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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

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)

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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 – 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