Intrusive igneous structures form when magma cools beneath Earth's surface, creating unique formations like dikes, sills, and batholiths. These structures vary in size and shape, from thin sheets to massive irregular bodies, and play a crucial role in shaping Earth's crust.
The formation of intrusive igneous rocks involves complex processes like magmatic stoping and can result in economically significant mineral deposits. These rocks also create distinctive landforms like laccoliths and volcanic necks, which offer insights into Earth's geological history and provide valuable resources.
Intrusive Igneous Structures
Characteristics of intrusive igneous structures
- Intrusive igneous structures form when magma solidifies beneath Earth's surface, intruding into pre-existing country rock
- Cooling occurs slowly, allowing formation of large, visible crystals (granite, diorite)
- Dikes are tabular, sheet-like intrusions that cut across layering of surrounding rock, forming when magma fills vertical or near-vertical fractures, ranging in thickness from centimeters to hundreds of meters (Shiprock dike, New Mexico)
- Sills are tabular, sheet-like intrusions parallel to layering of surrounding rock, forming when magma intrudes along bedding planes or horizontal weaknesses, ranging in thickness from centimeters to hundreds of meters (Palisades Sill, New Jersey)
- Batholiths are large, irregular intrusions with surface area greater than 100 km², forming deep within Earth's crust, often associated with mountain-building processes, composed of coarse-grained igneous rocks (Sierra Nevada Batholith, California)
Process of magmatic stoping
- Magmatic stoping occurs when magma rises through crust by breaking off and engulfing blocks of surrounding country rock, happening when magma is less dense than surrounding rock
- Broken blocks of country rock, known as xenoliths, become incorporated into magma
- As magma rises and cools, xenoliths may partially melt or react with magma, contributing to overall composition of resulting igneous rock
- Stoping plays significant role in formation of plutons (large, intrusive igneous bodies), allowing magma to create space for itself as it rises through crust, contributing to expansion and growth of plutons over time
Landforms of intrusive igneous activity
- Laccoliths are dome-shaped intrusions that form when magma is injected between layers of sedimentary rock, uplifting and deforming overlying strata into dome shape (Henry Mountains, Utah; Black Hills, South Dakota)
- Volcanic necks are vertical, pipe-like structures representing solidified conduits of former volcanoes, forming when magma solidifies within vent of volcano
- Erosion of surrounding, less-resistant rock leaves behind prominent, often cylindrical landform (Ship Rock, New Mexico; Devil's Tower, Wyoming)
Economic significance of intrusive igneous rocks
- Intrusive igneous rocks, particularly felsic composition, often associated with valuable mineral deposits
- Hydrothermal fluids (hot, mineral-rich solutions) circulate through intrusive body and surrounding rock, concentrating minerals in economically significant quantities
- Common mineral deposits associated with intrusive igneous rocks:
- Porphyry copper deposits formed by concentration of copper minerals in and around porphyritic intrusions (Bingham Canyon Mine, Utah; Chuquicamata Mine, Chile)
- Skarn deposits formed by interaction of hydrothermal fluids with carbonate rocks, resulting in formation of calc-silicate minerals and ore minerals containing valuable metals (copper, gold, zinc)
- Pegmatites are very coarse-grained intrusive rocks that form from late-stage crystallization of magma, containing rare elements (lithium, beryllium, tantalum) and gemstones (tourmaline, topaz)
- Intrusive igneous rocks are important sources of dimension stone used in construction and decorative applications (granite countertops, building facades)