Geology 100: Planet Earth

Lecture 26: Topic 18 (Glaciers/begin)

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12/3 and 12/4
Topic 16: Reading - Chapter 22

I) What is a glacier?

- A body of ice that exists year around, and flows.
- Two Main Categories of glacier:
* Alpine Glaciers (also called Mountain Glaciers or Valley Glaciers): occur in high mountains, where it is cool enough at high elevations; they flow down valleys).
These currently occur in the Alps, Rockies, Andes, Himalayas, etc.
* Continental Ice Sheets: Thick lenses of ice (up to 2 to 4 km thick) that cover substantial regions of continents. At present, there are two examples, Greenland and Antarctica.

II) Formation and Movement of a Glacier

- Glaciers form when snow accumulates sufficiently above the snow line, so that at the base of the pile, pressure causes snow to recrystallize to form ice.
- A snow blanket is originally very porous with lots of air between flakes.
- As it is buried, it compacts. The points of the flakes melt away and the grains pack together more tightly. The rsulting material is "firn."
- As it is buried even deeper, the ice crystals recrystallize, forming a mass of ice with almost no pore space.

- Ice is very weak, so gravity causes the ice to start flowing. Ice crystals deform and slip past one another. Ice crystals are so weak, that they can change shape easily.

* For Valley Glaciers: Ice "river" flows down the slope. The toe (= nose, or termination) of the glacier can flow significantly downhill even protruding below the snowline.

* Region where snow accumulates and new ice forms is zone of accumulation. Region where ice melts or evaporates and no new ice forms is zone of ablation. Ablation involves three phenomena: melting, sublimation, and calving.

* For continental glaciers, the snow accumulates in a polar region. But the pile can only become so high before it begins to spread out laterally, because ice is so weak. So when the ice mass becomes significantly thick, it begins to flow sideways to form a wide sheet. The process is analogous to pancake syrup spreading out over the surface of a pancake, as you continuously pour over the center of the pancake.

- Advance vs. Retreat: If the position of the toe moves progressively further from the zone of accumulation with time, we say that the glacier is advancing. If the position of the toe moves progressively closer to the zone of accumulation with time, we say the glacier is retreating. Note that ice cannot flow back up hill, so when we say a glacier retreats, we are not saying that the ice starts flowing backwards, only that the rate of melting is exceeding the rate of new supply, so the position of the toe moves. In effect, the position of the toe is due to the dynamic balance between the supply of new ice flowing toward the toe from the zone of accumulation, and the rate at which ice melts away at the toe.

III) Erosional Features of Glaciers

A) Process of erosion:
1) Pressure of moving glacial ice can pluck rock and debris up from the ground and carry it along.
2) Also, once the debris is embedded in the ice, the moving ice acts as a rasp (sandpaper) that can grind away bedrock.

B) Consequences of Erosion: Glacial erosion creates:
* glacially polished surfaces
* striations (scratches or grooves in bedrock)

C) Landforms formed by erosion:
- In mountains (Alpine glaciation), you get distinctive landforms:
* U-shaped valley: A valley, carved by a glacier, whose sides are very steep and whose floor is rounded, so that overall, the valley has a U-shape in profile; such valleys contrast with the V-shaped valleys carved by rivers)
* hanging valley: a side valley whose floor does not intersect the floor of the main valley; typically, a waterfall may spill out of the side valley)
* arete: a knife edge ridge between two valleys carved by glacier
* cirque: a bowl-shaped depression formed at the head of an Alpine glacier.

IV) Depositional Features of Glaciers

A) Types of Sediment
- Glaciers are, effectively, so viscous that they can carry sediment of any size (clay- to sand- to boulder-sized).
- Till: At the toe, the glacier drops this sediment. The resulting unsorted debris (meaning, a mixture of grain sizes) is called till. (Note that sorting refers to the degree to which sediment has been winnowed so that it contains only one grain size. Well sorted sediment has only one grain size. Ice does not sort sediment because it is solid, and can carry material of any size.
- Outwash Gravel: Meltwater streams may wash through some of the till and sort out sand and gravel which it deposits in a wide plain to the front of the glacier. This moderately sorted sediment is called outwash, and the flat plane is called an outwash plain.
- Meltwater Lake Bed: In the quiet water of a meltwater lake, fine-grained clay can settle out to form mud.
- Loess: Strong winds blow off of the glacier (because glacier cools the air and cold air moves to lower elevations at the front of the glacier). These winds pick up the fine sediment (silt and clay) and carry it beyond the outwash plain to where it is deposited. The resulting sediment is called loess.

B) Depositional Landforms:
- Till can make up distinct landforms:
* terminal moraine: if the nose position is fixed for a long time, a large pile develops. he glacier acts as a conveyor belt, bringing the sediment. If the glacier then retreats, the resulting ridge is called a terminal moraine.
* ground moraine: if an advancing glacier overrides the till pile, it smears it out on the ground surface, creating ground moraine.
* drumlin: sometimes the advancing ice molds the till into a streamlined hill (a drumlin).

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