Intrusive and Extrusive Igneous Rock Structures

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Intrusive and Extrusive Igneous Rock Structures

Basalt dikes hosted in a granitoid pluton, with
metasediment roof pendant Wallowa Mts, Oregon
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Igneous Structures

• Intrusive (Plutonic)
• Magma cools slowly at depth
• Characteristic rock texture
• Characteristic structures

• Extrusive (Volcanic)
• Magma cools quickly at surface
• Characteristic rock textures
• Characteristic structures

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Igneous Structures

• Intrusive
• Batholith
• Stock
• Lopolith
• Laccolith
• Volcanic neck
• Sill
• Dike
• Extrusive
• Lava flow or plateau
• Volcano (many types)
• Crater
• Caldera
• Fissure

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Intrusive Igneous Structures

• Contacts (boundary between two rock bodies) can
  be
• Concordant
• Does not cross cut country rock (surrounding
  rock) structure, bedding, or metamorphic fabric
• Ex laccolith, sill
• Discordant
• Cross cuts country rock structure
• Ex dike, batholith, stock

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Intrusive Igneous Structures

• Categorized by depth of emplacement

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Intrusive Igneous Structures Large Scale

• Major scale intrusive bodies Plutons
• Batholith gt100 km2 in map area (usually
  discordant)
• Stock lt100 km2 in map area
• Lopolith dish-shaped layered intrusive rocks
  (concordant)

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Intrusive Igneous StructuresIntermediate Scale

• Concordant intrusives
• Sill tabular shape
• Laccolith mushroom-shaped
• Roof pendant (remaining country rock)
• Discordant intrusives
• Dike tabular shape
• Volcanic neck cylindrical

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Intrusive Igneous Structures Small Scale

• Apophyses
• Irregular dikes extending from pluton
• Veins
• Tabular body filling a fracture (filled with 1-2
  minerals)
• Xenoliths
• Unrelated material in an igneous body
• Autoliths
• Genetically related inclusions (related igneous
  material)

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Extrusive Igneous Structures

• Volcanism
• Directly observable petrologic process
• Redistributes heat and matter (rocks) from the
  interior to the exterior of the earths surface
• Occurs in oceanic continental settings
• Volcano
• Anywhere material reaches earths surface

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Extrusive Igneous Structures Scale

• Large scale structures
• Lava plateau (LIP flood basalt)
• Ignimbrite (ash flow tuff pyroclastic sheet)
• Intermediate scale structures
• Shield volcano
• Composite volcano (stratovolcano)
• Caldera, crater
• Lava flow or dome
• Small scale structures
• Tephra (pyroclastic material)
• Lava flow features
• Cinder cone

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Extrusive Igneous Structures Eruption Styles

• Effusive Eruptions
• Lava flows and domes
• Erupted from localized fissures or vents
• Generally low silica content (basalt, primitive
  magma)
• Explosive Eruptions
• Tephra (fragmental material)
• Pyroclastic falls or flows
• Erupted from vents
• Generally high silica content (felsic, recycled
  magma)
• Photo glossary of volcano terms

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Extrusive Igneous Structures Eruption Controls

• Two main controls on eruption style
• VISCOSITY
• A fluids resistance to flow
• Determined largely by fluid composition
• DISSOLVED GAS CONTENT
• Main magmatic gasses H2O, CO2, SO2 (or H2S)
• At high pressure, gasses are dissolved in the
  magma
• At low pressure (near surface), gasses form a
  vapor, expand, and rise boiling
• Interaction controls eruption style
• Gas bubbles rise and escape from low viscosity
  magma EFFUSIVE ERUPTION
• Gas bubbles are trapped in high viscosity magma
  increase of pressure EXPLOSIVE ERUPTION

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Extrusive Igneous Structures Eruption Controls

• Two main controls on eruption style
• VISCOSITY and DISSOLVED GAS CONTENT

• In general, both viscosity and gas content are
  related to magma composition
• High silica content gt higher viscosity, more
  dissolved gas
• Low silica content gt lower viscosity, less
  dissolved gas

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Types of Volcanic Products Effusive

• Lava Flow
• Dominantly basalt (low viscosity and gas)
• Thin and laterally extensive sheets
• Pahoehoe flows smooth, ropey flows
• Aa or block flows rough and irregular flows
• Baked zones oxidized zones due to contact with
  high temperature lava flow

• Lava Dome
• Dacite or rhyolite (high viscosity, low gas
  content)
• Thick,steep-sidedflows

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Types of Volcanic Products Explosive

• Pyroclastic particles
• Fragmental volcanic material (TEPHRA)
• Vitric (glass shards)
• Crystals
• Lithic (volcanic rock fragments)
• Broken during eruption of magma
• Typically higher silica, high gas content
• Categorized by size
• Ash (lt 2.0 mm)
• Lapilli (2-64 mm)
• Blocks and bombs (gt64 mm)

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Types of Volcanic Products Explosive

• Pyroclastic fall (mainly Ash fall)
• Material ejected directly from volcano (fallout,
  air fall)
• Ash, lapilli (pumice, scoria), blocks, and bombs
• Sorted (small particles carried further)
• Laterally extensive, mantles topography
• Pyroclastic flow (nueé ardante or ignimbrite)
• Fast moving, high density flow of hot ash,
  crystals, blocks, and/or pumice
• Follow topographic lows
• Can be hot enough after deposition to weld, fuse
  vitric fragments

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Types of Volcanic Products Explosive

• Hydroclastic Products
• Water-magma interaction (phreatomagmatic) causes
  explosive fragmentation
• Typically basaltic lavas
• Any water-magma interaction (sea floor, caldera
  lake, groundwater)

• Great volumes of hydroclastics on the sea floor
  and in the edifice of submarine volcanoes
• Highly subject to alteration gt clay minerals,
  microcrystalline silica, and zeolite

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Styles of Volcanic Eruption Effusive

• Lava Plateaus and Flood Basalts (LIPs)
• Generally low viscosity, low gas content effusive
  lava flows (basalt)
• Hot spot and continental rift settings
• Great areal extent and enormous individual flows
• Erupted from fissures
• Examples (no modern)
• Columbia River Basalt Group
• Deccan Traps

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Styles of Volcanic Eruption Effusive

• Shield volcanoes
• Generally low viscosity, low gas content effusive
  lava flows (basalt)
• Hot spot and continental rift settings
• Central vent and surrounding broad, gentle
  sloping volcanic edifice

• Repeated eruption of mainly thin, laterally
  extensive lava flows
• Modern examples
• Mauna Loa, Kiluaea (Hawaii)
• Krafla (Iceland)
• Erta Ale (Ethiopia)

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Styles of Volcanic Eruption Effusive

• Submarine eruptions and pillow lava
• Generally low viscosity, low gas content effusive
  lava flows (basalt)
• Divergent margin (mid-ocean ridge) settings
• Produces rounded pillows of lava with glassy
  outer rind
• Can produce abundant hydroclastic material
  (shallow)
• Modern examples
• Loihi, Hawaii

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Styles of Volcanic Eruption Explosive

• Cinder cone
• Generally low viscosity, high gas content
  (basalt)
• Subduction zone settings (also continental rifts
  and continental hot spots)

• Small, steep sided pile of loose tephra (mainly
  lapilli, blocks, and bombs)
• Scoria or cinder
• Often form on larger volcanoes (shield or
  stratovolcano)
• Modern example
• Parícutin, Mexico

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Styles of Volcanic Eruption Explosive

• Composite cones and Stratovolcanoes
• Generally higher viscosity, high gas content
  (andesites)
• Dominantly subduction zone settings

Mayon Volcano Philippines

• Composed of layers of loose pyroclastic material
  (fallout and flows) and minor lava flows, some
  shallow intrusions
• Form from multiple eruptions over hundreds to
  thousands of years
• Examples
• Mt. St. Helens, Mt. Rainier (USA)
• Pinatubo (Indonesia)

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Styles of Volcanic Eruption Explosive

• Calderas and pyroclastic sheet (ignimbrite)
  deposits
• Generally high viscosity, high gas content
  (rhyolite)
• Subduction zone and continental hot spots

Crater Lake, Oregon

• Form by collapse of volcano following evacuation
  of the magma chamber
• Often produce widespread ash, ignimbrite
  (pyroclastic flow)
• Examples
• Krakatoa, Indonesia (modern example)
• Crater Lake, Yellowstone (USA)

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Volcanic Hazards

• 500 million people live in high hazard regions
• Eruptions and hazards are largely predictable
• Main hazards
• Tephra (mainly ash)
• Lava flows
• Pyroclastic flows
• Lahar
• Avalanche/landslide
• Volcanic gas
• Tsunami
• Climate change