What is the primary difference between basal sliding and internal deformation?

Basal sliding involves the entire mass of ice moving over the bedrock facilitated by meltwater lubrication, whereas internal deformation is the slow movement of ice crystals sliding past one another within the glacier due to gravity and pressure.

How does the Pressure Melting Point (PMP) affect glacial movement?

The PMP is the temperature at which ice melts under pressure; if the base of a glacier reaches this temperature, meltwater is produced, enabling basal sliding. If the temperature is below PMP, the glacier is frozen to the bed and moves only via internal deformation.

What is the difference between glacial till and fluvio-glacial deposits?

Glacial till is unsorted, unstratified debris deposited directly by the ice, containing a mix of sizes from clay to boulders. Fluvio-glacial deposits are sorted and stratified by meltwater, which carries and deposits material based on the water's energy levels.

What is a common misconception regarding 'glacial retreat'?

A common error is assuming that the ice itself moves backward during retreat. In reality, the ice always flows downslope due to gravity; 'retreat' simply means the front of the glacier is melting faster than the ice is being replaced from upstream.

Why is it incorrect to assume all glaciers erode the landscape equally?

Cold-based glaciers are frozen to the bedrock and move very slowly with minimal basal sliding, meaning they often preserve the underlying landscape rather than eroding it, unlike highly erosive warm-based glaciers.

What error occurs when identifying the cause of a landform without considering equifinality?

Equifinality means different processes can produce similar results; failing to consider this may lead to an incorrect conclusion about a landscape's history, such as assuming a depression is always erosional when it could be depositional (e.g., a kettle hole).

Define 'Firn' (or Névé) in the context of glacial formation.

Firn is intermediate-stage snow that has survived at least one summer season and has been compacted into a denser, granular form, but has not yet reached the density of true glacial ice.

What is 'Entrainment'?

Entrainment is the process by which a glacier captures and incorporates loose rock and sediment into its ice mass, which can then be transported supraglacially, englacially, or subglacially.

Define 'Abrasion' and its characteristic evidence.

Abrasion is the mechanical scraping of bedrock by debris frozen into the base of a glacier. Its primary evidence includes polished rock surfaces, striations (fine scratches), and the production of rock flour.

Why does glacial ice often appear blue?

As snow is compacted into ice, air bubbles are squeezed out. The resulting dense ice absorbs longer wavelengths of light (reds) and scatters shorter wavelengths (blues), giving it a distinct blue appearance.

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Glaciated Landscape & Change

Glacial Processes

Summary

Glacial processes encompass the dynamic systems of ice formation, movement, and the geomorphic actions of erosion, transportation, and deposition. These processes are driven by gravity and thermal regimes, specifically the relationship between pressure and the melting point of ice, which dictates how glaciers reshape the Earth's surface across various spatial scales.

1. Definition & Core Concepts

Glaciers as Open Systems: Glaciers function as open systems with inputs (snowfall, avalanches), throughputs (ice movement), and outputs (meltwater, calving, evaporation). The balance between these is known as the glacial budget or mass balance.
Ice Formation Process: Glacial ice forms through the compaction of snow over time. As layers accumulate, snow is compressed into firn (or névé), eventually squeezing out air pockets to form dense, blue-tinted glacial ice.
Accumulation and Ablation: The accumulation zone is the upper part of the glacier where inputs exceed outputs, while the ablation zone is the lower part where melting and calving exceed accumulation. The equilibrium line marks the boundary where these two are equal.

Cross-section of a glacier showing accumulation and ablation zones, the equilibrium line, and movement vectors including basal sliding.

2. Underlying Principles of Movement

Gravity and Shear Stress: Glacial movement is driven by gravity acting on the mass of the ice. As the weight and slope angle increase, the shear stress at the base increases, eventually overcoming the friction of the bedrock.
Pressure Melting Point (PMP): The PMP is the temperature at which ice melts under a specific pressure. Because water expands when it freezes, increasing pressure actually lowers the melting point (e.g., at 200 atmospheres, ice melts at $-1.85$ Celsius).
Basal Sliding vs. Internal Deformation: In warm-based glaciers, meltwater at the base acts as a lubricant, allowing the ice to slide over the bedrock (basal sliding). In cold-based glaciers, the ice is frozen to the bed, and movement occurs only through the slow shifting of ice crystals (internal deformation).

3. Methods of Glacial Erosion

4. Key Distinctions: Thermal Regimes

5. Exam Strategy & Tips