7.1 Metamorphic processes and conditions
3 min read•july 22, 2024
Metamorphism transforms existing rocks without melting, driven by changes in , , and fluids. This process alters mineral composition, texture, and structure, occurring in Earth's crust and upper mantle. It's a key mechanism in rock formation and transformation.
Factors like temperature, pressure, fluids, and parent rock composition control metamorphism. Different types include contact, regional, and hydrothermal metamorphism. Mechanisms like and solid-state diffusion work together to create new minerals and textures in metamorphic rocks.
Metamorphic Processes and Conditions
Definition of metamorphism
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- Transformation of pre-existing rocks into new rocks through changes in physical and chemical conditions without melting
- Involves recrystallization, solid-state diffusion, deformation, and
- Results in changes to mineral composition, texture, and structure of the original rock (parent rock)
- Occurs within the Earth's crust and upper mantle
- Driven by changes in temperature, pressure, and the presence of fluids
Factors controlling metamorphic processes
- Temperature
- Higher temperatures increase the rate of metamorphic reactions
- Facilitates recrystallization and solid-state diffusion
- determines temperature changes with depth in the Earth's crust
- Pressure
- Higher pressures can change mineral stability and influence metamorphic reactions
- Lithostatic pressure is exerted by the weight of overlying rocks
- Directed pressure is applied in a specific direction, often resulting from tectonic forces (compression, extension)
- Fluids
- Play a crucial role in facilitating metamorphic reactions and transporting elements
- Water lowers the melting point of rocks and enhances the mobility of elements
- Carbon dioxide and other volatiles influence mineral stability and facilitate metasomatism
- Composition of the parent rock
- Determines the types of minerals that can form during metamorphism
- Influences the metamorphic grade and the resulting mineral assemblages
Types of metamorphism
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- Occurs when rocks are heated by the intrusion of magma or lava
- Affects a relatively small area around the intrusion (aureole)
- Characterized by the formation of high-temperature, low-pressure metamorphic minerals (hornfels)
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- Occurs over large areas due to heat and pressure during mountain-building events or deep burial
- Affects extensive regions of the Earth's crust
- Characterized by the formation of metamorphic rocks with distinct mineral assemblages and textures (schists, gneisses)
- Often associated with the development of and lineation
- Hydrothermal metamorphism
- Occurs when rocks are altered by the interaction with hot, ion-rich fluids
- Typically associated with volcanic or magmatic activity, or deep circulation of groundwater
- Can lead to the formation of ore deposits and the alteration of rocks near hydrothermal vents
- Characterized by the introduction or removal of elements, leading to changes in the chemical composition of the rocks
Mechanisms of metamorphic change
- Recrystallization
- Formation of new minerals or changes in the size, shape, or orientation of existing minerals
- Driven by the minimization of surface energy and the reduction of strain energy
- Can lead to the development of larger, more stable mineral grains
- May result in the formation of metamorphic textures (foliation, lineation)
- Solid-state diffusion
- Movement of atoms within a solid material in response to chemical potential gradients
- Occurs at the atomic scale and is driven by differences in chemical concentration and temperature
- Allows for the exchange of elements between minerals, leading to changes in mineral composition
- Plays a crucial role in the formation of metamorphic minerals and the equilibration of mineral assemblages
- Interaction between recrystallization and solid-state diffusion
- Recrystallization creates new mineral surfaces and grain boundaries, facilitating solid-state diffusion
- Solid-state diffusion provides the necessary elements for the growth of new minerals during recrystallization
- The combined effects contribute to the overall equilibration of metamorphic rocks and the development of characteristic metamorphic textures and mineral assemblages
Key Terms to Review (18)
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.
Differentiation: Differentiation is the process by which different minerals or materials segregate from one another during the formation of rocks, especially in the context of metamorphism. This process leads to the development of distinct mineral compositions and textures within rocks, influenced by factors such as temperature, pressure, and the presence of fluids. Understanding differentiation is crucial for grasping how metamorphic rocks evolve and the conditions that affect their formation.
Foliated: Foliated refers to a texture of metamorphic rocks characterized by the parallel alignment of mineral grains, which often results from directed pressure during metamorphism. This alignment can create distinctive layering or banding in the rock, making foliated metamorphic rocks easily identifiable. The degree of foliation can vary from subtle to pronounced, depending on the intensity of the metamorphic conditions involved.
Foliation: Foliation is a textural feature in metamorphic rocks characterized by the parallel alignment of mineral grains or layers, which occurs due to directed pressure during metamorphism. This alignment often results in a layered appearance and can significantly influence the physical properties of the rock, such as its strength and how it breaks. The development of foliation is closely linked to the metamorphic processes that occur under varying pressure and temperature conditions.
Geothermal gradient: Geothermal gradient refers to the rate at which temperature increases with depth below the Earth's surface, typically expressed in degrees Celsius per kilometer. This concept is crucial in understanding metamorphic processes as it influences the conditions under which rocks undergo metamorphism, including changes in mineral composition and texture due to heat and pressure.
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.
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.
James Hutton: James Hutton was a Scottish geologist, often referred to as the 'Father of Modern Geology,' who lived in the late 18th century. His groundbreaking ideas about the Earth’s processes and time laid the foundation for understanding the rock cycle and the dynamic systems that govern geological changes.
Marjorie C. L. B. H. E. K. Hall: Marjorie C. L. B. H. E. K. Hall is a notable figure in the field of geology, particularly recognized for her research and contributions to the understanding of metamorphic processes and conditions. Her work has shed light on how various factors, including temperature, pressure, and the presence of fluids, influence the metamorphic transformation of rocks, leading to the formation of distinct metamorphic textures and minerals.
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.
Metasomatism: Metasomatism is the process of chemical alteration of a rock by the introduction or removal of chemical components, typically through hydrothermal fluids. This process can lead to significant changes in the mineral composition and texture of the rock, influencing its properties and behavior under varying pressure-temperature conditions. Metasomatism often occurs in metamorphic environments, linking it to the broader concepts of metamorphic processes and the classification of metamorphic facies based on specific pressure-temperature conditions.
Non-foliated: Non-foliated refers to a type of metamorphic rock that does not exhibit a layered or banded appearance. Unlike foliated rocks, which show distinct layers due to the alignment of mineral grains under pressure, non-foliated rocks are typically formed from minerals that do not align in a preferred orientation. This can occur under conditions of high temperature and pressure where the original rock's minerals recrystallize without directional stress.
Pressure: Pressure is the force exerted per unit area within a material, often measured in pascals (Pa). In the context of geological processes, pressure plays a critical role in metamorphism, influencing how rocks respond to changes in their environment. This can lead to various metamorphic processes, such as foliation and the formation of new minerals, as rocks are subjected to elevated pressures beneath the Earth's surface.
Recrystallization: Recrystallization is the process where minerals in a rock undergo a transformation, resulting in the formation of new crystals from existing ones, often due to changes in temperature, pressure, or chemical conditions. This process can lead to the alteration of sedimentary rocks into metamorphic ones, showcasing how dynamic Earth processes contribute to the cycling of materials and the evolution of rock types over geological time.
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.
Tectonic setting: A tectonic setting refers to the geological environment in which tectonic processes occur, including the arrangement and movement of Earth's lithospheric plates. It encompasses features like plate boundaries, subduction zones, and rift valleys, influencing the formation of geological structures such as mountains and earthquakes. Understanding the tectonic setting helps explain how metamorphic processes develop under specific conditions.
Temperature: Temperature is a measure of the average kinetic energy of particles in a substance, indicating how hot or cold that substance is. In geology, temperature plays a crucial role in determining the behavior and properties of magma, as well as influencing metamorphic processes that affect the formation of rocks. Higher temperatures can lead to changes in mineral stability and the melting of rock, which are essential for understanding various geological phenomena.