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

A balanced glacier creates a moraine of till at the terminus. With more ablation, the glacier retreats, leaving till and potential stratified drift.

Step-by-step solution

Understanding a Balanced Glacier Budget

When a glacier has a balanced budget, it means that the amount of snow and ice accumulation equals the amount of melting and sublimation, resulting in no net change in the glacier's size or position. At the terminus, you would expect to find a moraine, which is a build-up of debris deposited at the glacier's edge. This moraine is typically composed of till, a mixture of unsorted sediment and rock particles deposited directly by the glacier.

Impact of Ablation Exceeding Accumulation

When ablation (melting and sublimation) exceeds accumulation (snowfall and ice gain), the glacier will retreat. This means the glacier's terminus will move backward as it loses more mass than it gains. The loss in ice mass will result in less forward movement of the glacier due to decreased gravitational pull.

Expected Deposits with Increased Ablation

As the glacier retreats due to ablation exceeding accumulation, it will leave behind smaller deposits of unsorted till. These deposits may form features known as recessional moraines, which are remnants of the glacier's previous positions. Additionally, streams from the melting glacier might lead to deposits of stratified drift, which are sorted and layered sediments carried and deposited by glacial meltwater.

Key concepts

Moraine

A moraine is a geological feature formed by the accumulation of debris at the edge of a glacier. The debris is primarily composed of rock particles and sediments that have been gathered by the glacier as it moves. Moraines are found at various points along a glacier's path, but are most prominently featured at its terminus—the glacier's end.

• Lateral Moraines: These form along the sides of a glacier, where stones and sediments are piled up, creating ridges.
• Medial Moraines: When two glaciers converge, their lateral moraines can merge to form a medial moraine.
• Terminal Moraines: These are found where the glacier deposited material at its furthest point of advance during its latest movement.

Understanding moraines helps scientists trace the historical path and size of glaciers.

Till

Till is an unsorted geological material that is deposited by a glacier. Unlike stratified drift, till consists of a mix of debris sizes, ranging from fine clay to large boulders. This debris is transported and eventually deposited directly by the ice.

• Characteristics of Till: It lacks sorting and layering. The sizes and types of materials within till can vary widely.
• Formation: As glaciers advance, they pick up rocks and soil. When the glacier melts, it releases these materials, which form till.

The presence of till is a good indication of past glacial activity, offering clues about the direction and movement of ancient glaciers.

Stratified Drift

Stratified drift is different from till due to its sorted nature. It consists of sediments that have been transported and deposited by glacial meltwater, resulting in distinct layers. The process of sorting occurs because the flowing water segregates particles by size.

• Composition: Typically made of sand, gravel, and silt.
• Depositional Features: Includes features like outwash plains, composed of materials carried from the glacier and deposited outside of it.
• Importance: The layers found in stratified drift reveal information about past water flow and the dynamics of melting glaciers.

Through stratified drift, researchers can understand the hydrological patterns that existed during and after glacial movements.

Ablation

Ablation refers to the processes of loss of ice and snow from a glacier. This occurs through melting, sublimation (where ice turns directly into vapor), and calving (when chunks of ice break off). Ablation is a critical factor in determining the size and movement of glaciers.

• Effects of Ablation: Increased ablation can lead to glacier retreat as more ice is lost compared to what is gained through accumulation.
• Consequences: Leads to changes in the landscape, such as the formation of moraines and outwash plains.
• Monitoring: Understanding ablation rates helps scientists assess the sustainability of glaciers under climate change.

By studying ablation, we can predict future changes in glacier size and their impact on the environment.

Accumulation

Accumulation involves the processes by which glaciers gain ice and snow. This occurs primarily through snowfall, but can also include other forms of precipitation like sleet and hail. Accumulation is necessary for the maintenance and growth of glaciers.

• Impact on Glacier Health: Healthy glaciers have a balance between accumulation and ablation. When accumulation exceeds ablation, glaciers advance.
• Zone of Accumulation: Typically, the higher, colder areas of a glacier where snow persists year-round.
• Indicators: Thick, perennial snow cover is a sign of significant accumulation.

Understanding the balance between accumulation and ablation is crucial for predicting glacial movements and their environmental impacts.