Dynamic Planet/Glaciers

Glaciers are the topic of Dynamic Planet for the 2019 season. This was previously the event's topic during the 2013 and 2014 seasons.

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. Glacier thickness can range from as little as 50 meters or less, to well over 2 kilometers. They form from years of unmelted snow being compressed into dense, glacial ice.

All glaciers originate on land, but can flow into the sea. Glaciers which extend into the ocean should not be confused with Sea Ice, which is formed from seawater freezing rather than snow compressing.

Glaciers are immense bodies of ice, often several Gigatons in mass. As such, they leave behind a very unique landscape, with dozens of types of landforms.



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 ice shelves and ice rises) Greenland:	  1,784,000 Canada: 	  200,000 Central Asia:	  109,000 Russia:	          82,000 United States:	  75,000 (including Alaska) China & Tibet:	  33,000 South America:    25,000 Iceland:	  11,260 Scandinavia:	  2,909 Alps:  	   2,900 New Zealand:	  1,159 Mexico: 	  11 Indonesia:	  7.5 Africa: 	  10 The Worldwide Coverage info was taken from the following site: http://nsidc.org/cryosphere/glaciers/questions/located.html

Formation
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.

There are a wide variety of conditions which affect the formation and maintenance of glaciers.

Movement
Glaciers flow because of gravity. While other factors affect the rate and specific ways in which glaciers flow, the driving force can always be attributed to gravity. Whether it's simply pulling downhill on the ice or other ice uphill being dragged down and pushing more along, the weight of a glacier is what makes it move. Unsurprisingly, the steeper the slope of the mountain that the glacier rests upon, the faster it will flow. Glaciers generally cannot flow on level ground or uphill until they are over 60m thick.

The advance and recession of a glacier should not be confused with its flow. A glacier never flows backwards or uphill, but it can have a net loss of ice at its terminus to make it appear to recede further up the mountain. Advance and recession only has to do with mass balance and gain/loss of snow & ice, not with how it moves.

The actual way in which a glacier flows is dependent on far more factors. There are three main ways a glacier can flow, but can often include two or even all three methods if the environment changes substantially between different parts of the glacier.

Basal Sliding
This process involves the movement of the base of the glacier across the bedrock upon which it lies, usually incorporating meltwater. When comparing glaciers which undergo basal sliding, thinner, steeper glaciers are most active. There are 3 ways in which basal sliding is accomplished.


 * Basal Slip is when a thin layer of water between the ice and rock lubricates the glacier, allowing for faster flow. This meltwater can come from a variety of sources, including pressure-melting, percolation, and moulins & conduits. This is generally more applicable to smoother bedrock surfaces, but still constitutes the majority of basal sliding. Additionally, if enough meltwater is present, Basal Slip can allow for a surge to occur, where the rate of flow is several orders of magnitude higher than normal.


 * Enhanced Basal Creep is when the ice encounters a large obstacle. The large increase in pressure causes the ice to deform plastically around the obstacle.


 * Regelation Flow is when ice encounters a small bedrock obstacle. Rather than deforming around it, it melts under the pressure and refreezes on the other side. This only happens if the object is small enough to allow the latent heat on the lee side (refreezing) to be quickly conducted to the stoss size (melting) and assist further melting.

Internal Deformation
Also known as Creep, Internal Flow, Plastic Flow, and Plastic Deformation, this process involves ice crystals slowly sliding across each other from within the glacier. Ice can deform because it behaves plastically under standard glacier conditions, but can crack with large stresses. Internal deformation occurs in all types of glaciers, since it is not reliant on meltwater.

Bed Deformation
(Sometimes Subglacial Deformation) As its name suggests, Bed Deformation involves the shifting of softer sediments to allow the glacier to move downhill. Subglacial till is composed of unsorted sediments with a wide range of sizes, from boulders to clay. Finer sediments, such as clay and sand, deform readily when shear stress is applied and also have high power-water pressure (pressure of groundwater between particles). Much like basal sliding, bed deformation depends on meltwater at the base. Basal sliding is more efficient if water remains directly under the surface of the ice, whereas bed deformation is more prevalent where the sediment becomes saturated with water, reducing its strength.

Thermal Regime
Due to the importance of meltwater in two of the three methods of glacier flow, the thermal regime of a glacier cannot be ignored. Simply put, the temperature of a glacier, more specifically its base, determines its thermal regime. There are two sets of names for thermal regimes, which will be combined into one for the purposes of simplicity.

Cold-Based (Polar)
These glaciers are frozen effectively year-round, excluding any seasonal melting near the surface. Most importantly, the base of the ice is frozen. These are generally found at higher latitudes and have lower seasonal variations in temperatures. There is very minimal to no meltwater. They move exclusively through internal deformation without any basal slip or bed deformation. The ice is generally frozen to the rock.

Warm-Based (Temperate)
Also known as Wet-Based, glaciers of this thermal regime are characterized as being warm enough to have meltwater. They are generally at or very close to their melting point during the year throughout the entire thickness of the glacier. They are generally found at lower latitudes. The movement of these glaciers is largely through basal sliding (specifically basal slip). Meltwater plays a substantial role in the process, mainly coming from surface melt that is channeled to the bottom through moulins, tunnels, crevasses, and more. If the basal ice melts, either through temperature or pressure-melting, even more basal slip can occur. During the winter months, the glacier often refreezes to the bedrock, slowing the movement periodically. The meltwater of warm-based glaciers can also lead to an increase in plucking, leading to increased sediment transport.

Polythermal (Subpolar)
Polythermal glaciers are those which have components of both warm- and cold-based glaciers, which vary depending on the location. Realistically, most valley glaciers are polythermal, containing both elements of warm- and cold-based glaciers, depending on the area being looked at. They can range from mostly Warm-based to mostly cold-based.

Factors which Prevent Movement
While various factors contribute to the movement of glaciers, others directly oppose it. First and foremost, friction between the ice and bedrock is (usually) the biggest contributor to stopping a glacier. Another factor which counteracts glacial movement is debris at the terminus, such as terminal moraines, which provide an extra "wall" that the glacier has to push along. Finally, there is Ice Shelf Buttressing. Here, an ice shelf prevents an outlet glacier from advancing any further into the sea, slowing or stopping its flow.

Mass Balance
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. A glacier will advance when there is a net positive gain in ice, particularly at the terminus, making it grow further downslope. A glacier will retreat when the opposite occurs; it will melt away, leaving the terminus higher up the mountain. The visual appearance of advance and retreat should not be confused with the flow of a glacier, which is always downslope.

Glacier Parts
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 or equilibrium line, which moves during the summer.

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Glacial Morphology
Glaciers are generally classified by their location and general features, and fall into two categories based upon constrainment by underlying topography.

Constrained
Glaciers which are constrained by underlying topography are generally also called mountain glaciers as they are all confined by mountains. The only exception from this list would be ice fields.

Valley
Valley glaciers are a general group of glaciers which flow through the valleys of mountains. Sometimes they originate from cirque glaciers which have spilled down into the valley.

Piedmont
(French: Foot of Mountain) Piedmont glaciers are valley glaciers which have flowed out beyond the edge of the mountains and into an open plain. They are characterized by a fan or mushroom shape at the foot of the mountain.

Cirque
Cirque glaciers are generally the smallest type of glacier and form in bowl-shaped depressions in mountains. They can expand beyond their original confines and become a valley glacier.

Ice Field
Ice fields are large expanses of glaciers which cover mountains up almost to their peaks, leaving nunataks. This means that ice fields are still partially confined by the mountains they reside in. They can form when a large number of valley glaciers or even smaller ice fields join together.

Outlet
Outlet glaciers are a special form of valley glacier which drain ice from ice caps, ice fields, and Ice Sheets through narrow mountain passages. These can terminate both on land, into an ice shelf, or simply into the ocean.

Unconstrained
Unconstrained glaciers are generally called continental glaciers, although this more often refers to ice caps and ice sheets, and less so constituent ice streams.

Ice Cap
An ice cap is a dome-shaped mass of glacier ice that spreads out in all directions. Ice caps are usually larger than ice fields but always under 50,000 sq. kilometers. The dome shape refers to the fact that accumulation, if it occurs, is generally near the center, leading to a raised area which will flatten out by Internal Deformation.

Ice Sheet
An Ice Sheet is the same as an ice cap, except it is greater than 50,000 sq. kilometers. Ice Caps and Ice Sheets are referred to as Continental Glaciers.

Ice Stream
Ice Streams are special areas of ice caps and ice sheets with substantially increased rates of flow, upwards of 500-1000 meters per year. They are important for the mass balance of the ice caps and ice sheets, and are often riddled with crevasses and shear margins from the tension and shear stresses the ice undergoes.

Other Glacier Types
There are other classifications for glaciers which do not specifically belong to the morphological categories, or are simply subtypes of the aforementioned examples.

General
These glacier classifications do not refer to one specific type.

Tidewater
Any glacier which terminates in water, but does not extend far beyond the coast, is considered a tidewater glacier. These are generally valley and outlet glaciers. They calve at very high rates, creating lots of icebergs which can be a hazard to oceangoing vessels. They generally have high flow rates due to the calving.

Ice Shelf
An ice shelf is a glacier or ice sheet which as flowed out into the ocean. These are very thick and composed of glacial ice, and should not be confused with sea ice, which is thinner and made from seawater. The area where an ice shelf is connected to land is known as the grounding line. They are large and relatively permanent, but have been known to break away and disappear, as was the case with Larsen B in 2002. Ice shelves are also responsible for ice shelf buttressing, where outlet glaciers are held back by the sheer mass of the ice shelf.

Valley
These glacier classifications usually apply to Valley glaciers and their derivatives.

Hanging
A valley glacier which terminates at a hanging valley is a hanging glacier.

Branched-Valley
Any valley glacier which has a tributary glacier is a branched-valley glacier.

Niche
A niche glacier is very small glacier that occupies gullies & hollows on pole-facing slopes of a mountain which are covered by shadows. If the conditions become more favorable, it can develop into a cirque glacier.

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

Glacial Deposition
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.

Glacial Erosion
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.

Sediment Transport
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.

Subglacial Morphology
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Subglacial lakes 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.

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.

Glacier Fluctuations 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.

Glacial History of the Quaternary
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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.

Climatic Effects
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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?
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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.

Interglacial Periods
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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.



Vocabulary
For a list of vocabulary relating to glaciers, please see Dynamic Planet/Glacier Vocabulary.

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
 * CrazyPunyMan's Dynamic Planet Notes