Ethan Clarke
Plastic flow occurs when a material under severe stress starts to behave as a Newtonian fluid, leading to permanent deformation. This phenomenon is observed in both non-metallic and metallic materials, with the former tending to exhibit plastic flow faster. While high pressure can influence the flow of materials, the primary mechanism causing plastic flow is basal slip, which involves the movement of dislocations within the crystal lattice of the material. In the context of glaciers, basal slip occurs when the ice at the bottom slides over the underlying surface, facilitated by the presence of meltwater acting as a lubricant.
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What You'll Learn
High pressure increases the likelihood of plastic flow
Plastic flow occurs when a material under severe stress starts to behave as a Newtonian fluid. This is a form of plastic strain, or deformation, that is permanent and persists even after the stress is removed. It can occur under sustained or high forces. Materials that have undergone plastic flow may still have parts that are working elastically, while other areas have yielded.
While high pressure can influence the flow of ice, it is not the direct cause of plastic flow. The movement along the basal planes, or the bottom of the glacier, defines the flow mechanism. This movement is called basal slip and is facilitated by the presence of meltwater, which acts as a lubricant. In warm glaciers, the basal slip occurs most effectively at temperatures close to freezing.
The thickness of a glacier also plays a role in plastic flow. A minimum thickness of 22 meters is required for plastic flow in temperate glaciers. This is because the pressure at depth contributes to the stress required for plastic flow to occur.
In summary, high pressure increases the likelihood of plastic flow by increasing the stress on materials. This is particularly evident in glaciers, where the pressure at depth contributes to the deformation of the ice. However, it is important to note that high pressure is not the only factor influencing plastic flow, and the specific mechanisms can vary depending on the material and conditions.
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Basal slip is the primary cause of plastic flow in glaciers
Plastic flow occurs when a material under severe stress starts to behave as a Newtonian fluid and is a form of plastic strain. This is a deformation process that is permanent and persists on stress unloading. It can occur under sustained force as well as high forces. In the context of glaciers, plastic flow is caused primarily by basal slip, which occurs when the ice at the bottom slides over the underlying surface. This is facilitated by the presence of meltwater, which acts as a lubricant.
The movement of glaciers is influenced by various factors, including the accumulation of mass, ablation processes, and the slope of the underlying terrain. Glaciers accumulate mass in their upper portions through precipitation and wind-blown snow, while ablation occurs in the lower portions through melting, sublimation, and calving icebergs. The combination of these processes and the force of gravity leads to the downslope movement of glaciers.
Basal slip, or basal sliding, is a key mechanism in glacier flow. It occurs when the ice at the bottom of the glacier moves over the underlying bed, facilitated by the presence of water, which reduces friction and allows faster ice flow. This process is known as regelation, where the melting of ice under pressure generates a layer of water that helps the glacier move downslope. The roughness of the underlying rock surface, the amount of meltwater, and the weight of the glacier all influence the velocity of basal sliding.
While high-pressure levels can increase the likelihood of deformation, they are not the direct cause of plastic flow in glaciers. Instead, it is the movement along basal planes, facilitated by basal slip, that defines the flow mechanism. Basal slip occurs most effectively at temperatures close to freezing, and it is enhanced by the presence of water acting as a lubricant. This water lubrication can be provided by meltwater from the glacier itself or from water in the underlying sediment or bedrock.
In summary, basal slip is the primary cause of plastic flow in glaciers due to the movement it facilitates along the basal planes. This movement is influenced by factors such as temperature, water lubrication, and the weight of the glacier, all contributing to the overall flow of the glacier.
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Crevasses are cracks that form in the upper layers of glaciers
Crevasses are deep cracks that form in the upper layers of glaciers. They are not to be confused with plastic flow, which is the permanent deformation of a material under severe stress. While crevasses are related to plastic flow, they are not the cause of it.
Crevasses are formed due to the movement and behaviour of ice under varying topographical conditions. They are influenced by the tension in the brittle upper layer of the glacier as it moves over uneven terrain, causing it to crack while the lower ductile layer flows beneath. This movement creates shear stress, which causes breakage along the faces of the cracks. The size of crevasses depends on the amount of liquid water present in the glacier. They can be as deep as 45 meters and as wide as 20 meters.
There are several types of crevasses, including longitudinal, splaying, and transverse. Longitudinal crevasses form parallel to the flow of the glacier and are typically concave down, forming an angle greater than 45 degrees with the margin. Splaying crevasses appear along the edges of a glacier and result from shear stress from the margin and longitudinal compressing stress from lateral extension. Transverse crevasses are the most common type and form in a zone of longitudinal extension where the principal stresses are parallel to the direction of glacier flow, creating extensional tensile stress.
The formation of crevasses can be influenced by changes in velocity, such as when a glacier accelerates or decelerates due to topographical features. For example, if a glacier speeds up as it flows down a steep slope, the upper ice may not be able to keep up, resulting in cracks. Crevasses can also form when faster-moving ice collides with slower-moving ice, such as when a glacier moves over a bump or step in its path.
The presence of crevasses can be extremely dangerous for anyone attempting to navigate across a glacier. They are often covered by snow bridges, making them invisible and potentially lethal. Mountaineers and climbers use techniques such as roping together in teams and friction knots to minimize the risk of falling into hidden crevasses.
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Low pressure typically acts against the conditions needed for plastic flow
Plastic flow occurs when a material is subjected to stress beyond its elastic limit, causing it to deform permanently. This phenomenon is observed in various materials, including metals, ice, and non-metallic substances. While high pressure can increase the likelihood of deformation, low pressure typically acts against the conditions necessary for plastic flow to occur.
In the context of glaciers, the primary cause of plastic flow is basal slip, which happens when the ice at the bottom slides over the underlying surface. High-pressure levels at greater depths can make ice more susceptible to deformation, but the direct mechanism for plastic flow is the movement along basal planes facilitated by meltwater. Basal slip occurs when meltwater acts as a lubricant, enabling the glacier to slide over the ground, particularly at temperatures close to freezing.
However, low pressure generally reduces the stress on materials, making them less likely to undergo plastic flow. It acts against the conditions required for plastic flow by decreasing the likelihood of permanent deformation. This is because plastic flow is a form of plastic strain, which occurs when a material under severe stress starts to behave as a Newtonian fluid. While high pressure can contribute to plastic flow by increasing stress, it is not the sole or most direct cause.
The occurrence of plastic flow depends on multiple factors, including the material's properties and the presence of lubricants or water. For example, in the case of gears, plastic flow can be influenced by high contact stresses, low sliding velocities, and inadequate lubricating oil viscosity. Additionally, the hardening process of materials, such as case hardening or through hardening, can impact their susceptibility to plastic flow.
Overall, while high pressure can influence the likelihood of plastic flow, low pressure typically counteracts the conditions necessary for this phenomenon to occur.
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Plastic flow is associated with softer materials
Plastic flow is a deformation process that occurs when a material under severe stress starts to behave as a Newtonian fluid. It is a form of plastic strain that results in the gradual, permanent change in the dimensions of solid materials. While plastic flow can occur in any material, it is generally associated with softer materials or those that have not undergone a hardening process.
In the context of gears, plastic flow is often observed in softer materials, particularly when the hardening process has not been initiated. This can lead to surface deformation, such as rippling or ridging, caused by high contact stresses and the rolling and sliding action of tooth meshing during operation. To prevent failure in gears, it is crucial to reduce the load applied as the speed of the driven unit increases.
Basal slip, a mechanism where the ice at the bottom of a glacier moves over the underlying bed, is another example of plastic flow in softer materials. This typically occurs in warm glaciers, where the base is lubricated by meltwater, allowing the glacier to slide over the ground. High-pressure levels at greater depths also contribute to plastic flow by increasing the likelihood of deformation in the ice.
Plastic flow can also be observed in non-metallic materials, which tend to exhibit plastic flow faster than metallic materials. High temperatures are not necessary for plastic flow to occur, but they can accelerate the process. Overall, plastic flow is a critical phenomenon to consider in materials science, especially when dealing with softer or unhardened substances.
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Frequently asked questions
Plastic flow is a permanent deformation of a material under severe stress.
Basal slip is the main mechanism that causes plastic flow. It involves the movement of dislocations within the crystal lattice of the material.
Basal slip occurs when the base of a glacier is lubricated by meltwater, allowing the glacier to slide over the ground.
High pressure can contribute to plastic flow by increasing the stress on a material. However, it is not the only or most direct cause.
Yes, temperature and the rate of deformation can also affect plastic flow. Higher temperatures and faster deformation rates can accelerate the process.
