3.3 Intrusive igneous structures and landforms
3 min read•july 22, 2024
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
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- 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 , 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 , 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 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)
Key Terms to Review (18)
Assimilation: Assimilation is the geological process where a body of magma incorporates surrounding rock material, leading to changes in its composition and characteristics. This process can significantly affect the evolution of igneous rocks, as the added materials alter the chemical makeup and mineral content of the magma, influencing the resulting intrusive structures and landforms, as well as the differentiation processes within the magma itself.
Batholith: A batholith is a large mass of intrusive igneous rock that forms from the crystallization of magma deep within the Earth's crust. These immense structures often cover over 100 square kilometers and can extend several kilometers into the crust, showcasing a variety of rock types. Batholiths play a crucial role in understanding the geological history of an area, as they are often associated with mountain-building processes and can reveal information about past volcanic activity.
Contact Metamorphism: Contact metamorphism is a type of metamorphism that occurs when rocks are heated and altered due to their proximity to hot magma or lava. This process typically leads to localized changes in the mineral composition and texture of the surrounding rocks, resulting in the formation of metamorphic rocks that reflect the conditions near the intrusive igneous structures that caused the metamorphism. The relationship between contact metamorphism and these structures highlights how heat transfer from magma can dramatically influence nearby rock formations.
Continental crust: Continental crust is the thick part of the Earth's crust that forms the continents and is composed primarily of granitic rocks. It is generally older and less dense than oceanic crust, playing a crucial role in shaping the Earth's surface and influencing geological processes. This type of crust supports a wide variety of landforms and features, including mountains, valleys, and plateaus, and is significant in understanding igneous activity and tectonic interactions.
Crystallization: Crystallization is the process by which solid crystals form from a homogeneous solution, melt, or gas, typically as minerals precipitate from magma or fluids. This process is fundamental to the formation and classification of minerals, as well as playing a vital role in the rock cycle, where it leads to the creation of various rock types.
Dyke: A dyke is a type of intrusive igneous structure formed when magma intrudes into pre-existing rock layers and solidifies, creating a vertical or steeply inclined wall of igneous rock. These formations are important indicators of the geological history and processes that have shaped the Earth’s crust, as they often cut through other rock types and can help reveal the nature of the surrounding geological environment.
Geological time: Geological time refers to the vast timescale used by geologists and paleontologists to describe the timing and relationships of events in Earth's history. It encompasses the major periods, epochs, and eras that illustrate the planet's formation, evolution of life, and significant geological events. Understanding geological time is crucial for interpreting intrusive igneous structures and landforms, as it provides context for how these features developed over millions of years.
Igneous Classification: Igneous classification refers to the categorization of igneous rocks based on their origin, texture, and mineral composition. This classification helps in understanding how the rocks formed, whether from cooling magma beneath the surface or lava erupting at the surface. Intrusive igneous structures, such as plutons and batholiths, play a key role in this classification by illustrating how these rocks solidify slowly over time, leading to larger crystal formations compared to their extrusive counterparts.
Laccolith: A laccolith is a type of intrusive igneous rock formation that occurs when magma pushes its way between layers of sedimentary rock, causing the overlying rock layers to bulge upward. This structure typically forms a dome shape and can create significant landforms on the surface. Laccoliths are important in understanding how magma behaves during its ascent and the resulting geological features created by this process.
Mineral composition: Mineral composition refers to the specific types and proportions of minerals present within a rock or geological material. This term is crucial for understanding the characteristics and properties of igneous rocks, as it influences their behavior during formation, their appearance, and their stability in different environments. A rock's mineral composition can provide insights into its origin and the conditions under which it was formed, playing a significant role in defining intrusive igneous structures and landforms.
Pluton: A pluton is a large, intrusive igneous rock body that forms from the cooling and solidification of magma beneath the Earth's surface. These formations can vary in size and shape, and they contribute significantly to the geological features of an area, often leading to the creation of mountain ranges and other landforms. Plutons are primarily classified based on their size and relationship to surrounding rock, which helps in understanding the geological history and processes involved in their formation.
Relative Dating: Relative dating is a method used to determine the chronological order of geological events and formations without assigning exact numerical dates. This technique relies on the principles of stratigraphy and the relationships between rock layers, fossils, and geological features to establish a sequence of events in Earth's history.
Rock Cycle: The rock cycle is a continuous process that describes the transformation of rocks through various geological processes, including formation, breakdown, and reformation. It connects igneous, sedimentary, and metamorphic rocks through mechanisms such as weathering, erosion, and plate tectonics, illustrating how one type of rock can change into another over time. This cycle highlights the dynamic nature of the Earth's crust and is essential in understanding how geological features are formed and altered.
Sill: A sill is a tabular sheet of igneous rock that forms when magma intrudes into existing layers of rock and solidifies horizontally between them. This formation typically occurs at shallow depths within the Earth's crust, and sills can be found in various geological settings, providing insights into the history and movement of magma beneath the surface.
Solidification: Solidification is the process through which molten rock (magma or lava) cools and transforms into solid rock. This transformation is critical in the formation of various intrusive igneous structures and landforms, which arise when magma solidifies beneath the Earth's surface, leading to the creation of distinct geological features.
Stock: In geology, a stock is a large, irregularly shaped body of intrusive igneous rock that has formed beneath the Earth's surface. Stocks are typically less than 100 square kilometers in area and are thought to be the remnants of magma that solidified after rising through the crust but did not reach the surface. Understanding stocks helps in identifying the processes of magma intrusion and the formation of various landforms associated with volcanic activity.
Subduction Zone: A subduction zone is a geological feature that occurs at convergent plate boundaries, where one tectonic plate is forced beneath another into the mantle. This process leads to the formation of deep ocean trenches, volcanic arcs, and earthquake activity, connecting it to various geological phenomena including volcanic eruptions and seismic events.
Texture: In geology, texture refers to the size, shape, and arrangement of the grains, crystals, or particles within a rock or soil. This characteristic plays a crucial role in understanding the formation processes and the history of the material, influencing properties like porosity, permeability, and overall behavior of rocks and soils. Texture can provide insights into the environment of formation and the processes that shaped the rock or soil over time.