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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
Origin of Magma• Magma is rock melted by earth’s
internal heat• Magma results from melting of the
crust and upper mantle• Earth’s internal temperature
increases with depth from 20-30°C per kilometer
• Known as geothermal gradient
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)
Pluton Classification• Plutons classified by size, shape,
and relationship to surrounding rock layers
• Types of Plutons:• Sills• Laccoliths• Dikes• Batholiths• Stocks
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
Sill
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
Perfect SillSedimentary layers are visible above and below the sill
Sill
http://z.about.com/d/geology/1/0/J/S/sill.jpg
Laccolith
http://www.tulane.edu/~sanelson/images/laccolith.gif
Bear Butte, a laccolith near the Black Hills in Wyoming, has been exposed by weathering
Dikes• 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
Ship Rock, NM Dike
Dike in Sedimentary Layers
http://www.rosssea.info/geology.html
Notice how the igneous intrusion (dike) has cut across existing sedimentary layers
Dike
Batholith• Largest plutons, often form
mountain ranges• Surface exposure > 100 square
kilometers (km2)• Stocks are < than 100 km2
Batholiths can form the cores
of mountains
Batholiths
Half Dome at Yosemite, CA is part of a batholith complex
Idaho features some large batholiths
Igneous Stock•Devil’s Post Pile in California is an igneous stock
•Note the 6-sided columnar jointing of the basalt
http://www.anthroarcheart.org/tblt7.htm