroclastic materials – “fire fragments”• Types of pyroclastic debris
• Ash and dust - fine, glassy fragments
• Pumice - porous rock from “frothy” lava
• Cinders - pea-sized material
• Lapilli - walnut-sized material
• Particles larger than lapilli• Blocks - hardened or cooled lava• Bombs - ejected as hot lava
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A volcanic bomb
Bomb is approximately 10 cm long
Figure 5.7
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Profiles of volcanic landforms
Figure 5.12
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Formation of Crater Lake, Oregon
Figure 5.22
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Figure 5.24
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Figure 5.25A
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Formation of a volcanic neck
Figure 5.27
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Other volcanic landforms
Pyroclastic flow• Felsic and intermediate magmas• Consists of ash, pumice, and other debris• Material ejected at high velocities• Example = Yellowstone plateau
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Other volcanic landforms
Fissure eruptions and lava plateaus• Fluid basaltic lava extruded from crustal
fractures called fissures• Example = Columbia River Plateau
Lava domes• Bulbous mass of congealed lava
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Other volcanic landforms
Volcanic pipes and necks• Pipes - short conduits that connect a magma
chamber to the surface• Volcanic necks (e.g., Ship Rock, New
Mexico) - resistant vents left standing after erosion has removed the volcanic cone
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Figure 5.39
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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 plutons
• Shape - tabular (sheetlike) vs. massive• Orientation with respect to the host
(surrounding) rock• Concordant vs. discordant
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Intrusive igneous activity
Types of intrusive igneous features• Dike – a tabular, discordant pluton• Sill – a tabular, concordant pluton (e.g.,
Palisades Sill in New York)• Laccolith
• Similar to a sill
• Lens or mushroom-shaped mass
• Arches overlying strata upward
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Igneous structures
Figure 5.28 B
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A sill in the Salt River Canyon, Arizona
Figure 5.30
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Figure 5.31A
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Intrusive igneous activity
Intrusive igneous features continued• Batholith
• Largest intrusive body
• Surface exposure > 100+ km2 (smaller bodies are termed stocks)
• Frequently form the cores of mountains
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Batholithsof western
NorthAmerica
Figure 5.32
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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
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Distribution of some of the
world’s major volcanoes
Figure 5.34
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Plate tectonics and igneous activity
Igneous activity at plate margins• Spreading centers• Subduction zones: “ring of fire”
Intraplate igneous activity• Hot spots: Hawaii, yellowstone
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Volcanoes and climate
The basic premise• Explosive 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 – 1815• Krakatau, Indonesia – 1883
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Volcanoes and climate
Modern examples• Mount St. Helens, Washington - 1980• El Chichón, Mexico - 1815• Mount Pinatubo, Phillippines - 1991