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Glaciers

 

Glaciers

Glaciers form when climate in a region, or on a worldwide basis, changes enough that the accumulation of snow is very much greater than the loss of it due to melting or evaporation, over a long period of time. Glaciers are an excellent example of an equilibrium process on the Earth. Glaciers tend to accumulate snow near their sources or centers and lose snow though melting at their edges or termini.

When the rate of melting is equal to the rate of accumulation the glacier neither grows nor recedes, and is in an equilibrium state.

– If the rate of melting exceeds the rate of accumulation, the glacier begins to recede, indicating a movement towards a new equilibrium at which the rates will be balanced again.

– Conversely if the rate of accumulation is greater than the rate of melting, the glacier grows.

The growth or recession of glaciers on a worldwide basis has a significant effect on the rest of the Earth’s surface. Currently the glaciers and polar ice caps of the world contain about 44 billion cubic kilometers of the Earth’s water, roughly 3% of the total water on the planet and 85% of the total fresh water.

Glaciers and Glacial Ice

As snow accumulates it gets buried under additional snow and is compacted. Fresh snow as it accumulates on the grounds contains about 5% water and 95% air. The first state of compaction is granular snow or ice that contains about 50% water and 50% air. As further burial occurs the granular ice becomes firn, which is about 25% air and 75% water. The final stage of compaction is glacial ice, which is almost like a rock with closely packed grains of ice and 5-20% air in the form of air bubbles distributed throughout the ice. Obviously as the snow goes through the transformation from snow flakes to glacial ice it becomes tremendously compacted and loses about 80-90% of its initial volume.

                                                         

Glacial Formation

There are two factors that lead to the creation and growth of glaciers; low temperatures and adequate supplies of snow. For enough snow to accumulate to form a glacier ground temperatures must be low enough that snow remains on the ground year round. This does not necessarily mean that the air temperature is below the freezing point of water continuously, but it must remain below freezing for most of the time. Currently glaciers are more common in the upper latitudes because the temperature is lower on a continual basis than at the lower latitudes, due to the fact that the inclination of the Sun is low and the amount of solar energy reaching the ground is less than it is at lower latitudes.

The temperature variation as a function of elevation is shown for a hypothetical mountain in the central plains area of the U.S. the temperature varies from 23 o C at sea level to –10 o C at 15,000 feet. The vegetation style changes from deciduous (leaf bearing, oak), to coniferous (cone bearing, pine) and disappears at about 8,000 ft. The snow line occurs at about 8500 ft. Clearly above about 8500 ft. a glacier could be sustained at this location, if there was enough accumulation of snow.

                                                   

The accumulation of snow is the other important factor in the formation and growth of glaciers. Often cold climates tend also to be dry, making the formation of a glacier unlikely. Even mountainous areas near sources of moist air are not always populated by glaciers. If moist air moves off the ocean onto the shore and encounters a large mountain snow accumulation will occur on the windward side, however the moisture is mostly gone by the time the air reaches the leeward side of the mountain. Glaciers will likely form only on the windward side of the mountain. A good example of this effect is the contrast between the eastern and western sides of the Cascade Mountains in Oregon.

Ablation and Accumulation

Glaciers grow or recede as a function of the balance between accumulation and ablation of snow and ice. Accumulation of snow occurs at the upper portions of the glacier. After accumulation the snow is buried and begins to flow down slope towards the terminus of the glacier.

                                       

The glacial budget is the total of accumulation and ablation for a glacier. The glacial budget of Antarctica between 1945 and 1995 decreased due to a warming of the ocean around West Antarctica by about 3 oC, resulting in a loss of more than half of the floating ice shelf portion of the region, including a single iceberg more than 75 km long. Although the shrinkage appears to have stabilized, the loss of glacial ice in this region remains a concern because it may be a precursor to a more catastrophic worldwide ablation event.

Glacial Movement

Glaciers behave almost like the rock in the mantle of the Earth, a very viscous fluid that flows slowly down slope. Glaciers move by basal slip along the base of the glacier and by plastic flow within it, as shown in Figure 7-15. Basal slip is the movement of ice along the interface between the glacier and the bedrock by slippage along a layer of water. The layer of water is caused by the pressure exerted on the ice along the base of glacier, which causes a small amount of melting of the glacier. It is the same effect that makes ice skating possible. Plastic flow occurs as the pressure of the snow and ice at the top of the glacier causes the ice at depth to deform and move down slope in an effort to reduce the pressure on it. On a microscopic level the individual grains in the ice change their shape in order to accommodate the pressure.

 

Types of Glaciers

There are two types of glaciers; alpine glaciers (also called valley glaciers) and continental glaciers. Continental glaciers tend to occur in high latitude regions where temperatures are extremely low, and alpine glaciers occur almost anywhere in the world. Alpine glaciers are much smaller than continental glaciers. Greenland is covered by a continental glacier that is up to 9500 ft, in elevation covering an area of about 13 million square kilometers.

Alpine glaciers form in river valleys in mountainous regions, and in many ways resemble river systems. Alpine glacier form high in mountain valleys and often extend many kilometers down the valley. Snow accumulates in the upper parts of river and tributary valley. Often a single mountain will numerous tributaries separated by sharp narrow ridges called aretes emanating from the highest peak in the area, called the horn, as shown below. At the source of the alpine glacier, below the horn is a bowl-shaped valley called a cirque. The main valleys alpine glaciers are thicker and erode deeper into the river valley than the tributaries. This leads to the tributaries being suspended above the main valley in hanging valleys. The aretes between the tributary valleys are cut off by the ice in the main valley and are called truncated spurs. The features described here are erosional features that illustrate the strength of glaciers as agents of erosion.

                                           

 


Continental glaciers are much larger than alpine glaciers and are not constrained by existing topographic features like alpine glaciers. The two largest continental glaciers are in Greenland and Antarctica and they cover virtually the entire continent. Continental glaciers reach thickness of almost 10,000 feet and completely flatten the topographic features over which they flow, due to the tremendous pressure the mass of ice exerts on the bedrock beneath it. Continental glaciers move more slowly than alpine glaciers because they do not flow down slopes like alpine glaciers but rather flow outward from the central course area. Most of the midwestern part of North America is due to the erosion of the topographic features by a continental glacier that covered about half of North America during the Pleistocene Ice Age. The polar regions contain large continental glaciers called ice caps. Ice caps are not usually anchored on bedrock but are actually huge floating ice masses.

                                           


Glacial Depositional Features

Alpine glaciers leave a complex array of depositional structure behind when they recede. Glacial sediments are called drift. The poorly sorted sediment that is deposited directly by the ice is called till. The most pervasive depositional feature of alpine and continental glacier is the moraine. Moraines are deposits of sediment that are carried along at the base, sides and front of the glacier. Glacial moraine sediment is characterized by poorly sorted material ranging from rock flour up to coarse cobbles as large as a washing machine. The ground moraineis carried along between the bottom of the glacier and the bedrock over which it flows. The ground moraine is partially trapped within the lower part of the glacier and acts as a huge abrasive blanket that scours the bedrock below the glacier as it passes over it. Terminal moraines are ridges of poorly sorted sediment and debris that are pushed along the ground at the end of the glacier as it advances. Terminal moraines can be more than one hundred feet high and can extend for many kilometers. Recessional moraines are smaller moraines that form at the terminus of a receding glacier. Recessional moraines are formed by brief periods of advance during the recession of a glacier. Both continental and alpine glaciers display ground, lateral, terminal and recessional moraines. Alpine glaciers also display lateral moraines which are similar to ground moraines in terms of their grain size distribution and the fact that they are partially embedded in the glacier, but are located at the sides of the glacier. Medial moraines are another feature of alpine glaciers that form when a tributary of an alpine glacier coalesces with the main valley trapping the lateral moraines of the tributaries within the main valley of the glacier.

 

                                          

Significant amounts of water are carried beneath glaciers and additional melt water flows in front of glaciers even if they are not receding, and the water carries with it some of the finer sediment contained by the glacier. Much of the water-borne sediment is deposited out in front of the terminal and recessional moraines along the outwash plain. The sediment is composed of well sorted sand-sized particles that are deposited by the braided streams of melt water that flow from the glacier. Another depositional feature formed by melt water is the esker. Eskers are small sinuous ridges of well sorted sediment that are deposited in tunnels beneath the glaciers by the melt water that flows through the tunnels. Boulders as large as houses are often found in areas which were once covered by glaciers. The boulders, called erratics were carried along on top of the glacier and deposited when the glacier melted and receded. One unusual depositional feature usually associated with continental glaciers is the drumlin. Drumlins are elliptical mounds of glacial till that are aligned parallel to the movement direction of glacial movement. One end of the drumlin is steeply inclined and the other is gently sloped. It is thought that the steep side is pointing in the direction of the glacial movement.

                                               



Source http://www.indiana.edu/~geol116/Week11/wk11.htm

 

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