Grade 11

Advanced

9 min

Lesson 10 of 12

Glacial Landscapes and Dynamics

Not Started

Investigates the power of ice in carving alpine and continental landforms.

How can a slow-moving river of ice, moving only centimeters a day, carve through solid granite mountains more effectively than the world's most powerful rivers?

Learning Objectives

1.Explain the mechanics of glacial plucking and abrasion in carving the lithosphere.
2.Identify and describe macro-scale erosional landforms including cirques, arêtes, and U-shaped valleys.
3.Distinguish between unsorted glacial till in moraines and sorted glacio-fluvial deposits.

The Sculptors: Plucking and Abrasion

Glaciers erode the landscape through two primary mechanisms. Plucking occurs when meltwater penetrates joints in the bedrock, freezes, and bonds the rock to the glacier. As the ice moves, it 'plucks' large fragments away. Abrasion is the 'sandpaper' effect. Rocks embedded in the glacier's base grind against the bedrock. This process creates glacial striations (scratches) and rock flour, which gives glacial lakes their turquoise hue. The rate of erosion

E

can be simplified as a function of ice velocity

v

and debris concentration

c

E \propto v \cdot c

Quick Check

Which process is responsible for creating smooth, polished rock surfaces with long parallel scratches?

Think about the 'sandpaper' effect mentioned in the text.

Answer

Abrasion

Alpine Erosional Landforms

As glaciers move downslope, they transform V-shaped river valleys into broad U-shaped valleys (glacial troughs). At the head of the glacier, rotational scouring creates a cirque—a bowl-shaped depression. When two cirques erode back-to-back, they form a sharp, knife-like ridge called an arête. If three or more cirques meet, they create a pyramidal peak or 'horn.' The depth of these features is often influenced by the thickness of the ice, where the downward pressure $P$ is calculated by $P = \rho g h$ , where $\rho$ is ice density, $g$ is gravity, and $h$ is ice thickness.

Calculate the pressure exerted at the base of a glacier that is 200 meters thick. Assume ice density $\rho = 917 \text{ kg/m}^3$ and $g = 9.8 \text{ m/s}^2$ .

1. Use the formula $P = \rho g h$ . 2. Substitute the values: $P = 917 \cdot 9.8 \cdot 200$ . 3. Result: $P = 1,797,320 \text{ Pascals}$ (or approx $1.8 \text{ MPa}$ ).

Quick Check

If you see a sharp, jagged ridge separating two glacial valleys, what landform are you looking at?

It is a French word meaning 'fishbone' or 'edge'.

Answer

An arête

Depositional Dynamics: Moraines vs. Outwash

Glacial deposits are categorized by how they were transported. Till is material deposited directly by the ice; it is unsorted and contains everything from clay to giant boulders. This forms moraines: lateral (sides), medial (middle), and terminal (the furthest point of advance). Conversely, glacio-fluvial deposits are moved by meltwater. Because water loses energy selectively, these deposits are stratified (layered) and sorted by size. These form outwash plains (sandur) in front of the glacier.

A geologist finds a sediment layer containing a mix of fine silt and 2-meter wide granite boulders with no distinct layering.

1. Observe the sorting: The sediment is 'unsorted' (mixed sizes). 2. Observe the structure: There is no 'stratification' (layering). 3. Conclusion: This is glacial till, likely part of a moraine, deposited directly by ice rather than meltwater.

The mass balance of a glacier is $B = A - C$ , where $A$ is accumulation and $C$ is ablation.

1. If $A > C$ , the glacier has a positive mass balance and advances. 2. The ELA is the specific altitude where $A = C$ . 3. If regional temperatures rise, the ELA shifts to a higher altitude, increasing the area of the ablation zone and causing the glacier to retreat.

Key Takeaways

1Plucking and abrasion are the primary erosional mechanisms, driven by ice movement and basal meltwater.
2U-shaped valleys and cirques are the 'fingerprints' of past glacial activity in mountainous regions.
3Glacial till is unsorted and unstratified, while glacio-fluvial outwash is sorted and layered by meltwater energy.
4The terminal moraine marks the maximum historical extent of a glacier's advance.

Quick Check

Multiple Choice

Which landform is created by the intersection of three or more cirques?

Multiple Choice

Why is glacio-fluvial sediment 'sorted' while glacial till is 'unsorted'?

True/False

A medial moraine is formed when two lateral moraines from merging glaciers join together.

What's Next?

Review Tomorrow

In 24 hours, try to sketch a mountain range and label a cirque, an arête, and a U-shaped valley from memory.

Practice Activity

Use Google Earth to find the 'Aletsch Glacier' in Switzerland. Identify the medial moraines and the terminal point where the meltwater begins.

Browse

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Glacial Landscapes and Dynamics

Learning Objectives

The Sculptors: Plucking and Abrasion

Alpine Erosional Landforms

Depositional Dynamics: Moraines vs. Outwash

Key Takeaways

Quick Check

What's Next?

Glacial Landscapes and Dynamics

Learning Objectives

The Sculptors: Plucking and Abrasion

Alpine Erosional Landforms

Depositional Dynamics: Moraines vs. Outwash

Key Takeaways

Quick Check

What's Next?