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1
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
Epizonal Mesozonal Catazonal
Depth Shallow
<6-10 km
Intermediate
~8-14 km
Deep
>~12 km
Contacts Discordant Variable Concordant
Size Small to
moderate
Small to large Small to large
Contact
metamorphism
Very common Uncommon Absent
Age Cenozoic Mesozoic-
Paleozoic
Paleozoic or
older
• Categorized by depth of emplacement
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Intrusive Igneous Structures:
Large Scale
• Major scale intrusive bodies: Plutons
– Batholith: >100 km2 in map area (usually discordant)
– Stock: <100 km2 in map area
– Lopolith: dish-shaped layered intrusive
rocks (concordant)
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Intrusive Igneous Structures:
Intermediate 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 earth’s surface
– Occurs in oceanic & continental settings
• Volcano: – Anywhere material reaches earth’s 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 fluid’s 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 –> higher viscosity, more dissolved gas
• Low silica content –> 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- sided flows
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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 (< 2.0 mm)
• Lapilli (2-64 mm)
• Blocks and bombs (>64 mm)
Ash
Tephra Bombs
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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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• Hydroclastic Products – Water-magma interaction (phreatomagmatic) causes
explosive fragmentation
– Typically basaltic lavas
– Any water-magma interaction (sea floor, caldera lake, groundwater)
Types of Volcanic Products: Explosive
– Great volumes of
hydroclastics on the sea
floor and in the edifice of
submarine volcanoes
– Highly subject to alteration –> 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
Mauna Loa, Hawaii
– Repeated eruption of
mainly thin, laterally
extensive lava flows
– Modern examples:
• Mauna Loa, Kiluaea
(Hawaii)
• Krafla (Iceland)
• Erta Ale (Ethiopia)
20
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)
SP Crater, Arizona – 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
22
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)
23
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
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