4.2 Igneous rock classification and textures
2 min read•july 24, 2024
Igneous rocks form from cooling magma or lava, creating diverse textures and compositions. The uses mineral content and texture to categorize these rocks, ranging from to based on silica levels.
Textures in igneous rocks reveal their cooling history. Rapid cooling produces fine-grained or glassy textures, while slow cooling forms coarse-grained rocks. Common examples include basalt, granite, obsidian, and pumice, each with unique properties and uses.
Igneous Rock Classification
Classification of igneous rocks
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- IUGS classification scheme employs mineral composition and texture as key parameters focuses on content and alkali feldspar to ratio (QAP diagram)
- Felsic rocks contain high silica content (>63% SiO2) appear light-colored due to abundance of quartz and feldspars (granite, rhyolite)
- Intermediate rocks possess moderate silica content (52-63% SiO2) blend light and dark minerals (diorite, andesite)
- rocks have low silica content (45-52% SiO2) appear dark-colored due to abundance of ferromagnesian minerals (gabbro, basalt)
- Ultramafic rocks contain very low silica content (<45% SiO2) appear very dark-colored composed mainly of and pyroxene (peridotite, dunite)
Textures in igneous rocks
- exhibits fine-grained crystals not visible to naked eye results from rapid cooling of magma (basalt, rhyolite)
- displays coarse-grained crystals visible to naked eye forms through slow cooling of magma (granite, gabbro)
- shows mixed grain sizes with large crystals (phenocrysts) in fine-grained matrix (groundmass) indicates two-stage cooling process (porphyritic andesite)
- contains small rounded cavities (vesicles) formed by trapped gas bubbles during rapid cooling common in volcanic rocks (pumice, scoria)
Formation of igneous textures
- Cooling rate effects determine crystal size rapid cooling produces fine-grained or glassy textures slow cooling forms coarse-grained textures
- Magma composition influences crystal formation felsic magmas tend to form larger crystals due to higher viscosity mafic magmas often produce smaller crystals due to lower viscosity
- Volcanic environments promote rapid cooling resulting in fine-grained or glassy textures (obsidian, basalt)
- Plutonic environments allow slow cooling leading to coarse-grained textures (granite, diorite)
- Crystal growth factors include:
- Nucleation rate: frequency of crystal seed formation
- Growth rate: speed of crystal enlargement
- Available space: room for crystals to develop without interference
Identification of common igneous rocks
- Basalt exhibits mafic composition aphanitic texture dark color often contains vesicles used in construction (pavement, buildings)
- Granite displays felsic composition phaneritic texture light color contains quartz, feldspar, and mafic minerals widely used in countertops and building facades
- Obsidian shows felsic composition black or dark color formed by rapid cooling of lava historically used for tools and weapons
- Pumice presents felsic composition vesicular texture light color low density formed by rapid cooling of gas-rich lava used in abrasives and lightweight concrete
Key Terms to Review (24)
Aphanitic texture: Aphanitic texture is a fine-grained texture found in igneous rocks, characterized by crystals that are too small to be seen with the naked eye. This texture occurs when lava cools rapidly at or near the Earth's surface, allowing minimal time for crystal growth. Aphanitic textures are often associated with volcanic activity and indicate that the rock formed from quickly cooled magma or lava.
Batholith: A batholith is a large, deep-seated mass of intrusive igneous rock that forms when magma cools and solidifies below the Earth's surface. Batholiths are often composed of coarse-grained granite and can cover an area of over 100 square kilometers, making them some of the largest plutonic features on Earth. These massive formations provide insight into the geological processes that shape the Earth's crust and are significant in understanding the classification and texture of igneous rocks.
Bowen's Reaction Series: Bowen's Reaction Series is a scientific model that describes the order in which minerals crystallize from cooling magma, revealing the relationship between temperature and mineral formation. This series helps understand how igneous rocks form and provides insights into the mineral composition of these rocks, which in turn influences their weathering and erosion behavior in different environmental conditions.
C. W. M. van der Pluijm: C. W. M. van der Pluijm is a notable geologist recognized for his contributions to understanding igneous rock classification and textures. His work has emphasized the importance of texture in determining the formation processes of igneous rocks, linking mineral composition to their origin and cooling history, which helps in classifying these rocks into various categories based on their physical characteristics.
Crystallization: Crystallization is the process through which solid crystals form from a liquid or gas, often occurring as minerals cool and solidify from magma or as solutions evaporate. This process is crucial in the formation of various rock types and plays a significant role in the rock cycle, mineral formation, and the classification of igneous rocks based on their textures and composition.
Dike: A dike is a type of igneous intrusion that forms when magma rises through pre-existing rock layers and solidifies vertically or at a steep angle. This geological feature is essential for understanding the processes that shape the Earth's crust and how different types of igneous rock are formed. Dikes can vary in thickness and length, often cutting across the layers of surrounding rock, and they play a crucial role in the classification of intrusive igneous features.
Extrusive igneous rock: Extrusive igneous rock is formed from the cooling and solidification of molten lava on the Earth's surface. These rocks typically have a fine-grained texture due to the rapid cooling that occurs when lava erupts and comes into contact with air or water, preventing large crystals from forming. Their characteristics are essential in understanding volcanic activity and the geological processes that shape the Earth's crust.
Felsic: Felsic refers to a category of igneous rocks and minerals that are rich in silica and light-colored materials, primarily feldspar and quartz. This term is crucial for understanding the composition of the Earth's crust, as felsic rocks are prevalent in continental regions and are indicative of the processes that shape our planet's surface.
Glassy texture: Glassy texture refers to a unique characteristic of certain igneous rocks that exhibit a smooth, glass-like appearance due to rapid cooling of molten material. This rapid cooling prevents the formation of crystalline structures, resulting in a solid mass that is often translucent or transparent, resembling glass. This texture is commonly associated with volcanic rocks that cool quickly when lava erupts and comes into contact with water or air.
Intrusive igneous rock: Intrusive igneous rock is a type of rock that forms from the slow crystallization of magma beneath the Earth's surface. This process occurs over long periods, allowing large crystals to develop, which results in a coarse-grained texture. The characteristics of intrusive igneous rocks can provide insights into the conditions under which they formed and the type of magma involved.
IUGS Classification Scheme: The IUGS Classification Scheme is a systematic method for classifying igneous rocks based on their mineral composition and texture. This classification helps geologists communicate about the types of igneous rocks and understand their formation processes, which is crucial for studying the Earth's crust and volcanic activity.
Lava flow: A lava flow is a stream of molten rock that erupts from a volcano and moves down its slopes or spreads across the ground. These flows can vary in speed, composition, and temperature, influencing the formation of different volcanic landforms and types of igneous rocks as they cool and solidify.
Mafic: Mafic refers to a type of igneous rock that is rich in magnesium and iron, characterized by a darker color and higher density compared to felsic rocks. Mafic rocks are primarily found in the Earth's mantle and oceanic crust, and they play a significant role in understanding the planet's composition and internal structure.
Olivine: Olivine is a magnesium iron silicate mineral that is commonly found in igneous rocks and is a significant component of the Earth's upper mantle. Its distinctive green color and granular appearance make it easily identifiable, and it plays a crucial role in understanding both the composition of the Earth and the classification of igneous rocks, particularly basalt and peridotite.
Phaneritic texture: Phaneritic texture refers to a coarse-grained igneous rock texture where individual crystals are large enough to be seen with the naked eye. This texture is formed when magma cools slowly beneath the Earth's surface, allowing ample time for crystals to grow. As a result, phaneritic rocks typically exhibit a mosaic of interlocking mineral grains, which can provide insights into the cooling history and mineral composition of the rock.
Plagioclase: Plagioclase is a group of important tectosilicate minerals that are commonly found in igneous rocks. They are feldspar minerals characterized by their sodium and calcium content, with varying compositions that affect their physical properties and the types of rocks they form. Plagioclase plays a significant role in determining the classification of igneous rocks based on their mineral content and texture.
Porphyritic texture: Porphyritic texture refers to a type of igneous rock texture characterized by the presence of large crystals, known as phenocrysts, embedded within a finer-grained matrix or groundmass. This texture indicates a two-stage cooling process where the magma initially cools slowly underground, allowing larger crystals to form, followed by a rapid cooling phase that creates the finer-grained component. Understanding this texture helps in classifying igneous rocks and provides insights into their formation conditions.
Pyroclastic flow: A pyroclastic flow is a fast-moving current of hot gas and volcanic matter, known as tephra, that flows down the slopes of a volcano during explosive eruptions. These flows can reach speeds of up to 700 km/h (about 435 mph) and can be extremely destructive due to their high temperatures and mobility. Understanding pyroclastic flows is crucial as they shape volcanic landforms, influence the composition of igneous rocks, and are closely linked to the properties of magma that contribute to explosive eruptions.
Qapf diagram: The qapf diagram is a useful graphical representation that helps classify igneous rocks based on their mineral composition, specifically focusing on the relative proportions of quartz, alkali feldspar, plagioclase, and other volcanic minerals. This diagram allows geologists to categorize rocks into different groups, such as granite or basalt, and understand their formation processes by examining their mineral content and texture. By plotting the composition of an igneous rock on this diagram, geologists can quickly determine its classification and related characteristics.
Quartz: Quartz is a common and abundant mineral composed of silicon dioxide (SiO₂), known for its hardness and resistance to weathering. This mineral plays a vital role in Earth's structure, forming the primary component of many rocks and influencing the classification and formation of minerals, as well as contributing to the development of various rock types, including igneous and sedimentary rocks.
Sill: A sill is a tabular, horizontal intrusion of igneous rock that forms when magma intrudes between layers of pre-existing rock. Sills are significant because they provide insight into the processes of magma movement and crystallization beneath the Earth's surface, and they often exhibit specific textural characteristics that help classify the associated igneous rocks.
Solidification: Solidification is the process by which a substance changes from a liquid state to a solid state, often occurring as molten rock cools and crystallizes to form igneous rocks. This transformation is crucial in determining the mineral composition and texture of the resulting rock, as the rate of cooling influences crystal size and distribution. The nature of solidification directly impacts the classification of igneous rocks into categories such as intrusive or extrusive, depending on where and how they cool.
Ultramafic: Ultramafic refers to igneous rocks that contain a very high percentage of magnesium and iron-rich minerals, typically less than 45% silica. These rocks are primarily composed of olivine, pyroxene, and other ferromagnesian minerals, making them dense and dark in color. Their unique composition has significant implications for understanding the Earth's mantle, volcanic activity, and the formation of certain types of magma.
Vesicular texture: Vesicular texture refers to the characteristic appearance of igneous rocks that contain numerous small cavities or vesicles, formed by gas bubbles trapped in the molten rock during solidification. This texture is significant as it indicates the volcanic origin of the rock, showing that gas was present during the eruption and cooling process. The presence of these vesicles can affect the physical properties of the rock, including its density and strength.