Chapter 13: Glacial Processes, Patterns and Associated Landforms

Chapter 13: Glacial Processes, Patterns and Associated Landforms

Glaciation

• Earth once covered with glaciers

• Last glaciation ended around 10,000 years ago

• Current interglacial period – Holocene

• Two major glaciations have affected Ireland: The Munsterian, 300,000 to 132,000 years ago

The Midlandian, 79,000 to 13,000 years ago


Glaciers

• Rivers of ice that move slowly

• Move downslope under the influence of gravity and the pressure of own weight

• Form where rate of accumulation of snow and ice is greater than rate of melting

• Largest reservoir of fresh water on Earth


Types of glaciers

1. Valley glaciers: form in mountainous locations and move down valleys

2. Continental ice sheets/glaciers: enormous areas of glacial ice and snow


Causes of an ice age

• Change in the Earth’s orbit around the sun

• Change in the angle of the Earth’s axis


Processes of glacial erosion

Glaciers erode the landscape they travel over in two ways:

1. Plucking

2. Abrasion


1. Plucking

Most effective where: • Rock is well jointed

• Rock has already been weakened by freeze-thaw action

• Bottom of glaciers scrape along valley floors – creates friction causing melting around the base of the glacier

• Meltwater refreezes

• Freezes around the rocks on the valley floor and these become part of the glacier

• Glacier advances

• Newly trapped rock is plucked out of the valley floor

• New material is then used in process of abrasion


2. Abrasion

• Occurs when the bedrock beneath the glacier is eroded by the debris/material embedded in the sides and bottom of the glacier

• ‘Sandpaper effect’ – scrapes the rock over which it is travelling and leaves scratches or grooves in the rock

• Striations – show the direction of the ice flow


Processes of glacial transportation

1. Basal sliding• Friction between the base of the glacier and the

valley floor

• Meltwater acts as a lubricant

• Glacier slides downslope under the influence of gravity

2. Internal flow• Ice crystals react to pressure and gravity

• Melting and refreezing

• Ice changes internally over time without completely melting or breaking


Factors affecting the rate of glacial erosion

• Thickness of ice

• Topography

• Geology

• Gradient

• Accumulation and ablation


Landforms of glacial erosion

1. Cirque

• Birthplace of a glacier

• Depression in a mountain

• Three steep sides

• Also called a corrie or coom

2. Arête

• Narrow ridge

• Formed when two cirques formed side-by-side or back-to-back


Landforms of glacial erosion (continued)

3. Pyramidal peak

• Formed when three or more cirques are eroded back-to-back or side-to-side around the sides of a mountain

• Isolated peak in the centre



4. U-shaped valley

• Glaciers take the easiest route as they move from upland areas down through their valleys

• Route is often a pre-existing V-shaped river valley

• Glacier moves through this valley, changing the shape from a V-shape to a U-shape

• Erode vertically and laterally

• Valleys have steep sides and flat floors

• Glacier cuts off interlocking spurs of the V-shaped valley leaving truncated spurs



5. Ribbon lakes• Long, narrow lakes found in glaciated U-shaped

valleys

• As a glacier advances through its valley it abrades the landscape

• Soft rock is abraded faster and easier than the harder resistant rock forming basins

• Within the rock basin meltwater and rainwater accumulates forming a ribbon lake

6. Fjords• Drowned U-shaped valleys

• Result of melting glaciers


Glacial deposition

• Dropping or laying down of sediment that was once transported by a glacier

• Lowland areas

• Deposited material is called glacial drift

• Material deposited directly by ice is called till or boulder clay

• Material deposited by glacial meltwater is known as fluvio-glacial deposits


Examples of landforms of glacial deposition

• Moraines

• Drumlins

• Erratics


Examples of landforms of fluvio-glacial

deposition

• Eskers

• Outwash plains

• Kames and kettleholes


Landforms of glacial deposition

1. Moraine

• Deposited debris

• Various sizes ranging from large boulders to fine rock flour

• Material may be angular or rounded in shape


Five types of moraine

I. Lateral moraine

II. Medial moraine

III. End/terminal moraine

IV. Ground moraine

V. Englacial moraine



2. Drumlins

• Oval-shaped hills consisting of boulder clay

• Show direction of glacier movement

• Occur in swarms or cluster

• ‘Basket of eggs’ topography

• ‘Drowned drumlins’ – as the ice melted sea levels rose and the drumlins appear as islands in the sea



3. Erratics

• Material transported by the glacier and are said to be ‘out of place’ when deposited

• Provide information about the direction of the glacier and how far it has travelled – crag and tail

• Esker – long winding ridges of stratified sand and gravel that wind its way across lowland areas



4. Outwash plains

• Glaciers melt

• Release vast amounts of water

• Spreads outwards beyond the end/terminal moraine

• Carries large volumes of rock and gravels and sands



5. Kames

• Piles of sediment consisting of gravels and sand

• Deposited along the front of a retreating glacier



6. Kettle holes

• Blocks of ice separate from the main glacier

• Buried partly in meltwater sediments

• Blocks of ice melt leaving depressions or holes

• Fill with water – form kettle hole lakes

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Chapter 13: Glacial Processes, Patterns and Associated Landforms