Chapter 4 Volcanism and Extrusive Rocks

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Chapter 4Volcanism and Extrusive Rocks
GEOL 101 Introductory Geology
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Nature of Volcanic Eruptions

• Viscosity of magma controls the nature of a
  volcanic eruption
• Factors controlling viscosity
• Composition of the magma
• Temperature of the magma
• Dissolved gases in the magma
• Explosiveness violence of a volcanic eruption

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Nature of Volcanic Eruptions

• Viscosity measure of a materials resistance to
  flow (e.g., higher viscosity materials flow with
  great difficulty)
• Factors affecting viscosity
• Temperature - hotter magmas are less viscous
• Composition
• higher silica (SiO2) content higher viscosity
  (e.g., felsic lava, rhyolite)
• lower silica content lower viscosity, fluid
  behavior (e.g., mafic lava, basalt)

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Nature of Volcanic Eruptions

• Factors affecting viscosity (continued)
• Dissolved Gases
• Gas content affects magma mobility
• Magma gases expand nears the Earths surface due
  to decreasing pressure
• The violence of an eruption is related to how
  easily gases escape from magma

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Nature of Volcanic Eruptions

• Summary
• Fluid lavas (basaltic) generally produce quiet
  eruptions
• Hawaiian Islands
• Highly viscous lavas (rhyolite or andesite)
  produce more explosive eruptions
• Mount Saint Helens

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Nature of Volcanic Eruptions
Magma Compositions
Bomb is approximately 10 cm long
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Materials extruded volcano

• Lava Flows
• Basaltic lavas are much more fluid
• Types of basaltic flows
• Pahoehoe lava (twisted or ropey texture)
• aa lava (rough, jagged blocky texture)
• Dissolved Gases
• One to six percent of a magma by weight
• Mainly water vapor and carbon dioxide

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Pahoehoe lava flow
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Typical aa flow
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Materials extruded from volcano

• Pyroclastic materials Fire fragments
• Ash and dust - fine, glassy fragments
• Pumice - porous rock from frothy lava
• Lapilli - walnut-sized material
• Cinders - pea-sized material
• Particles larger than lapilli
• Blocks - hardened or cooled lava
• Bombs - ejected as hot lava

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Volcanic bomb
Bomb is approximately 10 cm long
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Volcanoes

• General Features
• Opening at the summit of a volcano
• Crater steep-walled depression at the summit,
  generally lt 1 km diameter
• Caldera a summit depression typically lt 1 km
  diameter, produced by collapse following a
  massive eruption
• Vent opening connected to the magma chamber via
  a pipe

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Volcanoes

• Types of Volcanoes
• Shield volcano
• Cinder cone
• Composite cone (Stratovolcano)
• Comparison of Size

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Volcano size comparison
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Volcano Types

• Shield volcano
• Broad, slightly domed-shaped
• Composed primarily of basaltic lava
• Generally cover large areas
• Produced by mild eruptions of large volumes of
  lava
• Example Mauna Loa on Hawaii

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Volcano Types

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

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Sunset Crater Flagstaff, AZcinder cone
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Volcano Types

• Composite cone (Stratovolcano)
• Most are located adjacent to the Pacific Ocean,
  Ring of Fire
  (e.g., Fujiyama, Mt. St. Helens)
• Large, classic-shaped volcano (1000s of feet
  high several miles wide at base)
• Mt. Rainer, WA - over 14,000 feet
• Composed of interbedded lava flows and layers of
  pyroclastic debris

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Volcano Types

• Composite cones (continued)
• Most violent type of activity
  (e.g., Mt. Vesuvius)
• Often produce a nueé ardente
• Fiery pyroclastic flow made of hot gases infused
  with ash and other debris
• Move down the slopes of a volcano at speeds up to
  200 km/hr
• May produce a lahar volcanic mudflow

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Composite volcano
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Mount Saint Helens typical composite
volcano
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Mount Saint Helens, WA
Before May 18, 1980
After May 18, 1980
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Mount Saint Helens

• Post May 18, 1980

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Mount Saint Helens nueé ardente
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Other volcanic landforms

• Calderas
• Steep-walled depressions at the summit
• Size generally lt1 km in diameter
• Pyroclastic flows
• Associated w/ felsic intermediate magma
• Consists of ash, pumice, and other fragmental
  debris
• Material is propelled from the vent at a high
  speed

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Formation of Calderas
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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
• Explosive eruptions of gas-rich magma
• Volcanic pipes and necks
• Pipes short conduits that connect a magma
  chamber to the surface
• necks resistant vents remaining after erosion
  has removed the volcanic cone (e.g., Ship Rock,
  New Mexico)

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Columbia Lava Plateau
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Lava dome on Mt. St. Helens
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Plate tectonics Igneous activity

• Global distribution of igneous activity is not
  random
• Igneous activity along plate boundaries
• Spreading centers
• Subduction zones
• Intraplate (Hotspots)
• Most volcanoes are located within or near ocean
  basins
• Basaltic rocks (oceanic and continental
  settings), granitic rocks (continental only)

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Global Distribution of volcanoes
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Spreading centers

• The greatest volume of volcanic rock is produced
  along the oceanic ridge system
• Mechanism of spreading
• Lithosphere pulls apart
• Less pressure on underlying rocks
• Results in partial melting of mantle
• Produces large amount of basaltic magma

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Subduction zones

• Occur in conjunction with deep oceanic trenches
• Descending plate partially melts
• Magma slowly moves upward
• Rising magma can form either
• An island arc if in the ocean
• A volcanic arc if on a continental margin
• Associated with the Pacific Ocean Basin
• Ring of Fire
• Worlds most explosive volcanoes

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Volcanoes at Pacific Northwest
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Submarine Eruptions
Form pillow structure.