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 that 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 budget results in a terminal moraine; a negative glacier budget leads to retreat and could form recessional moraines.

Step-by-step solution

Understanding a Balanced Glacier Budget

If the budget of a valley glacier is balanced, it means that the amount of ice being added to the glacier (accumulation) is equal to the amount being lost through melting and other processes (ablation). In this steady state, the terminus of the glacier remains in the same position over time.

Balanced Glacier Terminus Features

With a balanced glacier budget, a prominent feature at the terminus would be a terminal moraine. This is an accumulation of debris and sediment that marks the furthest advance of the glacier when it remains in a relatively fixed position over time.

Changing to a Negative Glacier Budget

If the glacier's budget changes so that wastage exceeds accumulation, this indicates that more ice is melting than is being replaced. This situation is described as having a negative glacier budget.

Effects on the Glacier Terminus

When the wastage exceeds accumulation in a glacier, the terminus or the end of the glacier begins to retreat. This means that the visible end of the glacier moves uphill as the ice melts faster than it is replenished.

Formation of Recessional Moraines

Under the new conditions of a negative budget where the glacier is retreating, a recessional moraine may form. These deposits occur as the glacier periodically pauses during its retreat, depositing material that indicates temporary positions of the glacier terminus.

Key concepts

Valley Glacier

A valley glacier is a river of ice that forms in mountainous regions. Picture it like a natural ice conveyor belt that slowly moves through a valley. Valley glaciers are especially exciting to study because they dramatically reshape landscapes over long periods. Their movement is responsible for carving out deep valleys and creating sharp mountain ridges. Valley glaciers flow downhill due to gravity, but their progress can vary based on the amount of snowfall they receive and the warmth of the climate they exist in. Whenever you see their path in a mountainous region, you can expect unique geological formations created from their journey.

Glacier Terminus

The glacier terminus is the edge or the end of a glacier. Think of it as the glacier's front door that can shift over time. The terminus is affected by the glacier's budget, which is the balance between ice accumulation and ice loss. If there is more accumulation than melting, the terminus advances; if there is more melting than accumulation, it retreats. When a glacier's budget is balanced, the terminus holds steady, maintaining its position over time. The position of the glacier terminus also influences the deposition of various landforms, such as terminal moraines and recessional moraines.

Terminal Moraine

A terminal moraine is created at the farthest reach of a glacier's advance. It resembles a natural barricade made of rocks, soil, and debris deposited by the glacier. Imagine this as the mound left when a bulldozer pushes forward and then stops. Terminal moraines offer crucial insight into the historical extent of glaciers. They mark the point where a glacier remained stationary for a significant period when its budget was balanced. These landforms not only provide history lessons about glaciation but also shape the natural landscapes by creating features like ridges and hills.

Recessional Moraine

As a glacier retreats, it often doesn't do so in one smooth motion. Instead, it pauses intermittently, leaving behind a series of smaller ridges parallel to the terminal moraine; these are known as recessional moraines. Each ridge signifies a point where the glacier paused long enough to deposit materials. Think of these as small vestiges of the glacier's journey, retracing its steps yet steadily moving backward. Recessional moraines offer insight into the retreat pattern of a glacier and thus are key indicators of past climatic changes. They paint a picture of the glacier's step-by-step retreat, making them essential for understanding the dynamic history of glaciers.