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)

Earth’s Geothermal Gradient

Temperature increases with depth within the earth

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

Summary of Igneous Bodies