Powerpoint Presentation Physical Geology, 10/e

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Volcanism and Extrusive RocksPhysical Geology
13/e, Chapter 4
Tim Horner CSUS Geology Department
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Volcanism and Earths Systems

• Atmosphere originally created from gases released
  by magmas
• 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 is produced when magma reaches Earths
  surface
• Explosive eruptions can produce rapidly cooled
  rock fragments called pyroclasts
• Size range 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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Crater Lake, Oregon
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Eruptive Violence and Physical Characteristics of
Lava

• Violence of eruptions controlled by
• Amount of Dissolved gases in the magma
• Water vapor, carbon dioxide, sulfur dioxide, etc.
• The more dissolved gases, the more fluid the lava
• Ease/difficulty of gases escaping to atmosphere
• Viscosity- a fluids resistance to flow
• Silica content
• Higher silica contents produce higher viscosities
• Lava temperature
• Cooler lavas have higher viscosities

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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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Extrusive Textures

• Texture - appearance of a rock with respect to
  size, shape and arrangement of its grains
• Texture terms
• Glassy - glass without mineral crystals
• ex Obsidian
• Fine-grained - most crystals lt 1 mm
• ex Basalt
• Porphyritic - larger crystals in a matrix of much
  finer crystals or glass
• ex Andesite

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Extrusive Textures

• Vesicular/Frothy - trapped gas bubbles
• Vesicular basalt
• Pumice
• Fragmental - particles blasted apart by explosive
  eruptions
• Dust and ash (lt2 mm)
• Cinders (2-64 mm)
• Blocks and bombs (gt64 mm)

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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
• Also called 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 (e.g., Pacific Ring of Fire) ex
  Cascade volcanos
• 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
• Submarine eruptions
• Nearly always basaltic
• Mid-ocean ridge eruptions
• Pillow basalts

Columbia river flood 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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Olympus Mons on Mars
Volcanic eruption on Io
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Lava domes on Venus
Lava channel in volcanic plains on Moon