Physical geology lecture - Glacial systems

What is a glacier? A pile of moving ice that formed from past accumulated snow falls.

Image of an iceberg in the foreground that calved off of the ice wall front of the glacier in the background. The ice wall is about 60-80 feet high. The whiter parts are where recent calving has occurred, whereas to the left you can see an older, dirtier and weathered part of the wall that has been more stable. In the background is an alpine ridge with small cirques and glaciers feeding this larger one. This photo is from the inner part of Hornsund fjord in Spitsbergen.

Alpine vs. continental glaciers?

Why study glaciers?

Glacial dynamics:

Transformation: snow -> granular snow -> firn -> glacial ice. Takes 12-200 years to complete. Processes involved include melting and refreezing and deformation, compaction and recrystallization.
ice movement mechanisms:
- creep.
- internal slip.
- basal slip.
- hybrids.
- average rate 12 m/yr.
ablation: loss of ice volume from glacier by melting, sublimation, calving. Sensitive to climatic parameters.
advances and retreats:
- frontal iceflow > frontal ablation produces advances.
- frontal iceflow = frontal ablation produces equilibrium, stable front and development of an end moraine.
- frontal iceflow < frontal ablation produces retreats.
surface and sub-ice drainage: evident by the number of streams/rivers that exit the front of the glacier and by the swallow holes on the surface of the glacier.
crevasses: deformation in the upper brittle crust: extensional cracks. Location and patterns a function of underlying bedrock geometry and the flow pattern in the ice.
surges:
- Iceland, 1963-64, ice front moved forward 8 km total with local rates of forward movement of 5 m/hour. many other examples.
- can flow across a valley and form an ice dam.
- what causes them?
isostatic depression and rebound: When a icecap grows it depresses the crust slowly due to its weight. This is because the crust if floating in a sense. When ablation dominates and the icecaps is melted, then the crust rebounds, but over a time frame of thousands of years. It is similar to unloading and loading a boat, but where the boat floats in very viscous fluid so that it takes time for the response. Since we have seen deglaciation in the last 10,000 years isostatic rebound is much more common at this point.
outwash and sedimentation: glaciers are very efficient erosion and transport agents, and so the meltwaters often have a lot of debris to work with.

Components of glacial systems:

glacial components: ice, firn, snow, water, rocks, air. How are the rocks incorporated into the glacier?
erosional land forms: cirques, aretes, horns, U-shaped valleys, hanging valleys, striations.
depositional land forms and deposits:
- moraines: lateral, median, end, terminal, recessional, push.
- sub-ice features: drumlins and eskers.
- outwash: braided rivers and lakes with varves.

In this photo can be seen quite a few components of an alpine glacial system. In the foreground is a glacial outwash stream bringing sediment down to the fjord. To the right it is cutting into the toe of some blocky moraine material (till). The calving glacial front is from a glacier that surged several years prior to this picture. On top you can see some of the dirty rock debris it carries. Much of this sediment load is deposited into the fjord waters. A close look shows a small piece in the process of calving off. Small ice bergs from prior calving floats in the fjord water out front. In the background is an alpine ridge carved by past glacial action. This image is from the St.Jons fjord in Spitsbergen.

This is a view from Wedel Jarlsberg Land in Spitsbergen of a icefalls cutting through the rock from a higher glacial plateau. Because of the bending of the ice slab as it goes over the underlying rock ledge, and becuase of an increase in velocity, crevasses open up at the top, and close at the bottom of the ice falls. These crevasses shown are big enough to drop a car into. Note also the fresh snow covering the ice. This means that some of the crevasses are undoubtedly hidden beneath snow bridges. When traveling on glaciers this is one of the major hazards one must look for.

This is a picture of glacial ice showing trapped air bubbles inside. It is these types of bubbles that record past climate information. Note that there are two populations of bubbles. One of these is marked by elongate shapes and are larger. These have the shape they do from shear or creep within the ice that deforms the bubbles. The origin of the smaller, round bubbles is enigmatic to me, but mightbe do to latter recrystallization processes.

This is a view of part of nothern Norway from the plane. The glacial ice is gone, but the landscape here is very much a product of glacial processes. To the right center can be seen a cirque with two glacial lakes in it (tarns). The depressions for these lakes were scooped out by the eroding ice. This cirque is hanging above the fjord, which was also once occupied by ice. Other small cirques and tarns are also evident. Note that the top of the mountain is flat. This is likely an erosional surface formed by an older ice cap that existed. When the ice cap disappeared Alpine glaciers developed in the next phase of glaciation and cut down into this older surface. Hence the topography is a bit different than with Alpine glaciers cut into mountains forming aretes and horns.

Pleistocene Ice Age:

Pleistocene (initiates 1.8 Ma) vs. Holocene (since 10,000 years ago)
Two recent major pulses:
- Wisconsin, 10,000-30,000
- Sangamon interglacial epoch.
- Illinioan 850,000-35,000
are older pulses, and interglacial periods.
Little Ice Age: circa 1400-1900.
overall pattern is of major advances and retreats with minor advances and retreats. This is suggestive of different factors operating at different time scales.

Older Ice Ages:

Huronian (2.1 billion years ago).
Vendian and the snowball earth: 600 Ma.
Gondwana (Permo-Carboniferous), around 275 Ma.
- Dwkya tillite in South Africa.
- Tachir tillite in India.

What are some of the potential causes of Ice Ages?

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