4.4 Intrusive igneous features and plutons

Magma intrusions shape Earth's crust, forming diverse igneous features. Dikes, sills, laccoliths, and batholiths each have unique characteristics, influenced by magma pressure, rock properties, and cooling rates. These structures play a crucial role in shaping landscapes and geological processes.

Plutons, ranging from small bodies to massive batholiths, have significant economic importance. They're sources of valuable ore deposits, geothermal energy, and building materials. Understanding plutons helps us tap into Earth's resources and comprehend its geological history.

Intrusive Igneous Features

Types of intrusive igneous features

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• Dikes cut across existing rock layers vertically or near-vertically forming narrow and elongated intrusions (San Rafael Swell, Utah)

• Sills intrude horizontally parallel to existing rock layers often between sedimentary strata (Palisades Sill, New Jersey)

• Laccoliths form dome-shaped intrusions concordant with surrounding rock layers at the base causing uplift of overlying rocks (Mount Hillers, Utah)

• Batholiths create large irregular-shaped intrusions covering areas greater than 100 square kilometers often forming mountain range cores (Sierra Nevada Batholith, California)

Formation of intrusive igneous features

• Magma emplacement occurs when magma intrudes existing rock driven by pressure differences and rock weakness

• Cooling rates vary slower in larger intrusions and faster in smaller ones affecting crystal size and texture

• Surrounding rock properties influence intrusion shape and orientation determining paths of least resistance for magma flow

• Depth of emplacement affects pressure and cooling rate influencing the resulting igneous feature type

Plutons and Economic Importance

Characteristics of plutons

• Size ranges from small bodies to massive batholiths with stocks smaller than 100 square kilometers and batholiths larger

• Shape varies from irregular to roughly circular influenced by surrounding rock structure appearing tabular domed or branching

• Composition typically felsic to intermediate with common rock types granite granodiorite and diorite while mafic compositions less common but possible

• Internal structure may show layering or zonation containing xenoliths (fragments of surrounding rock)

Economic importance of intrusions

• Ore deposits form as hydrothermal fluids concentrate valuable minerals with porphyry copper deposits associated with granitic intrusions and gold silver and other precious metals often found in veins

• Geothermal resources develop from heat of cooling plutons creating geothermal systems used for power generation and direct heating

• Building materials sourced from granite and other plutonic rocks used in construction with dimension stone industry relying on quarried plutonic rocks

• Groundwater resources accumulate in fractured plutonic rocks serving as aquifers important for water supply in some regions (Yosemite National Park)