Discussion 6

John Aubert  American River College Glacial Processes and Landforms Physical Geography Lecture Notes Page 1 of 3 [email protected]

TIP: Scan these notes before this topic is covered in lecture! TIP: Review your notes as soon as possible after every lecture!

Glacial Processes and Landforms (flowing ice as an agent of erosion)

I. Introduction: Ice is one of the flowing substances that erode, transport, and deposit earth materials to create

terrestrial landforms. The others are running water, wind, and waves. Glaciers have in the past covered vast areas of the world, especially compared to the present. Glacial activity has been particularly high over the course of the last 1.65 million years. Despite the relative lull at present, glaciers have been responsible for shaping many landscapes we see today.

A. Two Main Types of Glaciers

1. Alpine (high elevation): Alpine glaciers form for the most part in mountainous

areas, filling narrow valleys with "rivers" of ice. Orographic precipitation is the primary factor involved in their formation. Starts out as a cirque.

2. Continental (high latitude): Continental Glaciers (or ice sheets) are huge

expanses of ice many kilometers across (and thick) which form in cold, polar regions. The only two ice sheets in existence today are Greenland and Antarctica.

B. Formation of Glaciers 1. Annual snowfall must exceed snowmelt: In simple terms, glaciers will form

when more snow falls than melts.

2. Snowline: The term snowline is loosely applied to mean the elevation in a mountainous

environment above which snow remains until the next year's first snow. It is only above this line that there is the possibility for snow accumulation (and glacial development)

3. Transformation to Ice: When layers of snow accumulate, a slow process begins

which ultimately results in the transformation of that snow to ice. This process is one of densification and expulsion of air. When there are no longer air spaces between the ice crystals, the snow has become glacial ice.

II. Glacial Geomorphology: There are many similarities between alpine and continental glaciation; we

will focus mostly on alpine as it presents a good model. See continental below.

A. Erosion: Glaciers have amazing erosive powers, not readily noticeable.

1. Types of Erosion a) Abrasion: The glacial ice collects gravel, boulders, sand, etc and drags this

material along the bedrock surface scraping off rock particles. The very fine material is called glacial flour. This process often leaves scars in the rock called striations and polished surfaces called glacial polish.

b) Plucking - See text

2. Erosional Landforms

a) Cirque (depression left by a cirque glacier) b) Horn c) Arete d) U-shaped valley (also called trough)

(i) Hanging Valley (ii) Fiord (glacial trough filled by sea)

e) Striations (scratches) and glacial polish

John Aubert  American River College Glacial Processes and Landforms Physical Geography Lecture Notes Page 2 of 3 [email protected]

TIP: Scan these notes before this topic is covered in lecture! TIP: Review your notes as soon as possible after every lecture!

B. Transport 1. Mass Balance: A glacial system has inputs and outputs.

a) Accumulation Zone: The area above the snowline is termed the

accumulation zone. Snowfall exceeds snowmelt here.

b) Ablation Zone: The area below the snowline is termed the ablation zone.

Snowmelt (and icemelt) exceeds snowfall here.

c) Equilibrium Line: The boundary between the ablation zone and the

accumulation zone, roughly synonymous with the snowline. Snowfall equals snowmelt here.

d) Flow: Ice flows from the accumulation zone to the ablation zone. Carries

glacial till like a conveyer belt.

Mass Balance of an Alpine Glacier (sideview diagram)

2. Rate of Flow: (Between a few centimeters to 100m/day) 3. Evidence of Flow

a) Crevasses: Cracks (or crevasses) form in the glacier where the glacier pulls

away from the mountain, where it turns corners, and where it flows down a steep portion (icefalls).

b) Experimental Evidence: Stakes were pounded in a straight line across

glacier (perpendicular to flow). All stakes moved in relation to the stationary points alongside the glacier; the stakes in the middle flowed fastest over the next several years (friction is least in the center of a glacier).

C. Deposition (and Depositional Landforms): When a glacier melts it drops its load. 1. Types of Glacial Sediment

a) Till: This is the term for sediment deposited by glaciers. It is commonly

unsorted (made up of randomly mixed sediments of all sizes) and unstratified (unlayered). This is different from sediments deposited by running water, which are often sorted and stratified.

b) Glacial Flour: Finely ground rock dust, created thought glacial abrasion.

c) Loess: Glacial flour that has been blown and deposited by wind.

ZONE OF ACCUMULATION

ZONE OF ABLATION

Equilibrium

Line

Snowline

John Aubert  American River College Glacial Processes and Landforms Physical Geography Lecture Notes Page 3 of 3 [email protected]

TIP: Scan these notes before this topic is covered in lecture! TIP: Review your notes as soon as possible after every lecture!

2. Moraines: Mounds of deposited glacial till. a) Medial: Glacial till transported and deposited down the center of a glacier. b) Lateral: Glacial till transported and deposited down the side of a glacier.

c) Terminal (or recessional): Glacial till transported and deposited at the end

of a glacier. Moraines (Mapview)

3. Erratics (or ice rafted boulders): Single large boulders deposited by glaciers.

Often look out of place and often are of a different rock type than the local bedrock. III. Continental Glaciation - Common Landforms (See text for all of these)

1. Esker 2. Kettle 3. Drumlin 4. Terminal Moraine 5. Recessional (or End) Moraine 6. Outwash Plain

IV. Pleistocene (called the Ice Age; 1.65 million - 10,000 years ago):  There were a series of glaciations during this time. These are referred to as glacials (when glaciers

advance) and interglacials (when glaciers retreat).

 As recently as 15-18, 000 years ago, a massive continental ice sheet covered much of N. America from Canada southward into the present day United States. These ice sheets scoured off all regolith, soils, and plants in their path.

V. Current Trends: Observations indicate that a large percentage of glaciers and ice sheets worldwide

are currently retreating, leading to rises in sea level.

Sample Questions: Questions similar to these will be on your exam. As you study you should anticipate

how I might use these questions to create new questions on the same concepts.

1

For a glacier to form:

A. snowfall must exceed snowmelt B. snow must fall in all months of the year C. snowfall must equal snowmelt D. snowmelt must exceed snowfall

2 Much of northern North America (Canada and Northern US) was covered by glaciers as recently as _________________ years ago.

3 Which is the best definition of the term “snowline”?

GOOD NEWS!!! The practice questions above - plus many, many more – can be found in the online practice quizzes discussed in your

syllabus and in class. You can take each quiz multiple times, and each time you will get some new questions. Once submitted, the quizzes are graded automatically, with the correct answers provided immediately. This is a great way to prepare for the exams!!!

SERIOUSLY, TAKE THE ONLINE PRACTICE QUIZZES!!!

FLOW

FLOW

Lateral

Moraine

Medial

Moraine

Terminal

Moraine