Question

If the budget of a valley glacier were balanced for an extended span, what feature would you expect to find at the terminus of the glacier? Now assume 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

Balanced budget forms a terminal moraine; excess ablation forms recessional moraines as the glacier retreats.

Step-by-step solution

Understand Balanced Glacier Budget

A balanced glacier budget means that the rate of snow and ice accumulation equals the rate of ablation (melting and sublimation). This results in a steady glacier terminus that remains in the same position over an extended period.

Expectation with Balanced Budget

With a balanced glacier budget, the feature you would expect to find at the glacier's terminus is a terminal moraine. This moraine forms as the glacier deposits its carried debris at a consistent location over time, marking the farthest advance of the glacier.

Change in Glacier Budget - Ablation Exceeds Accumulation

If the budget changes and ablation (melting) exceeds accumulation (snowfall and ice build-up), the glacier begins to retreat. This means the glacier's terminus will move backward as the glacier shrinks.

Deposits Formed under Excess Ablation

As the glacier retreats, it leaves behind deposits called recessional moraines. These form from the debris that the glacier was carrying and now dumps incrementally as it retreats, marking different positions of the terminus as it recedes over time.

Key concepts

Glacial Ablation

Glacial ablation refers to the process where a glacier loses mass. This can occur through melting, sublimation (where ice turns into vapor without becoming liquid first), or calving, which is when chunks of ice break off. As ablation increases, the glacier loses more ice and snow than it gains. When ablation exceeds accumulation, the glacier will start to shrink and retreat.

• Melting - When the glacier ice turns into water.
• Sublimation - When ice goes directly from solid to vapor.
• Calving - Large pieces of ice break off into water bodies.

Understanding glacial ablation is essential for learning how glaciers change and evolve over time, and how they can influence the landscape around them.

Glacial Accumulation

Glacial accumulation is the process of snow and ice building up on a glacier. This happens when snowfall is higher than melting and evaporation. During periods of cold temperatures, especially winters, the accumulation can be substantial, leading to the glacier growing in size and potentially advancing:

• Snowfall - The primary method of accumulation.
• Freezing rain or snow - Adds to the glacial mass as temperature drops.

With a balanced glacier budget, accumulation and ablation are equal, keeping the glacier stable. But if accumulation surpasses ablation, the glacier can grow and advance.

Terminal Moraine

A terminal moraine is a ridge of debris formed at the terminus of a glacier. It's created when a glacier moves down and pushes rocks, sediments, and other materials, depositing them at the point of its maximum advance. If a glacier has a balanced budget, you’ll likely find a terminal moraine at its end. This terminal moraine marks the furthest edge of a glacier's reach.

• Characteristic shape - Hilly and ridged.
• Indicator of past glacial extent - Reflects the furthest the glacier advanced.

Tracking terminal moraines helps geologists understand past glacier movements and interpret climatic conditions over time.

Recessional Moraines

As glaciers retreat due to increased ablation, they leave behind a series of recessional moraines. These are similar to terminal moraines but differ in formation sequence. Recessional moraines indicate the intermittent stop points of a glacier as it melts and moves back:

• Sequential accumulation - Formed as the glacier halts temporarily.
• Marker of glacier retreat - Each line represents a pause in retreat.

By studying recessional moraines, researchers can map out historical glacier retreat patterns, offering insights into the pace of glacial changes and past climate variations.