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 °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 A sill results from molten magma injected into sedimentary layers Sills are parallel to the layers Sedimentary layers

Perfect Sill Sedimentary layers are visible above and below the sill Sill

Laccolith 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 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

Summary of Igneous Bodies