Question

If the budget of a valley glacier were balanced for an extended time span, what feature would you expect to find at the terminus of the glacier? Is it composed of till or stratified drift? Now assume that the glacier's budget changes so that ablation exceeds accumulation. How would the terminus of the glacier change? Describe the deposit you would expect to form under these conditions.

Short answer

If balanced: terminus has a moraine composed of till. With more ablation: glacier retreats, deposits could still be till.

Step-by-step solution

Understanding Glacier Budget

A glacier's budget refers to the balance between accumulation (the gain of ice and snow) and ablation (the loss of ice and snow). If the glacier's budget is balanced, it means that the amount of snow and ice being added is equal to the amount being lost.

Identifying Deposits at the Glacier Terminus

When a glacier's budget is balanced over an extended period, it remains relatively stable, leading to the formation of a terminal moraine at its terminus. This moraine is composed of till, which is an unsorted mixture of sediment deposited directly by the glacier.

Changing Budget to More Ablation

If the glacier's budget changes such that ablation exceeds accumulation, the glacier will retreat. The terminus will move backward, and any existing moraines might be exposed or shifted.

Forming Deposits under New Conditions

Under these conditions of more ablation, you might still expect the formation of moraines, but they could be less prominent as the glacier retreats. The deposits could be a mixture of till, as before, depending on how quickly the retreat happens and whether meltwater is actively reworking sediments into stratified drift.

Key concepts

Glacier Ablation

Glacier ablation is a crucial process in understanding how glaciers change over time. Ablation refers to the loss of ice and snow from a glacier. This can happen through melting, sublimation (where ice turns directly into vapor), or calving (where chunks of ice break off). When ablation increases, it indicates that more ice and snow are being lost than gained, causing the glacier to shrink or retreat. Glacial ablation can fluctuate due to various factors:

• Temperature: Higher temperatures increase melting and, thus, ablation.
• Sun exposure: Glaciers in sunny areas might experience faster melting.
• Wind: Strong winds can enhance sublimation and ice loss.

The rate of ablation is critical for projecting the future size and behavior of glaciers. If ablation consistently exceeds accumulation, the glacier will progressively get smaller.

Glacier Accumulation

Glacier accumulation is the process of snow and ice building up in a glacier. This occurs primarily through snowfall but can also happen through other means, such as wind-drifted snow. Accumulation is vital for a glacier's growth and sustainability. Several factors influence accumulation:

• Snowfall: High snowfall regions are typically associated with significant glacier accumulation.
• Temperature: Colder temperatures favor accumulation since less ice and snow melt.
• Terrain: Areas with shelter from wind might experience more tight-packed snow deposits.

A balanced glacier budget occurs when accumulation equals ablation. When accumulation is greater, the glacier advances, gaining mass and size. This balance plays a crucial role in the glacier's stability over time.

Terminal Moraine

Terminal moraines are fascinating geological features formed by glaciers. These are ridges made up of debris (till) that a glacier pushes to its end, or terminus, when it reaches its furthest forward point. Characteristics of terminal moraines include:

• Composition: They are primarily made of till—a mixture of rock, sand, and clay that the glacier has carried along its journey.
• Shape: Terminal moraines form as distinct ridges standing at the most advanced position of the glacier.
• Stability: In balanced glacier budgets, terminal moraines mark points where the glacier was steady for extended periods.

If the glacier retreats due to more ablation, these moraines might be left behind or moved. New deposits formed during retreat could include stratified drift if meltwater sorts sediments by size and density. Terminal moraines offer a record of past glacier extents and help geologists understand historical climate variations.