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Intrusive
Igneous
Rocks
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Igneous Rocks
• Magma is molten rock below the Earth’s
surface.
• Lava is magma on the Earth’s surface.
• Igneous rocks form
from cooling of:
(1) magma = intrusive
OR
(2) lava = extrusive
Magma Formation
• Magma collects in pools just beneath
the surface
• Why does magma rise?
– Melt is less dense than surrounding solid
rock!
• Animation: Density & magma movement
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Magma Formation
• Rocks melt at 650°-1000° C
• Exact temperature is controlled by:
– Pressure
– Dissolved gases (water vapor)
– Types of minerals present
Magma Formation
• Sources of heat:
1. Heat from
below:
Geothermal
gradient
• 30°/km
2. Mantle
plumes
Where does magma form?
1. At mantle plume
2. In subduction zone
3. Beneath mid-ocean
ridge
4. Beneath continental
rift
*mostly at plate margins (#2,3,4)!
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Characteristics of magma
• Consists of three components:
– Liquids (melt) composed of mobile ions
– Solids (“crystals”)
– Volatiles (gases) dissolved in the melt…
• water vapor (H2O)
• carbon dioxide (CO2)
• sulfur dioxide (SO2)
Characteristics of magma
• Cooling of magma results in
systematic arrangement of ions into
orderly patterns.
– What do we call these solids?
• Silicate minerals crystallize in a
predictable order!!!
(Bowen’s Reaction Series)
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Bowen’s Reaction Series
Discontinuous
Branch
Continuous
Branch
Lessons from Bowen’s Reaction
Series
•
Variety of igneous rocks is produced by large
variety of magma compositions
•
Separation of early-formed ferromagnesian
minerals from a magma body increases the
silica content of the remaining magma
•
Minerals melt in the reverse order of that in
which they crystallize from a magma
Igneous Activity and
Plate Tectonics
•
Igneous activity occurs
primarily at or near tectonic
plate boundaries!
•
Divergent boundaries 
mafic composition
–
From partial melting of the
asthenosphere
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Igneous Activity and Plate Tectonics
•
Convergent boundaries  intermediate
composition
–
From partial melting of basaltic oceanic crust
and overlying continental crust
Igneous Activity and Plate Tectonics
•
Adjacent to
Convergent
boundaries 
felsic
composition
–
From partial
melting of the
granitic
continental
crust
Igneous Activity and Plate Tectonics
•
Intraplate volcanism
– Mantle plumes produce
localized hotspots when
they produce magmas
that rise through the
crust
– Ex. Hawaii: mafic
Yellowstone: felsic
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Intrusive Bodies
• Named based on:
– Depth of formation
– Size
– Shape
– Relationship to country rock
Insert new Fig. 3.11 here
Shallow Intrusive
Bodies
•
Form <2 km beneath
Earth’s surface
•
Small scale
•
Likely internal
“plumbing” of
volcanoes
•
Chill and solidify
quickly  aphanitic
rocks
Insert new Fig. 3.11 here
Volcanic neck: Magma solidifies in throat of
volcano
Ship Rock, NM
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• Devil’s Tower,
Wyoming.
Shallow Intrusive Bodies
• More common tabular structures
form when magma fills cracks.
• Dikes – vertically oriented.
• Sills – horizontally oriented.
Dike
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Sill
Dike
Sill
Canary Islands, Basalt Cliffs
Volcano
Dike
Canary Islands, El Teide
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Dike
Canary Islands, El Teide
Deep Intrusive Bodies
• Pluton – body of
magma or igneous
rock that crystallized
slowly at depth
• At the surface they
are divided into:
– Stock (<100 km2)
– Batholith (>100 km2)
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Sierra Nevada
batholith
Lake Tahoe
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Yosemite National Park
Mt. Rushmore
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Extrusive Igneous
Rocks
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Igneous Review
• Magma is molten rock
below the Earth’s surface.
• Lava is magma on the
Earth’s surface.
• Igneous rocks form from
cooling of
(1) magma = intrusive
OR
(2) lava = extrusive
Lava flow
Lava flow
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Ashfall
Pyroclastic flow
Extrusive Igneous Rocks
• Can be observed directly
– Temperature measured
– Solidification of rock observed
• Compare with similar rocks where
volcanism is no longer active to
interpret ancient volcanic activity
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Extrusive Textures
• Size of grains
• Aphanitic – grains
<1mm
– Fast cooling
• Porphyritic – large
crystals in fine matrix.
– Slow then fast cooling
Extrusive Rock Composition
• Most extrusive rocks are aphanitic so
composition can be determined by color
Felsic
Quartz
K-feldspar
Na plagioclase feldspar
Mafic
Amphibole
Pyroxene
Ca plagioclase feldspar
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Rhyolite
Basalt
Andesite
Special Extrusive Textures
• Glassy – composed of glass
– Instant cooling: Ions frozen in place
– No minerals formed
– More likely to
form in viscous,
felsic magma
Obsidian
Special Extrusive Textures
• Vesicular – air bubbles (vesicles)
– Released pressure causes dissolved
gases to come out of solution
– As lava solidifies
air bubbles are
trapped
Vesicular basalt
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Pumice
Scoria
• More viscous magmas trap more
gas – form a froth
• Very low density
Special Extrusive Textures
• Fragmental –
consist of pyroclasts
– Angular rock
fragments formed by
a volcanic explosion
– Lava ejected into air
• Lava bombs –
cool rapidly in
flight
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• Volcanic
breccia –
composed of
large
fragments
• Tuff –
composed of
ash (>2 mm)
Volcanic Eruptions
• Provide information on the Earth’s
interior
• Vary in nature and violence
– Magma composition
Pyroclastic flow
Lava flow
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Kilauea, Hawaii
Mt. St. Helens, Washington
Type of explosion controlled by…
1. Viscosity (“stickiness”)- a fluid’s
resistance to flow
• Silica content
• Lava temperature
2. Amount of dissolved gases
• More dissolved gases  more fluid lava
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Gases
• Most gas released during an eruption is
water vapor
– Also carbon dioxide, sulfur dioxide,
hydrogen sulfide, hydrochloric acid
• Expanding, hot gases propel debris into
the atmosphere
• Origin of atmosphere and oceans
Explosiveness
• Mafic: low violence, lava flows,
divergent margins, intraplate zones
• Intermediate: moderate violence, lava
flows and pyroclastic explosions,
subduction zones
• Felsic: extremely violent, pyroclastic
explosions, intraplate zones
Volcano Anatomy
• Ejected volcanic material forms the
typical conical shape
• Vent – opening
• Crater – depression over vent
• Caldera – enlarged crater
– At least 1 km diameter
– Formed by explosion of summit or collapse
into magma chamber
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Caldera
Crater
• Crater Lake, Oregon – caldera.
• Animation.
Caldera Examples
Haleakala NP, Hawaii
Yellowstone NP, Wyoming
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• Extent of ash
and pyroclastic
debris from
Yellowstone
eruptions
• Yellowstone tuff
• 100s of meters
thick
Volcano Anatomy
• Volcanic domes – steep sided, dome or
spine shaped masses of volcanic rock
solidifying above a vent.
– Felsic or intermediate volcanoes – high
viscosity.
– “Corks” vent – traps gas and magma under
pressure.
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Volcanic domes, eastern California
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Types of Volcanoes
Shield Volcano
• Broad, gently sloping
• Made of solidified lava flows
• Low viscosity lava spread widely
and thinly
• Occur within oceanic plates or at
mid ocean ridges
• Ex. Islands of Hawaii
Animation
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Caldera
• Mauna Loa, Hawaii.
• Most lava
flows
underground
in lava tubes
• Lava makes
it to the sea
• Builds
islands
outward
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Shield Volcanoes
• 2 Types of Lava
– Pahoehoe: ropy
– A’a: rough and
jagged
Cinder Cone
• Constructed of
pyroclastic fragments
• Small size
• Steep slopes –
material falls close to
vent
• Mafic and
intermediate
• Animation
• Short life span.
• Easily eroded.
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Composite Volcano
• Alternating layers of
lava flows and
pyroclastic debris
• Intermediate
steepness
• Long life spans
• 100s to 1000s years
inactivity
Animation
Composite Volcano: Guatemala
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Composite Volcano
• Present at subduction zones
• Intermediate magma – andesite
• Variable temperature
– Hotter: lava flows
• Variable gas concentration
– More gas: pyroclastic explosion
Composite Volcanoes
• Most common type of volcano at convergent
plate boundaries
– Ex. Pacific Ring of Fire, Mediterranean
Other Eruption Types
• Flood eruptions
– Very fluid (basalts)
– Extremely large in
volume
– Create extensive lava
plateaus
Columbia river flood basalts
Pillow basalts
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Siberian Traps
Siberian Traps
• 2 million km2
• 5000+ m
thick
• 250 Ma
• Largest mass
extinction
• 90% of all
species
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