|Question Marathon Threads|
- 1 What are glaciers?
- 2 Where are glaciers found?
- 3 How do glaciers form?
- 4 Glacial Morphology
- 5 Glacial Geology
- 6 Factors that Cause Glacial Periods
- 7 Glacial History of the Quaternary
- 8 Climatic Effects
- 9 How are glaciers studied?
- 10 Interglacial Periods
- 11 Vocabulary
- 12 Resources
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. Kettle lakes are examples 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. Glaciers are found in/ around all seven continents. The total area is about 15,000,000 sq. kilometers.
Worldwide Coverage (sq.km):
Antarctica: 11,965,000 (without iceshelves and ice rises)
Central Asia: 109,000
United States: 75,000 (including Alaska)
China & Tibet: 33,000
South America: 25,000
New Zealand: 1,159
The Worldwide Coverage info was taken from the following site: http://nsidc.org/cryosphere/glaciers/questions/located.html
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.
First year glacial snow is known as neve. Accumulated snow that has survived one melt season is known as firn.
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.
A glacier's mass balance is defined as the difference between accumulation levels and ablation. Accumulation is the addition of snow or ice onto the glacier. Ablation is the depletion of ice from the glacier, through processes such as sublimation and evaporation.
In ~1930 Milutin Milankovitch proposed that variations in three parameters of the earth's orbit caused glacial fluctuations:
1. Orbital eccentricity - the orbit of the earth around the sun is not a circle, but is elliptical and also varies. This eccentricity is a minor cause for seasons.
2. Tilt variations in the axis of rotation (obliquity) - the tilt of the earth's rotational axis varies with time. A tilted axis is the primary cause of seasons.
3. Precession - the earth's axis of rotation wobbles which results in minor fluctuations in the amount of solar radiation we receive.
Milankovitch pacing seems to best explain glaciation events with periodicity of 100k, 40k, and 20k years. This pattern seems to fit the info on climate change found in oxygen isotope cores. However, there are some problems with the Milankovitch theories.
100,000 year Problem eccentricity variations have a significantly smaller impact on solar forcing than precession or obliquity and may be expected to produce the weakest effects. The greatest observed response is at the 100k year timescale, while the theoretical forcing is smaller at this scale, in regard to the ice ages. During the last 1 million years, the strongest climate signal is the 100k year cycle.
400,000 year Problem (aka stage 11 problem) eccentricity variations have a strong 400k year cycle. That cycle is only clearly present in climate records older than the last million years.
Stage 5 problem refers to the timing of the penultimate interglacial that appears to have begun 10k years in advance of the solar forcing hypothesized to have caused it.
Effect exceeds cause climate behavior is much more intense than calculated variations. Various internal characteristics of climate systems are believed to be sensitive to the insolation changes, causing amplification(positive feedback) and damping reponses(negative feedback)
Analyzing Oxygen Isotope Data
There are 3 stable isotopes of oxygen, 16O, 17O, and 18O. There is approximately 1 atom of 18O for every 500 atoms of the most abundant isotope, 16O.
H2O molecules containing the light isotope, 16O, are more active, evaporating slightly more readily than molecules containing the heavy 18O. Thus, the 18O/16O ratio in the water vapor is smaller than in ocean water; oxygen in water vapor is "lighter".
During precipitation, the heavy isotope, 18O, is concentrated in the rain or snow, leaving the vapor mass, a small reservoir, depleted in 18O. While crossing the ice sheet, both vapor and local precipitation become isotopically lighter. The depletion process is even more effective at lower temperatures, making winter snow isotopically lighter than summer snows.
In oxygen isotope analysis, variations in the ration of 18O to 16O by mass present in the calcite of oceanic core samples are used to find ancient ocean temperature change, and therefore climate change. Cold oceans are richer in 18O (which is included in the tests of the microorganisms, giving us calcite).
Temperature and climate change are cyclical when plotted on a graph of temperature vs. time. Temperature coordinates are measured by deviation from today's annual mean temperature, taken as zero. Ratios are converted to a percent difference from the ratio found in the Standard Mean Ocean Water (SMOW). Either form of the graph appears as a waveform with overtones. Half of a period is a Marine Isotopic Stage (MIS). It indicates a glacial (below 0) or interglacial (above 0). Earth has experienced 102 MIS stages; early Pleistocene stages were shallow and frequent while the latest were the most intense and widespread. Stages are numbered from the Holocene, which is MIS1. Glacials receive an even number and interglacials receive an odd number.
Glaciers are mainly classified based on size.
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.
Valley glaciers, the next largest glaciers, which flow through valleys in the mountains, and sometimes are cirque glaciers that have escaped their depression.
Piedmont glaciers Occur If valley glaciers flow out onto an adjacent plain.
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.
There are two important parts to glacial geology: erosion and deposition. These two processes cause the many features associated with glacial landscapes.
Moraine: any ridge or mound of glacial debris that is deposited in glaciated regions. Moraines can consist of boulders, gravel, sand and clay, among other sediments.
Terminal Moraine: deposited at the terminus (end) of the glacier, marking its furthest advance. Recessional moraines: related to them in that recessional moraines are ridges that are behind the terminal moraine- they mark other spots where the glacier had stopped in the past. Lateral moraines: are material that has been pushed off to the side of glaciers. Medial moraines: form when two glaciers converge. Ground moraine: the layer of till and other sediments underneath a glacier. Supraglacial moraines: accumulations of debris on top of the glacial ice.
Drumlins: elongated, streamlined hills made out of deposited till. The steeper side of the drumlin points in the direction in which the ice flowed.
Kames: smaller, irregularly shaped hills of deposit that accumulate as a glacier retreats.
Eskers: long, winding ridges of stratified deposit, left behind by glacial meltwater streams.
Kettles: often found as kettle lakes, formed by bits of glacial ice breaking off and forming depressions in the ground, which then melt.
There are many essential erosional landforms to know, many of them occurring as a result of alpine glaciation, rather than continental glaciation.
Arete- A sharp parallel ridge of rock that resists erosion, formed by two cirque glaciers coming together but not joining. The glaciers are usually flowing down opposite sides of a mountain.
Cirque- A large bowl shaped area carved out of a mountain by a moving glacier. They are bounded by a steep cliff know as a headwall.
Hanging Valley- Forms when a glacier erodes one area faster than the other. Found mostly towards the top of the mountain.
Horn- A pyramidal peak formed by three or more cirque glaciers meeting.
Roche moutonnée- A knob of bedrock carved into an asymmetrical hill.
Entrainment is the picking up of loose material by the glacier from along the bed and valley sides. Entrainment can happen by regelation or by the ice simply picking up the debris.
Basal ice freezing is thought to be made by glaciohydraulic supercooling, though some studies show that even where physical conditions allow it to occur, the process may not be responsible for observed sequences of basal ice.
Plucking is the process involves the glacier freezing onto the valley sides and subsequent ice movement pulling away masses of rock. As the bedrock is greater in strength than the glacier, only previously loosened material can be removed. It can be loosened by local pressure and temperature, water and pressure release of the rock itself.
Supraglacial debris is carried on the surface of the glacier as lateral and medial moraines.
Summer ablation, surface melt water carries a small load and this often disappears down crevasses.Subglacial debris is moved along the floor of the valley either by the ice as ground moraine or by meltwater streams formed by pressure melting.
Englacial debris is sediment carried within the body of the glacier.
End Moraine is englacial debris subsequently dropped as the glacier stops advancing.
Terminal Moraine is the end moraine farthest away from the glacier head. It marks the farthest extent for the glacier.
Recessional Moraines These are end moraines behind the terminal moraine that form when the glacier temporarily stops retreating and remains stationary or advances.
Till, or debris deposited directly by a glacier. It is unstratified and unsorted. Terminal and recessional moraines consist of till.
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.
Glacial History of the Quaternary
The Quaternary System is that lasted from the present to approximately 2.588 million years ago with the Neogene system before the Quaternary. The Quaternary System contains two series: the Holocene and the Pleistocene with the Holocene being the present. In this period, ice sheets were able to form in Greenland and Antarctica and the continents were formed to their present shape. As glaciers formed and later retreated, thousands of lakes and rivers were created all over the world. As the glaciers retreated the sea level rose and the amount of biological diversity in the oceans increased.
Glaciers are a very useful and abundant indicator for global climate change because they incase the air pockets from millions of years ago in the ice. Scientists are then able to drill out ice cores that hold these air pockets and study their air inside. The air can indicate what the atmospheric condition was like in the past, how the temperature variated, and different types of vegetation that were present millions of years ago. Glaciers can also indicate current climate change depending on where the snow line (firn line) is on a glacier and based on ice shelfs and how they are retreating. If the snow line on a glacier continues to move up the glacier then the ablation is greater than accumulation of snow and the glacier is retreating. This causes more water to be released from the glacier and to add to sea level and to form new lakes and rivers. Ice shelfs are able to indicate global climate change because if they continue to shrink and retreat that indicates that the global temperatures are increasing because the ice is melting. Retreat from any glacier can indicate climate change, but ice shelves are better at indication because they are located in places where the temperatures are normally colder.
How are glaciers studied?
Oftentimes the glacier's mass balance must be recorded on the ground, although satellites are sometimes used for rudimentary recording.
The two main processes used to determine ablation or accumulation are probing and crevasse stratigraphy, which can give accurate measurements of snowpack thickness.
Probing: researchers will place poles in the icepack at various points, at the beginning of the melt period or accumulation period. After a few months the researchers will return and look at the changes in levels of ice, by looking at the height of the ice along the pole.
Crevasse stratigraphy: researchers will find crevasses, then observe the number of layers that formed. Based on the layers the researchers will be able to determine how much snow accumulated. The layers are almost like layers in a tree trunk.
Glacial periods are characterized with large ice sheets and are normally known as ice ages. The periods between these are known as interglacial periods and currently we are in the Holocene interglacial period. The last glacial period was between 120,000 to 11,500 years ago and was during the Pleistocene Epoch.
Interglacial periods are caused by shifts in the Earth's orbit and this causes a change in the amount of solar radiation that hits the Earth. When the amount of solar radiation increases, this is when the Earth shifts from a glacial to interglacial period. The Quaternary has had multiple shifts between glacial and inter-glacial periods and in the middle of this period, the change cycle between glacial and interglacial changed every 100,000 years.
Antarctica is a great indicator if Earth is in a glacial or interglacial period because the amount of ice and snow on it indicates the amount of solar radiation that is hitting the Earth as well as the average temperature of the Earth.
CO2 is also an indicator of the changing from a glacial to interglacial period or vice versa. As the CO2 levels increase the Earth's average temperature will increase and it will move into an interglacial period whereas if the CO2 levels were to fall, the average temperature would fall and the Earth would change to a glacial period.
For a list of vocabulary relating to glaciers, please see Dynamic Planet/Glacier Vocabulary.