Question

Describe the relative rates of ice flow within the following parts of a glacier: (a) the bottom versus the top and (b) the edges versus the middle. Explain.

Short answer

The relative rates of ice flow within a glacier show variations between different parts: (a) at the bottom, the ice flow is generally faster due to higher pressures, warmer temperatures, and the possibility of basal sliding caused by melting and water lubrication, whereas the top exhibits slower flow due to less pressure and colder, more brittle ice; (b) the ice flow at the edges of the glacier is slower compared to the middle, as a result of increased friction from contact with the valley walls, while the middle part experiences less friction and flows more freely.

Step-by-step solution

Understand glacier flow

A glacier is a large body of ice that forms on land due to the accumulation, compaction, and recrystallization of snow. Glaciers flow under the influence of their own weight and gravity. This flow can be divided into two main types: internal deformation, which involves the movement of ice crystals, and basal sliding, which is the result of the glacier sliding over its bed.

Compare ice flow at the bottom versus the top of a glacier

The rate of ice flow within a glacier varies with depth. At the top (or surface) of a glacier, the ice is relatively cold, brittle, and experiences less pressure. It is therefore less prone to deformation and flows more slowly. On the other hand, ice at the bottom of a glacier is subjected to higher pressures due to the overlying weight of ice, and it is warmer due to geothermal heat from the Earth's interior. This causes the ice to be more plastic-like and able to flow more easily. Additionally, the high pressure at the base of the glacier may cause melting, leading to water acting as a lubricant between the ice and the bedrock, which can lead to basal sliding and increased rates of flow. In summary, the rate of ice flow at the bottom of a glacier is generally faster than at the top due to differences in pressure, temperature, and deformation.

Compare ice flow at the edges versus the middle of a glacier

The rate of ice flow also varies laterally across a glacier, with differences observed between the edges and the middle. The ice at the edges of a glacier interacts with the valley walls, which can create friction and slow down the flow of ice. This is known as the "no-slip condition." Conversely, due to the absence of valley walls restricting its movement, ice in the middle (or center) of a glacier experiences less friction and flows more freely. In summary, the rate of ice flow at the edges of a glacier is generally slower than in the middle due to the additional friction caused by contact with the valley walls.

Key concepts

Internal Deformation

Internal deformation plays a crucial role in the movement of glaciers. It occurs as ice crystals within the glacier shift and reorganize under stress. As gravity pulls the glacier downhill, the pressure from overlying ice layers press down, causing these crystals to slip past each other. This enables the glacier to bend and flow akin to a slow-moving river.

• At the core of internal deformation is temperature: warmer ice deforms more easily than colder ice.
• Additionally, increased pressure also enhances flow, particularly in the lower ice layers.

This process is a continuous cycle, with gradual adjustments helping the glacier to advance, retreat, or maintain its position based on climatic conditions.

Basal Sliding

Basal sliding is another significant mechanism of glacier movement. Unlike internal deformation, which involves movement within the glacier itself, basal sliding refers to the glacier moving over the ground beneath it. This movement is facilitated largely by the presence of meltwater at the glacier’s base, which acts as a lubricant.

• The meltwater is often a result of pressure-induced melting or geothermal heat.
• The water reduces friction between the ice and the bedrock, allowing the glacier to glide downhill.

This combination of sliding and flowing helps the glacier to move more effectively, particularly in warmer climates where more meltwater is generated.

Pressure and Temperature Effects

Pressure and temperature significantly influence how glaciers move. As pressure increases with depth within a glacier, the basal layers become warmer due to geothermal heat. This warmth makes the ice softer and more pliable, enhancing both internal deformation and basal sliding.

• High pressure at the glacier’s base encourages melting, producing the meltwater necessary for basal sliding.
• Conversely, colder temperatures near the surface mean the ice remains more rigid, slowing movement.

The debt and sheer volume of a glacier, therefore, dictate how it will move, with warmer and more pressured areas favoring faster movement.

Friction

Friction plays a critical role in modulating glacier speed. It primarily occurs at the glacier's edges, where the ice interfaces with valley walls, causing a slowdown.

• This is due to the no-slip condition, where friction is greater, impeding flow.
• In contrast, the central part of a glacier flows more freely since it experiences less frictional resistance.

The balance of friction across the glacier impacts its overall velocity, with less friction at the center promoting a faster flow compared to the edges, which encounter more resistance.

Ice Crystal Movement

The movement of ice crystals within a glacier determines how effectively it can deform and flow. As the glacier moves, these crystals undergo plastic deformation, changing shape without fully breaking.

• The ability to alter shape allows glaciers to contour to the underlying bedrock and landscape.
• Such movement is particularly pronounced at higher pressures and warmer temperatures, where the crystals can move past each other more easily.

Essentially, the way individual ice crystals move and adjust contributes to the glacier’s overall flow, facilitating both its stability and its responsiveness to environmental triggers.