Glaciers and glaciation

Motivation: why study them?

• curiosity...
• they are agents of landscape evolution
• local relevance: much glaciation in Michigan
• to understand climate changes, both short and long term, especially in the geologic time period called the Pleistocene (last 2 million years or so)
• 10% of all land surface is covered by ice today
• glaciers are the second largest reservoir of water, and they hold 90% of all available freshwater

There is ice over continents and over water. Much continental ice is over Greenland. The South Pole has continental ice with tongues of ice over water as well. The amount of glacier-covered land elsewhere is small. During ice ages, expansion of glaciers covers a lot of land.

What is a glacier?

A glacier is a land-based mass of ice that moves...basically a river of ice. The diagram below shows the general features of a glacier.

1. zone of accumulation: where precipitation occurs that "feeds" the glacier - the source,or zone of net gain
2. zone of ablation: where melting of ice occurs - the sink, or zone of net loss
3. equilibrium line: the boundary between the zones of accumulation and ablation - separates net gain from net loss
4. terminus: the downstream end of the glacier

Glaciers must have a source of precipitation and occur in places cold enough to produce glacial ice. In order for a glacier to grow, there must be more precipitation in winter than evaporation in the summer.

The forces drive the movement of a glacier are gravity-based:

1. elevation differences: like water, glaciers will move from high to low.
2. self-induced flow: even on a perfectly flat surface, a mound of ice will deform under its own weight. This is similar to putting a blob of pudding on a table; at first it may look like a mound but it quickly "oozes" forward around the sides.

How does a glacier move?

Two ways:
The whole glacier flows over land, called basal sliding.
Within the glacier, some parts move forward faster than others (the top and middle faster than the bottom and sides), called glacial creep.

How does basal sliding work?
Need to look at the stability field, or phase diagram of water below. This diagram shows what temperatures and pressures will produce different phases (that is, either vapor, liquid, or solid). From everyday experience, we know that high temperatures cause a vapor phase (think of water boiling); intermediate temperatures cause a liquid phase; and low temperatures cause a solid phase (think of ice in the freezer). You can see this on the stability field - follow the dashed green line from low to high temperatures. You will go from ice to water to vapor.

Another way to change the phase of water is to change the pressure. Increasing pressure will eventually force a vapor to become a liquid. Water is a tricky material. The liquid-ice phase change is backwards compared to most materials: when you increase the pressure on ice and keep the temperature the same, it becomes liquid water, not the other way around. See for yourself by following the dashed blue line from low to high pressure. You will go from solid to liquid. This is how glaciers "slide." The weight of the glacier pressing on bumps on the ground increases the pressure. The ice actually melts, goes around the bump, and refreezes on the other side of the bump where the pressure has once again dropped (see figure).

Rates of Movement:
Continental glaciers: 0.01-0.1 meter/day
Valley glaciers: 0.1-2 meters/day normally; 50-100 meters/day during certain times when the glacier is "surging"!!!

How is glacial ice made?

Normal snow is made into glacial ice, which is different from regular ice. The snow that falls has a density of typically 0.05 g/cm³. This snow becomes firn, which is denser than snow (0.50 g/cm³). It is also granular in shape and packed more orderly than snow. After more compaction occurs, ice is made. Its density is about 0.80 g/cm³. Lastly, after remelting, refreezing, and deforming, the normal ice becomes glacial ice, which is even denser: 0.90 g/cm³. Glacial ice can be very colorful, with blue (and sometimes green) hues.

Three types of sediments from continental glaciation

• till: deposited by ice (glacier grinds debris at its base forming clay-rich material, then till is left behind as glacier melts; or debris on the top of the glacier is dropped down after glacier retreats; or debris is pushed at the edge of the glacier as it advances); unsorted, unstratified (=no layering), all sizes present (from house-sized boulders to clays. Clays develop from the grinding of rock at the base of the glacier as the glacier moves forward)
• outwash: deposited by meltwater; well-sorted (usually only sand and gravel), stratified (sediments deposited by running water are deposited in layers).
• lake clays: in front of the edge of a glacier, there can be lakes made of the melting ice. These are called proglacial lakes. They are calm and thus collect small-sized particles, notably clays.

Glacial deposits can be transported huge distances.

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