Dynamic Planet/Glaciers

Glaciers are the topic of Dynamic Planet for the 2013 season.

What are glaciers?
Glaciers are large masses of snow and ice that have accumulated over years of snowfall and have flowed at some point in their lifespan. Their thickness can vary from as little as 50 to as large as thousands of meters. They form when snow does not fully melt during the warm part of the year, allowing the snow to accumulate and begin to compress.

Glaciers originate on land, but can flow into the sea. Glaciers can leave many unique landscapes, including new lakes, in their trails. The deposited material a glacier leaves is called a "moraine" or "glacial till". The front of a traveling glacier is called the "glacier head" while the back is called the "terminus." On earth, 99% of glacier ice is found in the polar regions.



Glaciers are a large part of erosion and deposition. They grind rock as they pass and also pick some sediment up and deposit them on the way. Glaciers can change V-shaped valleys formed by rivers into U-shaped valleys.

Glaciers are also important in lake formation. Eskers and kettle lakes are two types of lakes that are created by glaciers.

Where are glaciers found?
Glaciers can form anywhere that the average annual temp is low enough for snow to last all year round. These locations are normally in high latitudes or at high elevations. The direction that a mountain faces can also affect their formation. In the Northern Hemisphere, glaciers will often form on the Northern face of mountains, as the sun's rays will always be coming from the South.

How do glaciers form?
Glaciers form when snow and ice are able to remain throughout the year. Once snow begins to build up, it compresses the snow below it to form ice. Eventually the ice will reach a critical mass that will allow it to flow, and a glacier is born. Three mechanisms affect how glaciers are able to flow.

The first is the slope of the bedrock surface. The steeper this surface is the easier it is for a glacier to flow. Until a glacier is greater than 60m thick it is unable to flow on level ground or uphill.

The second mechanism of flow is internal deformation. Ice crystals inside the glacier can be flattened into sheets due to the immense pressure. This can allow ice crystals above them to slide on the flattened sheets in a form of plastic flow.

The amount of basal meltwater is the final mechanism that affects glacial flow. Basal meltwater is water that accumulates at the base of the glacier either due to surface meltwater that has percolated through the glacier or due to the pressure melting effect at the base of the glacier. Both of these help to lubricate the glacier allowing it to flow more easily. Basal meltwater can also lead to glacial surges, when the glacier moves very quickly (up to cms or ms more movement per day) due to this lubrication.

Glaciers can only flow forward, not backward. When melting is greater than the glaciers rate of flow, downwasting occurs.

Glacial Morphology
Glaciers are mainly classified based on size. A cirque glacier is the smallest and forms in a small bowl shaped depression in the mountains. These will be a few square kilometers in size. Cirque glaciers are also known an alpine glaciers. The next largest are valley glaciers, which flow through valleys in the mountains, and sometimes are cirque glaciers that have escaped their depression. If valley glaciers flow out onto an adjacent plain, they are known as piedmont glaciers.

Sometimes, a large number of glaciers are able to collect and join together, when this happens ice fields and ice sheets form. Ice fields are hundreds of square miles of glaciation, while ice sheets can cover thousands of square miles. These usually cover everything but the highest mountain peaks, which are known as nunatucks when they stick out of the ice like islands. Ice fields sometimes feed outlet glaciers, glaciers that occupy valleys that extend below the coverage of ice field.

Other glaciers include tidewater glaciers. These are when glaciers end up reaching the sea, but instead of spreading out to form ice sheets or shelfs, they terminate at the shoreline. They calve (break off edge pieces) rapidly, providing an efficient means of ice loss for ice sheets. Tidewater glaciers often have high acceleration rates.

Ice streams are important parts of glacial systems due to the fact that they discharge a majority of ice and sediment. They are narrow and flow at rates of .5-1 kilometers per year, a much faster rate than the glacier around them. Since they flow so much faster, they are heavily crevassed from eroding downwards as well. They are known to have abrupt shear margins.

Moving past ice streams, there are also subglacial lakes. They are, as the name suggests, bodies of freshwater that are contained deep within the layers of ice sheets. The largest known subglacial lake is Lake Vostok, located beneath the (East) Antarctic Ice Sheet. It is beneath more than 3 kilometers of ice, is 230km in length, has an area of 14000 square kilometers, and a volume of about 2000 cubic kilometers.

Glaciers themselves also have a few important parts. The top end of a glacier is known as its head, and the downhill end is known as the terminus. The terminal moraine is a large mass of debris that marks the glaciers furthest advance. Glaciers also have a zone of ablation, where snow melts in the summer, and a zone of accumulation, where is lasts all year. These two zones are separated by the snow line, which moves during the summer. Lateral moraines are material that has been pushed off to the side of glaciers and medial moraines form when two glaciers converge.

Glacial Geology
There are two important parts to glacial geology: erosion and deposition. These two processes cause the many features associated with glacial landscapes.

Factors that Cause Glacial Periods
Glacial periods are times in the Earth's history where average global temperatures were approximately 6 C lower and glaciers covered much of the planets surface. The last of these periods ended approximately 10,000 years ago. There are 6 main factors that contribute to global climate and can cause glacial periods: solar variability, insulation, dust, atmospheric composition, ocean current circulation, sea ice, and atmospheric circulation. All of these are natural processes and the only one that is affected by humans is atmospheric composition.

Resources

 * http://en.wikipedia.org/wiki/Glacier
 * http://scioly.org/w/images/7/78/Dynamic_planet_glaciers_smith.pdf
 * http://nsidc.org/glaciers/
 * http://www.hanksville.org/daniel/geology/glerosion.html
 * http://www.backyardnature.net/g/ice-ages.htm