The glacier as an open system; the glacial budget (accumulation and ablation); and the processes of glacial erosion, transport and deposition, including fluvioglacial processes.

What this dot point is asking

Eduqas wants you to treat the glacier as an open system, explain the glacial budget of accumulation and ablation, and explain the processes of glacial erosion, transport and deposition, including the role of meltwater in fluvioglacial processes.

The answer

The glacier as an open system

A glacier is an open system of inputs, stores, transfers and outputs. Snow accumulates in the zone of accumulation, is compacted through firn into glacier ice, and flows downslope under gravity into the zone of ablation where it is lost. Movement occurs by internal deformation (the ice deforms under its own weight) and, where meltwater is present, by basal sliding over a lubricated bed. The position of the snout reflects the budget over recent years: when accumulation outpaces ablation the glacier advances, and when ablation dominates (as under contemporary warming) it retreats.

Warm and cold-based ice

This distinction explains why alpine valley glaciers (warm-based) carve dramatic troughs while parts of the Antarctic ice sheet (cold-based) protect the rock beneath. Within a single glacier the regime can vary, so erosion is concentrated where meltwater allows sliding.

Glacial and fluvioglacial processes

Glacial erosion works by plucking (meltwater freezes onto jointed bedrock and the moving ice pulls away blocks) and abrasion (entrained debris grinds and striates the bedrock). Freeze-thaw weathering above the ice supplies angular debris that feeds abrasion. Transport carries debris supraglacially (on top), englacially (within) and subglacially (at the base), and the unsorted material is till. Deposition of this unsorted till occurs directly from the ice. Fluvioglacial processes involve meltwater, which sorts and rounds sediment and deposits it in layers, so fluvioglacial landforms (outwash, eskers, kames) are stratified and sorted, in contrast to the unsorted till of direct glacial deposition.

Examples in context

Example 1. Retreating alpine glaciers and the mass balance signal. Across the European Alps, warm-based valley glaciers such as those of the Mont Blanc massif have shown sustained negative mass balance for decades, with the equilibrium line rising and snouts retreating hundreds of metres. As the ice thins and the snout pulls back, it exposes freshly scoured bedrock, leaves behind unsorted till and reveals the U-shaped trough it carved, while meltwater streams rework the debris into sorted outwash downstream. The Alps are a clear contemporary example linking a negative glacial budget directly to landscape change, the connection Eduqas repeatedly tests.

Example 2. Cold-based ice in Antarctica. Much of the East Antarctic ice sheet is cold-based, frozen to its bed, so it moves almost entirely by internal deformation and erodes the rock beneath very little. Ancient landscapes and even preserved river valleys survive beneath the ice precisely because cold-based ice protects rather than carves. The contrast with warm-based alpine glaciers is the textbook way to show that the basal thermal regime, not just the presence of ice, determines whether a glaciated landscape is deeply eroded or preserved.

Try this

Q1. Define the equilibrium line of a glacier. [2 marks]

• Cue. The line on a glacier where accumulation equals ablation over a year, separating the zone of accumulation above from the zone of ablation below.

Q2. Explain why warm-based ice erodes more than cold-based ice. [3 marks]

• Cue. Warm-based ice has basal meltwater that lets it slide over its bed and supplies water for plucking, so it moves faster and erodes strongly; cold-based ice is frozen to its bed, moves only by internal deformation and barely erodes.