Intrusive Igneous Activity Plutons result from the cooling

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 2030°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 http: //www. psrd. hawaii. edu/Web. Img/Sill. gif Sedimentary layers

Perfect Sill Sedimentary layers are visible above and below the sill Sill http: //z. about. com/d/geology/1/0/J/S/sill. jpg

Laccolith Bear Butte, a laccolith near the Black Hills in Wyoming, has been exposed by weathering http: //www. tulane. edu/~sanelson/images/laccolith. gif

Ship Rock, NM 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

http: //www. rosssea. info/geology. html Dike in Sedimentary Layers Notice how the igneous intrusion (dike) has cut across existing sedimentary layers Dike

Batholiths can form the cores of mountains • Largest plutons, often form mountain ranges • Surface exposure > 100 square kilometers (km 2) • Stocks are < than 100 km 2

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

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/tblt 7. htm

Summary of Igneous Bodies