7.3 Classification of metamorphic rocks
3 min read•july 22, 2024
Metamorphic rocks transform under heat and pressure, changing their appearance and mineral makeup. These rocks are sorted into two main groups: foliated, with aligned minerals, and non-foliated, with a more uniform look. The original rock and metamorphic conditions determine the final product.
Foliated rocks include , , , and , each showing different levels of mineral alignment. Non-foliated rocks like , , and have distinct compositions and uses. Understanding these types helps geologists piece together Earth's history and processes.
Metamorphic Rock Classification
Classification of metamorphic rocks
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- Metamorphic rocks classified based on two main characteristics
- indicates physical appearance and arrangement of minerals
- Foliated rocks have a planar or layered appearance due to alignment of platy minerals (micas, chlorite)
- Non-foliated rocks have a uniform or granular texture lacking preferred mineral orientation
- Mineral composition determined by composition and metamorphic conditions (temperature, pressure)
- Specific mineral assemblages indicative of certain metamorphic grades and environments (, )
- indicates physical appearance and arrangement of minerals
Foliated metamorphic rock types
- Slate
- Fine-grained, low-grade metamorphic rock formed from shale or mudstone
- Well-developed allows rock to split into thin, smooth sheets (roofing slate)
- Phyllite
- Fine-grained, low to medium-grade metamorphic rock formed from slate under increased temperature and pressure
- Satiny sheen due to presence of tiny mica crystals aligned along foliation planes
- Schist
- Medium to coarse-grained, medium to high-grade metamorphic rock formed from phyllite or other rocks under higher temperature and pressure
- Contains visible mica flakes arranged in preferred orientation (muscovite, biotite)
- Common types include mica schist, garnet schist, and chlorite schist
- Gneiss
- Coarse-grained, high-grade metamorphic rock formed under high temperature and pressure conditions
- Exhibits distinct banding of light (quartz, feldspar) and dark (biotite, hornblende) minerals
- Common types include biotite gneiss, hornblende gneiss, and granite gneiss
Non-foliated metamorphic rock types
- Marble
- Formed from limestone or dolostone composed primarily of recrystallized calcite or dolomite
- Sugary or granular texture with interlocking carbonate crystals
- Often used as decorative building stone (Carrara marble, Yule marble)
- Quartzite
- Formed from quartz-rich sandstone composed almost entirely of interlocking quartz grains
- Extremely hard and resistant to weathering due to strong chemical bonds between quartz grains
- Often used as source of high-purity silica (glass making, abrasives)
- Hornfels
- Formed from various parent rocks through near igneous intrusions
- Fine-grained, dense, and massive appearance with splintery or conchoidal fracture
- Mineral composition depends on parent rock and composition of igneous intrusion (spotted hornfels, pyrite hornfels)
Parent rock vs metamorphic rock composition
- Parent rock composition largely determines mineral assemblage of resulting metamorphic rock
- Clay-rich rocks (shale, mudstone) metamorphose into slate, phyllite, and mica schist
- Quartz-rich rocks (sandstone) metamorphose into quartzite
- Carbonate-rich rocks (limestone, dolostone) metamorphose into marble
- Felsic igneous rocks (granite) metamorphose into gneiss
- (temperature, pressure conditions) also influences mineral assemblage and texture
- Low-grade metamorphism produces smaller, fine-grained minerals and well-developed foliation
- Medium-grade metamorphism results in larger crystals and more diverse mineral assemblages
- High-grade metamorphism produces coarse-grained minerals and less pronounced foliation
Key Terms to Review (24)
Amphibolite facies: Amphibolite facies refers to a specific set of metamorphic conditions characterized by moderate to high temperatures and pressures, typically ranging from about 500 to 700 degrees Celsius and pressures from 4 to 8 kilobars. This facies is primarily associated with the presence of amphibole minerals, particularly hornblende, and is indicative of a metamorphic environment where the original rock, or protolith, was subjected to significant alteration without melting.
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.
Foliated metamorphic rocks: Foliated metamorphic rocks are a type of metamorphic rock characterized by their layered or banded appearance, which results from the alignment of mineral grains under directed pressure. This texture develops during metamorphism when existing rocks are subjected to intense heat and pressure, causing minerals to recrystallize and align perpendicular to the stress direction, creating a distinct foliation. Foliation is a key feature that helps to classify these rocks and indicates the conditions of their formation.
Gneiss: Gneiss is a high-grade metamorphic rock characterized by its distinct foliation and banding, resulting from the intense heat and pressure applied to pre-existing rocks. This rock is typically formed from granite or sedimentary rock and displays a coarse-grained texture, making it easily identifiable. The unique textures and structures of gneiss, along with its classification as a metamorphic rock, stem from specific metamorphic processes and conditions that involve significant geological forces.
Granoblastic texture: Granoblastic texture refers to a type of metamorphic rock texture characterized by equidimensional, interlocking grains that are typically of similar size. This texture is often found in metamorphic rocks formed under high temperatures and pressures, where minerals grow together without any preferred orientation, indicating a uniform stress environment during metamorphism.
Greenschist facies: Greenschist facies is a set of metamorphic conditions characterized by moderate temperature and pressure, where minerals such as chlorite, albite, and actinolite are typically formed. This facies indicates a specific range of metamorphic conditions usually found in subduction zones or areas of regional metamorphism, reflecting the geological processes that transform pre-existing rocks into metamorphic rocks.
Hornfels: Hornfels is a fine-grained metamorphic rock that forms through the contact metamorphism of shale or other sedimentary rocks due to high temperatures and pressures. This rock is characterized by its dense, hard texture and often exhibits a variety of colors due to the mineral composition, which can include quartz, feldspar, and mica. The unique formation process of hornfels highlights the significance of temperature and pressure in metamorphic transformations.
Index minerals: Index minerals are specific minerals that form under particular pressure and temperature conditions during metamorphism, serving as indicators of the metamorphic environment in which they were formed. These minerals help geologists determine the metamorphic grade and conditions of a rock, which aids in classifying metamorphic rocks based on their formation processes.
Marble: Marble is a metamorphic rock formed from the recrystallization of limestone under heat and pressure, resulting in a dense and durable material. It is characterized by its fine to coarse crystalline texture and often exhibits a variety of colors due to mineral impurities. The transformation process gives marble its unique veined patterns, making it a popular choice for sculptures and architectural features.
Metamorphic facies: Metamorphic facies are distinct groups of metamorphic rocks that form under specific pressure and temperature conditions, reflecting the geological environment in which they developed. Each facies represents a specific range of metamorphic conditions that result in particular mineral assemblages, helping geologists understand the metamorphic history of rocks and the tectonic processes involved in their formation. By studying metamorphic facies, we can classify metamorphic rocks and relate them to the conditions present during their formation.
Metamorphic grade: Metamorphic grade refers to the intensity of temperature and pressure conditions under which metamorphic rocks form, indicating the degree of metamorphism a rock has undergone. Higher metamorphic grades signify more intense conditions, resulting in significant mineral changes and texture alterations in the rock, which help in classifying different types of metamorphic rocks based on their characteristics and formation processes.
Mid-ocean ridges: Mid-ocean ridges are underwater mountain ranges formed by the process of seafloor spreading, where tectonic plates pull apart and magma rises to create new oceanic crust. These features are significant because they represent the boundary between diverging tectonic plates, playing a crucial role in plate tectonics, geological processes, and the formation of metamorphic rocks under specific pressure and temperature conditions.
Mineralogy: Mineralogy is the branch of geology that studies the composition, structure, properties, and distribution of minerals. It focuses on understanding how minerals form, their chemical and physical properties, and how they are classified. This field is crucial for exploring various geological processes, including the formation of rocks and the transformation of materials in different environments.
Non-foliated metamorphic rocks: Non-foliated metamorphic rocks are a type of metamorphic rock that do not exhibit a layered or banded appearance. Instead, these rocks typically have a more uniform texture due to the absence of significant pressure during their formation, allowing minerals to crystallize without being aligned. They often form in environments where the temperature is high and the pressure is relatively low, resulting in distinct physical characteristics that differentiate them from foliated metamorphic rocks.
Parent Rock: Parent rock refers to the original rock from which a metamorphic rock is formed through the process of metamorphism. This term is crucial for understanding how different types of rocks evolve, as the characteristics of the parent rock heavily influence the properties and textures of the resulting metamorphic rock. Additionally, in igneous processes, parent rocks can be related to the source material from which magma is generated, connecting to the classification and textural aspects of igneous rocks.
Phyllite: Phyllite is a fine-grained metamorphic rock that is characterized by its shiny appearance and foliation, which results from the alignment of platy minerals, primarily mica. It forms under moderate temperature and pressure conditions, typically from the metamorphism of shale or mudstone, and exhibits a distinct sheen due to the presence of fine-grained mica. Phyllite represents an important stage in the metamorphic process, bridging the gap between slate and schist.
Quartzite: Quartzite is a hard, metamorphic rock formed from sandstone that has undergone high heat and pressure, resulting in a dense, durable material primarily composed of quartz. Its formation involves the recrystallization of the original sand grains, leading to a tightly interlocked structure that enhances its strength and resistance to weathering. Quartzite is commonly found in various geological settings and is often used in construction and decorative applications due to its appealing appearance and durability.
Regional metamorphism: Regional metamorphism is the process where rocks undergo significant changes in mineralogy and texture due to high pressure and temperature over large areas, typically associated with tectonic forces. This type of metamorphism is important as it helps in understanding the formation of various rock types and their association with geological processes like mountain building and plate tectonics.
Schist: Schist is a medium- to coarse-grained metamorphic rock characterized by its pronounced foliation and the presence of significant amounts of platy minerals such as mica. This foliation forms due to the alignment of these minerals under directed pressure and high temperatures during metamorphism, making schist an important example of how metamorphic processes and conditions create distinct rock textures and structures.
Slate: Slate is a fine-grained metamorphic rock that originates from shale through the process of metamorphism, characterized by its excellent foliation and ability to be split into thin, flat sheets. This rock is primarily composed of quartz and mica minerals and showcases a distinctive sheen when polished. Its formation under low-grade metamorphic conditions makes it an important example of how pressure and temperature influence rock properties.
Slaty cleavage: Slaty cleavage is a type of foliation found in metamorphic rocks, characterized by the ability to split into thin, flat layers. This texture occurs due to the parallel alignment of platy minerals, mainly mica, under directed pressure during metamorphism, leading to a distinct layering that allows for easy separation of the rock. The presence of slaty cleavage is a key feature that helps in identifying and classifying certain metamorphic rocks, particularly schists and slates.
Subduction Zones: Subduction zones are regions of the Earth's crust where one tectonic plate moves under another and sinks into the mantle. These zones are critical areas of geological activity, leading to the formation of deep ocean trenches, volcanic arcs, and earthquakes, as well as influencing metamorphic processes and rock classification.
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.
William H. P. C. W. W. M. R. M. C. S. S. A. K.: William H. P. C. W. W. M. R. M. C. S. S. A. K. refers to a classification system for metamorphic rocks based on various factors such as mineral composition, texture, and the conditions of metamorphism. This classification helps geologists understand the origin and evolution of these rocks, making it easier to identify their characteristics and their geological history.