Chapter 7 Fires Within: Igneous Activity

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Chapter 7 Fires Within Igneous Activity
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The Nature of Volcanic Eruptions

• Characteristics of a magma determine the
  violence or explosiveness of an eruption
• Composition
• Temperature
• Dissolved gases
• The above three factors actually control the
  viscosity of a magma

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The Nature of Volcanic Eruptions

• Viscosity is a measure of a materials
  resistance to flow
• Factors affecting viscosity
• TemperatureHotter magmas are less viscous
• CompositionSilica (SiO2) content
• Higher silica content higher viscosity
• Lower silica content lower viscosity

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The Nature of Volcanic Eruptions

• Dissolved gases
• Gases expand within a magma as it nears Earths
  surface due to decreasing pressure
• The violence of an eruption is related to how
  easily gases escape
• In summary
• Basaltic lavas mild eruptions
• Rhyolitic or andesitic lavas 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
• 16 by weight
• Mainly H2O and CO2

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Aa Lava Flow
Figure 7.5 A
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Materials Extruded from a Volcano

• Pyroclastic materialsFire fragments
• Types of pyroclastic debris
• Ash and dustFine, glassy fragments
• PumicePorous rock from frothy lava
• CindersPea-sized material
• LapilliWalnut-sized material
• Particles larger than lapilli
• BlocksHardened or cooled lava
• BombsEjected as hot lava

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A Volcanic Bomb
Figure 7.6 (top)
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Volcanic Structures

• General features
• Opening at the summit of a volcano
• CraterSummit depression lt1 km diameter
• CalderaSummit depression gt1 km diameter produced
  by collapse following a massive eruption
• VentSurface opening connected to the magma
  chamber
• FumaroleEmit only gases and smoke

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Volcanic Structures

• Types of volcanoes
• Shield volcano
• Broad, slightly domed shape
• Generally cover large areas
• Produced by mild eruptions of large volumes of
  basaltic lava
• Example Mauna Loa on Hawaii

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Mauna Loa, a Shield Volcano on Hawaii
Figure 7.9
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Volcanic Structures

• Cinder cone
• Built from ejected lava (mainly cinder-sized)
  fragments
• Steep slope angle
• Small size
• Frequently occur in groups

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Cinder Cone Volcano
Figure 7.12
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Volcanic Structures

• Composite cone (stratovolcano)
• Most are located adjacent to the Pacific Ocean
  (e.g., Fujiyama, Mount St. Helens)
• Large, classic-shaped volcano (1000s 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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Mount St. HelensPrior to the 1980 Eruption
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Mount St. Helens after the 1980 Eruption
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Profiles of Volcanic Landforms
Figure 7.10
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Volcanic Structures

• Nuée ardente
• Nuée ardenteA 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

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A Nueé Ardente on Mount St. Helens
Figure 7.15
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Volcanic Structures

• LaharVolcanic mudflow
• Mixture of volcanic debris and water
• Move down stream valleys and volcanic slopes,
  often with destructive results

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Other Volcanic Landforms

• Caldera
• Steep-walled depressions at the summit
• Generally gt1 km in diameter
• Produced by collapse
• Pyroclastic flow
• Felsic and intermediate magmas
• Consists of ash, pumice, and other debris
• Example Yellowstone Plateau

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Formation of Crater Lake, Oregon
Figure 7.18
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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
• Associated with explosive eruptions of gas-rich
  magma

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Other Volcanic Landforms

• Volcanic pipes and necks
• PipesShort conduits that connect a magma chamber
  to the surface
• Volcanic necks (e.g., Shiprock, New
  Mexico)Resistant vents left standing after
  erosion has removed the volcanic cone

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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
• ShapeTabular (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
• DikeA tabular, discordant pluton
• SillA 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 7.22 B
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A Sill in the Salt River Canyon, Arizona
Figure 7.23
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Intrusive Igneous Activity

• Intrusive igneous features continued
• Batholith
• Largest intrusive body
• Surface exposure gt100 km2 (smaller bodies are
  termed stocks)
• Frequently form the cores of mountains

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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 Worlds Major
Volcanoes
Figure 7.26
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Plate Tectonics and Igneous Activity

• Igneous activity at plate margins
• Spreading centers
• Greatest volume of volcanic rock is produced
  along the oceanic ridge system
• Mechanism of spreading
• Decompression melting occurs as the lithosphere
  is pulled apart
• Large quantities of basaltic magma are produced

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Plate Tectonics and Igneous Activity

• Subduction zones
• Occur in conjunction with deep oceanic trenches
• An island arc if in the ocean
• A volcanic arc if on a continental margin
• Associated with the Pacific Ocean Basin
• Region around the margin is known as the Ring of
  Fire
• Majority of worlds explosive volcanoes

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Plate Tectonics and Igneous Activity

• Intraplate volcanism
• Occurs within a tectonic plate
• Localized volcanic region in the overriding plate
  is 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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End of Chapter 7