Question

If the budget of a valley glacier were balanced for an extended span of time, what feature would you expect to find at the terminus of the glacier? Now assume the glacier's budget changes so that wastage 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 has a terminal moraine; with excess wastage, the glacier retreats, forming a recessional moraine and possibly outwash plains.

Step-by-step solution

Understanding a Balanced Glacier Budget

When the accumulation (gain) of snow and ice is equal to the wastage (loss) in a glacier, the glacier's budget is considered balanced. In this scenario, the terminus of the glacier, which is the end or the lowest point of the glacier, remains stationary because the input and output are equal.

Feature at the Terminus of a Balanced Glacier

With a balanced glacier budget, the terminus of the glacier would remain in a relatively stable position over time. Consequently, you would expect to find a terminal moraine, which is a build-up of debris and sediment that has been deposited at the glacier's terminus, marking the furthest advance of the glacier in its balanced state.

Conceptualizing Glacier Budget Change

If there is a change in the glacier's budget such that wastage (melting and sublimation) exceeds accumulation (snowfall and ice formation), the glacier is losing more ice than it is gaining. This imbalance results in the glacier retreating or moving back because the terminus moves backward as the ice melts.

Feature at the Terminus After Glacier Retreats

With the glacier now retreating due to wastage exceeding accumulation, the debris that was previously at the terminus is left behind. As a result, a recessional moraine may form. This type of moraine marks the positions to which glaciers temporarily stabilized during their retreat.

Expected Deposits Under New Conditions

As the glacier retreats, the meltwater may carry sediments away from the glacier's end, forming outwash plains or outwash fans. These are deposits made of sand and gravel that accumulate beyond the terminus as the glacier continues to lose mass.

Key concepts

Terminal Moraine Formation

A terminal moraine is a fascinating natural feature that forms at the end, or terminus, of a glacier. When a glacier's budget is balanced, meaning the amount of snow and ice accumulating is equal to the amount melting or sublimating, the terminus stays in one place. Over time, as the glacier slowly moves, it pushes rock, soil, and other debris forward. This debris, referred to as till, accumulates at the glacier's terminus. Due to the glacier's stability in a balanced state, this collection of debris indicates the furthest advance of the glacier.

• It's important to note that a terminal moraine is a type of end moraine, representing the farthest point reached by a glacier when it was in a balanced or advancing state.
• This moraine can significantly alter the landscape, often forming hills or ridges that impact local waterways and habitats.

Recessional Moraine Dynamics

Recessional moraines are created during a glacier's retreat phase when the glacier's budget becomes imbalanced. If wastage, or loss of ice due to melting, exceeds accumulation, the glacier moves back. Unlike terminal moraines, recessional moraines form as the glacier retreats and temporarily pauses.

• Each pause represents a time when the glacier's retreat slowed or stopped, allowing debris to accumulate at the terminus momentarily.
• These moraines offer a sequential record of the glacier’s retreat history, with multiple recessional moraines indicating a glacier that retreated in stages.

The presence of recessional moraines shows the dynamic and sometimes unpredictable nature of glaciers as they adapt to changing climatic conditions.

Outwash Plains Explained

Outwash plains, also known as sandur, are flat expanses of sand and gravel that form beyond a glacier's terminus, primarily during periods of retreat. As a glacier melts, its meltwater carries with it a multitude of sediments. These sediments are deposited over a broad area, creating a plain of sorted material.

• The process leads to distinctly layered deposits known as stratified drift, which differ from the unsorted till found in moraines.
• Outwash plains can cover vast stretches of land, particularly around large, retreating glaciers, and are an important part of the glacial landscape.

Due to their composition and the transport method, outwash plains are fertile grounds, often used for agricultural purposes. They provide valuable insights into past glacier movements and climatic conditions, making them important for geological studies.