Intrusive and Extrusive Igneous Rock Structures

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
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
MesozoicPaleozoic
Paleozoic or
older
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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,
steepsided
flows
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Types of Volcanic Products: Explosive
• Pyroclastic particles
– Fragmental volcanic
material (TEPHRA)
• Vitric (glass shards)
• Crystals
• Lithic (volcanic rock
fragments)
Bombs
Tephra
– 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
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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 deposition16to
weld, fuse vitric fragments
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 –> 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, Hawaii
• 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)
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
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Styles of Volcanic Eruption: Explosive
• Composite cones and
Stratovolcanoes
Mayon Volcano
Philippines
– Generally higher
viscosity, high gas
content (andesites)
– Dominantly subduction
zone settings
– 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
– 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)
Crater Lake,
Oregon
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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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