6 Glaciers

Anita Singh and Somvir Bajar

epgp books

 

 

Module 07: Glaciers

 

Objectives

  1. Understand glacier and differentiate the different types of glaciers
  2. Describe glacier formation and glacier movement
  3. Understand glacial budget and mass balance
  4. To study different types of glacier landforms

 

7.1 Glacier

 

Glacier is a mass of surface ice on land and formed by accumulation of snow. Glaciers flow downhill under gravity due to its own weight. Glaciers are maintained by accumulation of now and melting of ice. The melted ice discharged into the lakes or the sea. Glaciers grow and shrink in response to the climate change.

 

Meier (1974) provided a definition of a glacier:

 

“A glacier may be defined as a large mass of perennial ice that originates on land by the recrystallization of snow or other forms of solid precipitation and that shows evidence of past or present flow. The definition is not precise, because exact limits for the terms large, perennial, and flow cannot be set. Except in terms of size, a small snow patch that persists for more than one season is hydrologically indistinguishable from a true glacier. One international group has recommended that all persisting snow and ice masses larger than 0.1 square kilometers (about 0.04 square mile) be counted as glaciers. Hence, in the absence of an agreed-upon upper size limit for glaciers, a body of ice as large as the Antarctic Ice Sheet (slightly smaller than the conterminous United States and Europe combined) could properly be considered a glacier.”

 

  • Important facts about glaciers:
  • Glaciers store about 75% of the world’s fresh water.
  • Glacier ice crystals can grow to be as large as baseball.

The largest glacier in the world is the Lambert-Fisher glacier in Antartica.

 

Ice is a mineral as it possesses a definite chemical composition and atomic structure, and non- metallic inorganic and crystalline (Drewry, 1986, p.1). Selected mechanical and thermal properties of ice are shown in Table 1 (Drewry, 1986, p.2)

 

Sources of water in glaciers and Ice sheets

 

  1. a) Surface Melting
  2. b) Dissipation of mechanical heat
  3. c) Geothermal heat
  4. d) Groundwater flow and surface runoff
  5. e) Liquid precipitation

 

The blue color of glacial ice: Snowfall covers the surface and forms a layer of snow. Every year, snowfall makes layers and create pressure on the older layer beneath. As the time pass, the low-density snow transformed into much denser solid ice. Due to compaction, pressure

 

melting (as ice melts at 00C at atmospheric pressure, but under high pressure melts at a lower temperature) and refreezing reduce the air in the frozen mass and increase materials density. As, the density of ice increases, it reflects the more short wavelength of light, i.e. blue part of the spectrum. (Source- James Petersen, 2011, p. 394).

 

Area covered by glaciers

 

 

Table 1: Approximate worldwide area covered by glaciers (Source- Global Glaciers

 

Changes: facts and figures 2008; ttps://nsidc.org/cryosphere/glaciers/questions/located.html)

7.2 Glacier formation

 

Glaciers are masses of flowing ice that accumulated over a period of time and the snowfall is more than the melting of ice. The formation of glaciers starts when snow remains in the same area for a year and when enough snow accumulate and transform into ice. When a snow falls, its hexagonal ice crystal and called snowflakes, having a density of about 0.1 grams per cubic centimeter (0.06 oz/in3.) as shown in image 1.

 

Each year a new layer of snow is added and this layer compresses the older layer. The snowflakes are transformed into granular snow by melting and refreezing. With time, the grains grow and the air pockets between them decrease and become more compact as well as an increase in density. Due to the melting, refreezing, and pressure caused by new snowfall, the granular snow is converted into firn. The firn formation takes place after about 2 winters. Over a period of time and due to pressure, partial melting, and refreezing, the ferns grow together into larger interlocked ice crystals. When the ice crystals are more compacted and very little amount of airspace left between them, they are called glacial ice. The firn is about two-thirds as dense as water but the density of glacial ice is becoming 0.9 grams per cubic centimeter (0.52 oz/in3.). So, from snow into glacial ice, there is a lot of transformation (as shown in fig. 1) and increase in density. Glaciers are formed when snow remains in one place for a longer time and face many transformations. Glaciers size varies from a small football field to hundreds of kilometers long area. Glaciers are mostly present in Polar Regions like Antarctica, Greenland, and the Canadian Arctic, and occupy nearly 10% of the world’s total Land area (Source- NSIDC glaciers). Accumulation and ablation are two most important terms in the formation of glaciers. Accumulation area is the area where snowfall is accumulated (usually

higher elevation) and ablation area (usually lower altitude) where melting and evaporation takes place. Mostly the accumulation takes place in winter and ablation part takes place in summer. The glacier is in equilibrium when the snowfall equals snowmelt, when this equilibrium is disturbed either by excessive snowfall (called advances) or by excessive melting (called retreats). The glacial system is controlled by frozen precipitation and freezing temperatures.

 

7.3 Glacier movement

 

Glaciers flow downhill due to the gravitational force generated by their own weight. Glacial velocity depends upon the thickness of glacier, temperature, bedrock, and gradient. Movement takes place by one or combination of following three processes (Source http://geomorphology.org.uk/sites/default/files/intro_to_glaciers.pdf )

  • 1) Internal deformation: Glaciers have ice crystals and when there is slippage within or between ice crystals leads to glacier movement. The base having highest pressure and maximum deformation takes place at the base.
  • sliding: This involves the sliding of a glacier over its rocky base. The basal sliding is of three types- basal slip, enhanced basal slip, and regelation slip.
  • 3) Bed deformation: This is movement accomplished by the deformation of soft sediment or weak rock beneath a glacier.

A study conducted on the glacier movement in 1948 on the Jungfraufirn glacier, found that the ice near the surface is rigid and brittle. When the glacier flows rapidly at a location, friction creates giant cracks called crevasse (image 2). Moraine is another common feature of the glacier when it pushes or carries rocky debris as it moves. There are different types of moraine-like medial moraine (run down the middle of the glacier), lateral (run down along the edges) and terminus moraine (found at the terminus or snout of a glacier). Below the brittle zone, due to weight of the overlaying ice, the ice start flowing instead of breaking. This zone is called plastic zone and ice flow in this zone.

7.4 Types of glaciers

 

Different type of glaciers

 

Alpine glaciers: Alpine glaciers are found in mountain terrain and where the precipitation and temperature conditions present due to high elevation. Alpine glaciers occupy valleys usually that was created by steam erosion. In alpine glaciers, mass flow takes place downslope because of its own weight and gravity. Alpine glaciers created the characteristic rugged scenery, like the Himalayas, Sierra Nevada, Andes, and the Alps. The largest alpine glaciers are- Alaska and the Himalayas. Alpine glaciers have two zones- zone of accumulation and zone of ablation. The zone of accumulation is the colder, snowier and upper portion of the glaciers where accumulation exceeds ablation. While the zone of ablation is warmer, downslope portion, where ablation exceeds accumulation. The dominant accumulation season is winter and dominates ablation takes place in

 

summer. The equilibrium line in an alpine glacier is decided by accumulation and ablation. There are different factors which affect the equilibrium line like temperature, the interaction between latitude and elevation, amount of insolation, wind. Depending upon the climatic condition like more snowfall shifts the equilibrium line drop in elevation, while in a warmer climate it retreats to a higher elevation. The ice movement in alpine glacier takes place from the zone of accumulation to zone of ablation.

 

Continental glaciers: Continental glaciers exist where the appropriate conditions for ice formation occur because of high latitude. Image 3 shows continental glacier. The continental glacier is much thicker in the central part and thinner from the edges. The continental glaciers are larger than the alpine glaciers, about 96% of glacier area is occupied by ice sheets of Antarctica and Greenland. In continental glaciers, the ice and ice caps flow radially outward and downward in all directions from thicker side to thinner side. So, the zone of accumulation is the central part and zone of ablation is the surrounding.

Calving Glacier- Calving glacier has a terminus that ends in a body of water (river, lake, ocean) into and lose mass by calving means breaking off mass from main glacier and form icebergs.

 

They are widely distributed and fast flowing.

The correlation is linear between the calving rate and water depth.

The calving glaciers are significant because of glacier dynamics, non-climatic behavior, and socioeconomic impacts.

 

Niche glaciers: developed in small couloirs or depressions. “Niche glaciers are very small in size. Such glaciers occupy gullies and hollow on the slopes and appear as little more than large snow fields. They do not make image of a typical glacier. A niche glacier may develop into a corrie/cirque glacier if conditions are favorable, and it is common in glaciated areas to finds small niche glaciers between larger corrie/cirque glaciers” ((Source- Encyclopedia of Snow, Ice and Glaciers, source-, p.779).

 

Cirque glacier: Cirque glacier is a small glacier that forms within a cirque basin. The Frech term cirque originates from the Latin word circus meaning circle. They are among the most frequent types of glaciers found on Earth.

 

They are found in half-open, semicircular shaped niches, or hollows located on mountainsides or in upper part of valleys.They are very sensitive to climate change. They are vital water reservoir.

 

Hanging glacier- A glacier that originates high on the wall of a glacier valley and descends only part of the way to the surface of the main glacier. Avalanching and icefalls are the mechanisms for ice and snow transfer to the valley floor below.

 

Ice Cap- A dome-shaped accumulation of glacier ice and perennial snow that completely covers a mountainous area or island. The surface area is less than 50,000 Km2. Example- Iceland, Canadian Arctic.

 

Ice Field- A continuous accumulation of snow and glacier ice that completely fills a mountain basin or covers a low-relief mountain plateau to a substantial depth. When the thickness becomes great enough, tongues of ice overflow the basins or plateaus as Valley Glaciers.

 

Ice shelves- They are simply the floating sections of ice sheets. Example- Greenland and Ellesmere Island have many ice shelves.

 

Ice streams- They are fast-flowing ice channels bordered by slower-flowing ice with a velocity up to several km/yr. They have a slippery bed; the presence of sediments; the presence of subglacial troughs; presence and routing of subglacial meltwater.

 

Outlet glaciers-These drain ice from caps and ice sheets, and flow beyond ice cap or ice sheet margin. In this catchment area is not clearly defined, and they form valley and troughs. “The outlet glacier is confined to the narrow valley by the neighboring cliffs, and by lateral moraines deposited during a geologically recent period of glacier expansion. During that expansion, the outlet glacier reached the valley floor, pooled, and formed a piedmont lobe confined only by its own terminal moraine. In other words, it is a tongue of ice that extends radially from an ice dome. It may be identified within the dome as a rapidly moving ribbon of ice (an “ice stream”), while beyond the dome it typically occupies a shallow, irregular depression. The 700 km long Lambert Glacier, Antarctica, is one of the world’s largest outlet glaciers (Source- Encyclopedia of Snow, Ice and Glaciers, p. 799).

 

Piedmont glacier- A fan or lobe-shaped glacier, located at the front of a mountain range. It forms when one or more valley glaciers flow from a confined valley onto a plain where it expands. The 30-mile wide Malaspina is the largest in Alaska.

 

Polar Glacier- A glacier with a thermal or temperature regime in which ice temperatures always remain below the freezing point.

 

Reconstituted Glacier- A glacier formed below the terminus of a hanging glacier by the accumulation, and reconstitution by pressure melting (regelation), of ice blocks that have fallen and/or avalanched from the terminus of the hanging glacier. Also called Glacier Remaniè.

 

Rock Glacier- A glacier-like landform that often heads in a cirque and consists of a valley-filling accumulation of angular rock blocks. Rock glaciers have little or no visible ice at the surface. Ice may fill the spaces between rock blocks. Some rock glaciers move, although very slowly.

They are lobate or tongue- shaped landforms.

They have characteristics “wrinkled” patterns.

The downslope movement/flow of rock debris is estimated between few centimeters and decimeters per year.Transition phenomenon between glacier and non-glacier processes.

Image4 : Rock glacier, James Ross Island (Source- http://www.antarcticglaciers.org/modern-glaciers/types-of-glacier-2/types-of-glacier/)

Temperate Glacier- A glacier with a or temperature-regime in which liquid water coexists with frozen water (glacier ice) during part or even all of the year.

 

Tidewater Glacier- These are glaciers with a terminus that ends in a water body (like an ocean or a large lake). Tidewater glaciers are found in Alaska, the Canadian Arctic, Greenland, Svalbard, Novaya Zemlya, and the Antarctic Peninsula.

 

Typically, tidewater glaciers calve ice to produce icebergs. The ablation is mostly by calving.

Catchment glacier- “The catchment glacier represents the superficial area of a glacier from where the glaciers receive and release the snow/ice material in different form and maintain the equilibrium of a glacier. The catchment of a glacier may be formed by single or more ice body, slowly moving on the downward slope and from a huge glacier reservoir system. All processes by which snow or ice are added or removed from or to a glacier are considered on its catchment basis” (Source-Encyclopedia of Snow, Ice and Glaciers, p.127).

 

 

7.5 Glacial budget and mass balance

 

Glacial budget decided by the accumulation and ablation. If, there is more accumulation of ice, it shows a glacial advance, and if more melting or ablation leads to glacial retreats. The balance between both zones of accumulation and zone of ablation decides the glacial budget. The glacier advance, when there is more snowfall and less melting in the following summer. And if there is more melting as compared to snowfall, then the glacier retreats. The equilibrium line is decided by the zone of accumulation and zone of ablation (Image 5). The position of equilibrium line changes every year depending upon more snowfall or more melting. More snowfall and cooler summers favor the glacial advance, while less snowfall and warmer summer favor the glacial retreat.

 

Mass balance (b) is the product of accumulation (c) plus ablation (a).

=   +

 

Mass balance is usually given in meters water equivalents (m w.e.). The mass balance varies according to the time and space, as in winter more snowfall means more accumulation, while in summers more melting means more ablation. Fig. shows glacial mass balance. Fig. a shows the relationship between the altitude and temperature. Fig. b shows the surface mass balance and c shows annual net mass balance. Fig. D shows the mass balance (c+a) gradient curve. Glacier mass balance is measured by staking out a glacier. This method is time-consuming and needs to visit stakes several times a year. The stakes used for this purpose can be made up of wood, plastic, or bamboo. The stakes are inserted to a known depth into the glacier

 

Image 5: Cross-sectional view of an alpine glacier showing internal flow lines, zone of accumulation, snow line, and zone of melting (Source- http://opengeology.org/textbook/14-glaciers/)

 

7.6 Glacial landforms

 

Glacial landforms are grouped into erosional and depositional landforms. Erosional landforms are formed by removing materials and depositional landforms are formed due to retreating of ice. Erosional glacial landforms are formed by the erosion of bedrock. Glacial striation are formed  sharp-edged rock scrape across bedrock. The striation also tells about the direction of flow of ice, when ice disappeared from the area. The glaciers usually makes ‘U’ shaped valley, while rivers make ‘V’ shaped valley. When two U shaped valleys are next to each other, they formed a sharp ridge called an arête. Other landforms created by erosion are- horn, col, tarn, cirque, hanging valley and glacial trough.

 

Sediments are deposited by glaciers and ice is responsible for large amount of erosion. When ice moves it erode and when melt, leaves behind sediments called till. The common landform of glacial deposition is moraine. A moraine is unratified and unsorted deposits of sediments formed by the grinding and erosive effect of a glacier. The grinding process produce silt and melting water carries this silt and deposits in front of the glacier called sandur. Other landforms created by deposition are- glacial erratics, kettle, esker, and drumlin.

 

Moraine- A moraine is any material transported and deposited by a glacier. Like a river carries all sorts of debris and silts with its flow and creates deltas, likewise glacier carries rocks and soil and form moraine. A glacier acts like a conveyor belt and carries sediment along with it, and deposits at the end or sides of the glacier. Moraines are of different types depending upon the location as shown in image 6.

 

  1. a) Lateral moraine: A lateral moraine forms along the sides of a glacier.
  2. b) Medial moraine: A medial moraine is formed where two glaciers meet.
  3. c) Terminal moraine: Terminal moraine formed at very end of a glacier.
  4. d) Ground moraine: A blanket of glacier till deposited on all of the surfaces over which a glacier moves, typically by moving ice.

Image 6- Different types of moraines (Source: https://www.nps.gov/articles/denali.htm)

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