Alexei Volkov
Plastic flow is a geological phenomenon that occurs when rocks and ice move under intense pressure, causing them to flow like a highly viscous substance and resulting in irreversible changes to their shape. This process, known as plastic deformation, is of particular importance in the study of glacier flow and the behaviour of rocks and ice under high pressure and temperature conditions. It involves two main processes: intracrystalline gliding and recrystallization. The transition from elastic behaviour to plastic flow is a critical concept in understanding the mechanics of materials, especially in metal-forming processes and the behaviour of rocks within the Earth's crust.
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What You'll Learn
Rock plasticity theories
Plastic flow in geology refers to the deformation of rocks caused by a sustained force. Rock plasticity theories are based on the concept that rocks can exhibit plasticity, or irreversible deformation without fracture, under certain conditions. This goes against the conventional belief that rocks are brittle and typically fail by fracture.
There are several laboratory tests used to characterise rock plasticity, including confining pressure tests, pore pressure or effective stress tests, temperature-dependent tests, and strain rate-dependent tests. These tests help to determine the mechanical behaviour of rocks in terms of their strength and the applicability of plasticity theories.
Typical behaviours observed in rocks include strain softening, perfect plasticity, and work hardening. Rocks may exhibit strain softening, where they become more ductile with increasing deformation. Perfect plasticity refers to the ability of rocks to undergo deformation without fracture. Work hardening, on the other hand, describes the phenomenon where rocks become stronger with increasing deformation.
The governing equations and constitutive models for plasticity do not fully capture the complexities of plastic deformation in rocks. An additional kinematic assumption is required to account for both the elastic and plastic parts of the strain. Rock plasticity theories are developed based on specific requirements, including the presence of a linear elastic range and a well-defined elastic limit.
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Glacier flow
Ice deformation, or ice creep, is a process where the layers within an ice crystal shear parallel to each other without disrupting the crystal lattice. Ice creep is the mutual displacement of ice crystals relative to one another in response to applied shear stress, resulting in a slow forward motion in the direction of the ice-surface slope. This process is similar to the plastic flow of rocks within the Earth under extremely high pressures and elevated temperatures.
Basal sliding is the process by which a glacier slides over its bed, often on a thin film of lubricating meltwater. This sliding is accompanied by basal creep and regelation (refreezing). Subglacial bed deformation occurs when the glacier is decoupled from its bed, and the glacier flows through the deformation of underlying unfrozen subglacial sediments. This occurs when the water pressure in the pore spaces between sediment grains rises sufficiently to overcome the resistance between individual grains, allowing them to move relative to one another.
The rate of glacier flow varies, with ice streams and tidewater glaciers typically exhibiting the fastest ice flow velocities. Surging glaciers are characterised by flow instabilities, with long periods of slow flow punctuated by short phases of rapid ice flow.
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Intracrystalline gliding
Plastic flow is a phenomenon observed in materials such as rocks and ice when they are subjected to intense pressure and start flowing like a highly viscous substance. This results in plastic deformation, where the material does not return to its original shape even after the removal of pressure. In the context of intracrystalline gliding, this process specifically refers to the behaviour of ice crystals.
The term "gliding" aptly describes the relative motion of the layers within the ice crystal, which can be likened to the smooth, gliding motion of adjacent surfaces sliding past each other with minimal friction. This gliding motion allows for the redistribution of the ice crystal's structure, enabling it to adapt to the applied pressure without catastrophic failure or the formation of fractures. The ice crystal, through intracrystalline gliding, can adjust its internal arrangement to accommodate the forces acting upon it.
It is worth noting that while intracrystalline gliding primarily describes the behaviour of ice crystals, similar processes may occur in other crystalline materials under specific conditions. The study of intracrystalline gliding has provided valuable insights into the microscopic mechanisms of plastic flow in a broader range of materials, contributing to our understanding of material science and the behaviour of solids under pressure.
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Recrystallization
Plastic flow is a geological phenomenon that occurs when rocks and ice move under intense pressure, causing them to flow like a highly viscous substance and retain their deformed shape even when the pressure is removed. This process, known as plastic deformation, is crucial in understanding glacier flow and various geological processes such as rock folding and rock flow within the Earth's crust under high pressure and temperature conditions.
An essential aspect of plastic flow is recrystallization, a common microstructural transformation that occurs during deformation, metamorphism, and diagenesis of rocks. Recrystallization involves the rearrangement of crystal lattices, resulting in new grain boundaries and altered microstructures. This process can occur through various mechanisms, including grain boundary migration and new grain boundary formation, which can happen simultaneously or independently.
The specific recrystallization mechanisms depend on factors such as temperature, strain rate, and the presence of impurities. For example, at lower temperatures and faster strain rates, recovery occurs through strain-induced grain boundary migration recrystallization. As the temperature increases or the strain rate decreases, dynamic recrystallization takes place through progressive subgrain rotation. Further alterations in temperature and strain rate regimes influence the dominance of grain boundary migration and subgrain rotation mechanisms.
The study of recrystallization is crucial in understanding the behaviour of rocks under various geological conditions. It provides insights into the microstructural changes that occur during deformation and metamorphism, contributing to the development of domes, folds, and other geological features. By examining recrystallization mechanisms, scientists can also determine the temperature, pressure, and strain conditions that rocks have experienced, enhancing our understanding of Earth's geological history.
In summary, recrystallization is a fundamental process in plastic flow and rock deformation, playing a significant role in shaping the Earth's crust and contributing to our knowledge of geological processes and Earth's dynamic history.
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Flow plasticity theory
Plastic flow is a geological process that involves the movement of materials, particularly rocks and ice, under intense pressure. This movement results in an irreversible change in the shape of the material without any fracturing. In the context of geology, plastic flow is observed in processes such as rock folding and rock flow within the Earth's crust under extremely high pressures and elevated temperatures.
One of the fundamental assumptions in flow plasticity theory is the concept of a flow rule. In metal plasticity, the flow rule states that the plastic strain increment and the deviatoric stress tensor share the same principal directions. This assumption, known as the co-directionality assumption or the normality condition, simplifies the mathematical description of plastic deformation in metals.
However, when applying flow plasticity theory to rock plasticity, the pressure-dependence of the yield surface needs to be considered. This pressure-dependence leads to a relaxation of the co-directionality assumption. Instead, it is typically assumed that the plastic strain increment and the normal to the pressure-dependent yield surface share the same direction. This modified form of the flow rule is known as an associated flow rule.
The Prager consistency condition is another important aspect of flow plasticity theory. It is used to close the set of constitutive equations and eliminate unknown parameters. By examining the work done over a cycle of plastic loading and unloading, the validity of the associated flow rule can be justified.
Large deformation flow theories of plasticity often start with certain assumptions about the decomposition of the deformation tensor. One assumption suggests that the rate of deformation tensor can be additively separated into an elastic part and a plastic part. Alternatively, the deformation gradient tensor can be multiplicatively decomposed into an elastic component and a plastic component. Over time, the second assumption involving multiplicative decomposition has become more widely adopted for simulations and modelling.
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Frequently asked questions
Plastic flow is a process in geology where rocks and ice move under intense pressure, flowing like a viscous liquid and permanently deforming without fracturing.
Plastic flow is caused by the intense pressure and high temperatures acting on the material.
There are two main processes: intracrystalline gliding, where layers within a crystal shear parallelly without breaking the crystal lattice, and recrystallization.
An example of plastic flow is the movement of glaciers, where ice flows around and over obstacles, leading to deposition and the formation of new structures.
