Glacier Mass Balance
GMB is the relationship between accumulation and ablation of glaciers.
Accumulation = Inputs Ablation = Outputs
- When ablation > accumulation = negative mass balnce (therefore the glacier retreats).
- When ablation < accumulation = positive mass balance (therefore the glacier advances).
- When ablation = accumulation = glacier in equilibrium.
- A glacier forms when snow accumulates over time, turns to ice, and begins to flow outwards and downwards under the pressure of its own weight.
Processes of the Formation of a Glacier:
1) Snow falls onto the glacier, if the snow survives the winter months, its is considered to be firn. 2) Snow and firn are compressed by the overlying snow; forming one thickened mass of ice. 3) The pressure created from the overlying snow compacts the underlying layers. 4) The snow grains become larger ice crystals, randomly oriented in connected air spaces. 5) This dense glacial ice, looks blue.
GLACIERS HAVE DIFFERENT ZONES of ACCUMULATION and ABLATION and the ZONE of EQUILIBRIUM.
Glacial Mass Balance
Warm and Cold Based Glaciers
Snow that survives ONE MELT SEASON = Firn
- NÉVÉ: The young granulartype of snow, which has been partially melted, refrozen and compressed. This type of snow is associated with glacial formation throughout NIVATION.
Cold Based Glacier: Found in polar reigons (e.g Antartica), frozen to underlying bedrock, slow rates of accumulation and ablation, little movement and little erosion.
Warm Based Glacier: Found everywhere outside of polar reigons (e.g the Alps), not frozen to bedrock underneath, greater snowfall in winter and rapid ablation in summer resulting in imbalance. WBG move much more, therefore there is more erosion in these types of glaciers.
Warm and Cold Ice
- Warm ice has a temperature of around 0 degrees celcius throughout its depth and in summer is able to release lage amounts of meltwater.
- Temperatures in cold ice are permanently well below 0 degrees celcius, so, there's virtually no meltwater to facilitatemovement of a glacier.
- Short Term (yearly): daily, seasonally, yearly cycle.
- Long Term (multiple yearly): climate change, man made or cyclic.
- Niche Glaciers: Very small and lie in hollows and gulleys. Found in the Northern Hemishphere. E.g Bünsow Land, Norway.
- Cique/Corrie Glaciers: Larger than niche, but still comparitively small. They occupy armchair shaped hollows and sometimes overspill to feed larger glaciers in thr valleys. E.g Dane Glacier, Yosemite National Park.
- Valley Glaciers: They are larger masses of ice, move down an ice field- follow former river courses and are bounded by step sides. Most commonly pictured when glaciers are mentioned. E.g Glacier National Park, Montana.
- Piedmont Glaciers: Formed when valley glaciers extend out onto low land areas,spread out and they merge together. E.g Malospine Glacier, Alaska (3900 sq.km = largest in world).
- Ice Caps and Sheets: Huge areas of ice which spread outwards from cenetral domes. Exposed tops of mountains in ice sheets are called NUNATAKS, but the rest of the landscape is completely covered in ice. E.g Greenland Ice Sheet, Greenland.
CASE STUDY: Zemestan Glacier, Afghanistan
- Zemestan Glacier lies in the Wakhan Corridor, a stretch of land that extends from between China and Tajikstan.
- Out of the 30 glaciers in this reigon, 28 have been retreating over the last 40 years, and 2 have remained stationary.
- The largest retreat for any of the glaciers in this reigon was 36m per year. The smallest retreat for any of the glaciers in this reigon is 11m per year.
- The Zemestan Glacier is 5.3km long and 5.2km.sq, it begins at 5640m above sea level and terminates at 4800m above sea level.
- Zemestan Glacier has retreated at 17m per year. GLIMS (Global Land Ice Monitoring System) has recorded a total retreat of 460m, 9% of its total mass.
- The glacier has remained snow covered at higher elevations at the end of the summer.
- Indicates with the current climate change, the glacier will survive the temperature increase.
- Continued warming will increase the retreat and could threaten it's survival, glacier feed the Pamir River. The terminus is on a shallow slope and is not extensively crevassed.
Case Study: Hubbard Glacier, Alaska
- Hubbard Glacier is the largest calving glacier on the North American Continent (25% larger than Rhode Island).
- Its been advancing for the last 100 years, in the past 16 years, the glacier has twice cleared the sediments across the entrance to the mouth of the Russel Fjörd.
- Hubbar Glacier defying the global paradigm of valley or mountain glacier shrinkage, and retreat in response to global warming.
- Hubbard Glacier is the largect of 8 calving glaciers in Alaska that are increasing in total mass and advancing.
- All of these glaciers calve into the sea, are at the heads of land fjörds, have undergone retreats during the the last 1000 years, calve over repeatedly into into relatively shallow submarine moraines, and have unusually small ablation zones compared to their accumulation areas.
- The Hubbard Glacier has advanced at 24m per year, since 1895.
- Movement of ice is dependant upon temperature. This depends upon the pressure melting point of the glacier.
- Pressure Melting Point (PMP): the temperature at which the ice is on the verge of melting. A small increase in pressure can then lead to melting.
- PMP (Pressure Melting Point) is normally 0 degrees on the surface of the glacier, but it can be lower within a glacier, as is increased either by weight or movement.
Glacier can be cateogrised into:
- Temperate Glaciers: Meltwater actas as a lubricant and reduces friction. It uses four processes to move: basal flow (slipping), creep, extending and compressing flows and surges.
- Polar Glaciers: These move less quickly as they are frozen to their beds. They move by internal flow and occassionally creep and extend - compressing flow.
Both move rapidly on the surface and away from the valley sides. To be called a glacier, a mass of ice MUST be capable of movement occurs when the growing ice mass becomes too heavy to maintain it's rigid shape. Flow rates within the various reigons of a glacier aren't uniform, the middle of the glacier moves with the fastest speed.