Intrusive Igneous Activity

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Intrusive Igneous Activity

• Plutons result from the cooling and hardening of
  magma within the earth
• Exposed at surface after uplift and erosion
• Pluton refers to Pluto, the Roman god of the
  underworld

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Origin of Magma

• Magma is rock melted by earths internal heat
• Magma results from melting of the crust and upper
  mantle
• Earths internal temperature increases with depth
  from 20-30C per kilometer
• Known as geothermal gradient

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Where is Magma Produced?

• Subduction zones, friction between two tectonic
  plates
• Water decreases melting temp at subduction zones
• Subduction creates continental and marine
  volcanic arcs
• Hot spots, rising material melts due to lessening
  of pressure (decompression melting)

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Earths Geothermal Gradient
Temperature increases with depth within the earth
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Pluton Classification

• Plutons classified by size, shape, and
  relationship to surrounding rock layers
• Types of Plutons
• Sills
• Laccoliths
• Dikes
• Batholiths
• Stocks

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Sills Laccoliths

• A sill forms when magma is injected between
  sedimentary layers
• Sills resemble buried lava flows
• A laccolith is a type of sill that pushes the
  overlying rock layers into an arch shape

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Sill
A sill results from molten magma injected into
sedimentary layers Sills are parallel to the
layers
Sedimentary layers
http//www.psrd.hawaii.edu/WebImg/Sill.gif
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Perfect Sill
Sedimentary layers are visible above and below
the sill
Sill
http//z.about.com/d/geology/1/0/J/S/sill.jpg
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Laccolith
Bear Butte, a laccolith near the Black Hills in
Wyoming, has been exposed by weathering
http//www.tulane.edu/sanelson/images/laccolith.g
if
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Dikes
Ship Rock, NM Dike

• Dikes are plutons that form when magma is
  injected into fractures that cut across existing
  rock layers
• Dikes can radiate like spokes on a wheel from a
  volcanic neck

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Dike in Sedimentary Layers
http//www.rosssea.info/geology.html
Notice how the igneous intrusion (dike) has cut
across existing sedimentary layers
Dike
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Batholith
Batholiths can form the cores of mountains

• Largest plutons, often form mountain ranges
• Surface exposure gt 100 square kilometers (km2)
• Stocks are lt than 100 km2

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Batholiths
Idaho features some large batholiths
Half Dome at Yosemite, CA is part of a batholith
complex
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Igneous Stock

• Devils Post Pile in California is an igneous
  stock
• Note the 6-sided columnar jointing of the basalt

http//www.anthroarcheart.org/tblt7.htm
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Summary of Igneous Bodies