Glaciation

An ice age is a period of time in history when sheets of ice were found between 40� -60� latitude. This is a period of climatic ‘deep freeze’ which leads to the development of ice sheets, ice caps and valley glaciers.

Although the cause of ice ages is still a subject of controversy, an argument based on astronomical observations of the galaxy has gained credibility in recent years. The earth and its solar system are located asymmetrically within one limb of the Milky Way galaxy. The galaxy completes one rotation about once every 300 million years, taking the solar system through denser and thinner regions of interstellar dust and through changing gravity and magnetic fields. As with tidal processes, two disturbing phases appear to exist for each full cycle, so that every 150 million years a very slight change takes place in the solar system's galactic environment, possibly altering the earth's climate.

In addition, earth-based processes are also involved. Because of continental drift, periodic changes take place in the earth's geography, the effects of which can be understood by considering the changes that preceded the present ice age. These changes occurred about 60 million years ago, when a warm equatorial seaway called Tethys separated the northern landmass (Laurasia) from the southern one (Gondwanaland), bringing warm swirling currents to all the oceans. The old southern lands began drifting northward, however, so that Africa, Arabia, and India successively collided with Eurasia. Finally, Australia separated from Antarctica, allowing a cold current to circle the globe. One by one the former equatorial seaways were blocked by land. Each ocean was now isolated and connected with polar latitudes by great swirls of cold current.

This, then, is the very speculative ice age scenario: a slight external cooling because of the galaxy's rotation, as well as a favourable geographic-oceanographic setting. A chain reaction of cooling may then be initiated by minor variations in the earth's orbit.

Within each ice age are remarkable fluctuations known as glacials and interglacials: cold and warm phases that correspond to a cycle of about 100,000 years. Recognition of this glacial cycle required complex mathematical calculations, first worked out by a Yugoslav scientist, Milutin Milankovitch (1879-1958), who showed that the cycle has additional modulations that make it fluctuate considerably. They correspond to three variables in the earth's orbit.

Most important of these variations is the eccentricity cycle of 93,408 years—the variation of the orbit from its almost circular path. This affects the spin rate of the earth-moon system, which increases when the earth and moon are closer to the sun. The slower the spin rate, the stronger is the earth's magnetic field, which, in turn, tends to screen off the incoming particles of high energy from the sun, thereby cooling the climate.

The second of the orbital cycles is the change in the tilt of the earth's equatorial plane in relation to its orbital plane over a period averaging 41,000 years. About 25 percent of the glacial and interglacial temperature differences are due to this change, which varies from about 22� to 25�.

The third orbital phenomenon is the 25,920-year precession cycle, which is similar to the wobble of a spinning top. At present the earth's axis points to the Pole Star, and the Northern Hemisphere is closest to the sun in the winter, resulting in relatively mild summers and winters. About 11,000 years ago, however, the axis pointed so as to give the Northern Hemisphere colder winters and warmer summers. Because the winter ice in high latitudes remains far into the summer, this arrangement 11,000 years ago resulted in a secondary glacial episode, with intense droughts in the subtropics.

Another geographic element is also involved in the precession cycle. Besides the blocked seaways of the present ice age, most of the Northern Hemisphere is land, which generates a continental climate, whereas the Southern Hemisphere is encircled by a continuous seaway that provides far more maritime climates. If the land and sea were uniform in both hemispheres, the precession effect would be cancelled out.

Studies indicate that Milankovitch's cycles do not fully account for the timing of events in the recent glacial/interglacial cycle. Some researchers propose that other large-scale influences, including feedback from changes in ocean currents, are equally important.

Ice sheets are enormous masses of glacial ice and snow that cover over 50,000 square kilometres. These can be found in Antarctica and Greenland. The ice sheet of Antarctica is up to 4200 metres thick in places, covering nearly all of the land features except for the Transantarctic Mountains that poke up above the ice.

Ice caps are miniature ice sheets that cover less than 50,000 square kilometres. They form primarily in polar and sub-polar regions, occupying high and relatively flat areas.

Mountain glaciers develop in high mountainous regions, often flowing out of icefields that span several peaks or even an entire mountain range. The largest mountain glaciers are found in Arctic Canada, Alaska, the Andes in South America, the Himalayas in Asia, and on Antarctica.

The differences are as follows:

Mountain glaciers only develop in mountainous areas

Ice sheets are more than 50,000 kilometres in area

Ice caps are less than 50,000 kilometres in area

Ice sheets and ice caps can only cover relatively flat areas, unless they are deep enough to cover actual mountains.

 

Glaciers form by the snowfall of one year failing to melt by the next year, and for many years after, until a large amount of snow is built up. To enable the build up of snow, the glacier will require persistent sub-freezing temperatures to enable the build up snow. As time goes by, the snow will gradually compact to form granular snow, firn and glacial ice.

Glaciers are destroyed by the process of ablation, where snow or ice is lost by any means, such as melting and evaporation. For this to occur we would require temperatures of above freezing point. Glaciers also collect debris as they move by the process of plucking. When a glacier melts this debris is deposited. This causes what are known as erratic, where, for example, a boulder will be deposited on an otherwise flat plain.

Net growth of glaciers occurs when accumulation is greater than ablation.

Net loss of glaciers occurs when accumulation is less than ablation.

Debris collected by the glacier is lost when ablation occurs.

 

 

Ice is caused to move by gravity when on a hillside. When the ice has moved it leaves a near empty corrie, which is replenished when it snows. On the way downhill, or while it is still in the corrie, the glacier will pick up rocks which causes ablation.

The ice does not move at the same speed throughout. There is a kind of thalweg in the glacial flow. The slowest flows are at the edges of the glacial flow, and the fastest flows are in the middle of the glacial flow, just below the surface. This bears a great resemblance to a river flow. The evidence for the glacial flows not moving at the same speeds throughout is cracking in the ice.

 

 

Hard glacier ice is formed by the build up of snow on top of older snow. This process is known as nevee.

Snow

Granular Snow

Firn

Glacial Ice

The snow compacts under the weight of the fresh snow above it. After many years, so much snow would have fallen that the first layer of snow will be compressed, so making it more dense. When the weight of the snow is high enough, the snow will form granular snow, the granular snow will form firn, and the firn will form glacial ice. The densities of these are as follows:

Snow = 0.1

Granular Snow = 0.2

Firn = 0.55

Glacial Ice = 0.89

 

The air locked in the ice is what makes them not very dense. As we drop down from layer to layer, we find that the air has become locked in bubbles and is being force upwards by the great pressure of the ice above.

Ice is not as hard as rock, yet it is still able to erode rock. It does this through a number of ways, namely that the main body of ice procures rock and erodes other rock by abrasion. The rock is procured by plucking, which is the process of ice forming in cracks or fissures which is then dragged out of the rock face, forcing the rock it has frozen on to come out with it. This is particularly the case with the backwall of a corrie. Freeze-thaw weathering plays a major part in these processes, as it significantly weakens the rock, which will enable these processes to occur.

 

 

 

Rivers erode by using debris, which is carried in the river. Glaciers use the same method. The process used to erode in both cases is abrasion. However, the fundamental difference between the two is that glaciers are made of ice, which is a solid, so it is therefore able to support much greater weights. In a normal flow a river would only be able to transport silt and sand, because anything heavier would sink due to the forces of gravity, whereas a glacier would be able to carry large pebbles and huge boulders. This makes the eroding capability of a glacier much greater than that of a river.

A V-shaped valley is the way it is because the river erodes downwards, because the heavier debris is pulled to the river floor by gravity. As it erodes downwards the resulting walls at the side of the river collapse due to the processes of weathering. The debris is then transported away by the river and the process continues. The glacier however will cause a U-shaped valley to be formed, as the heavy debris it carries is not pulled to the floor by gravity. As the glacier travels down an existing V-shaped valley the process of abrasion will cause the valley walls to be eroded away into flat, vertical edges.

The maximum erosion for a river is at the river floor, as the heavier debris is normally at the bottom of a river due to the forces of gravity. As the debris gets pulled along it erodes downwards. The maximum erosion for a glacier however is at the sides, as that is where most of the debris is found, as the sides of a valley have a much larger surface area than the valley floor. The processes that contribute to making valleys certain shapes are gravity, erosion, abrasion, plucking and weathering.

 

 

 

Glaciers transports debris by collecting it in methods described earlier, such as plucking, and the debris will then be frozen inside a glacier until it is deposited. Material deposited by ice is known as till, and the landforms they produce are known as Moraines. Glacial deposition depends on a number of processes, depending on whether they operate under, on the surface or around the margins of a glacier. Sub-glacial deposition occurs in three ways, the first, undermelt, occurs when the melting of the underlying ice results in the deposition of material. The second process, basal lodgement, is when high friction between the bedload of the glacier and the bedrock results in smearing of material on the glacier bed. The final process of basal flowage is the squeezing of unconsolidated, soaked material into ice concavities. Supraglacial deposition occurs by either meltout or supraglacial flowage. Meltout occurs when material is deposited by the melting of a glacier’s surface, it is most commonly found in the snout of active glaciers. Supraglacial flowage is the movement down the ice surface of the debris rich upper parts of a glacier. Deposition can also occur around the margins of a glacier, through either the squeezing out of the till from under the ice or through meltout.

The method of transport differs from that of a river in that the method of transport is a solid (ice) and that of a river is a liquid (water). The solid is therefore able to carry heavier items such as boulders, whereas water can only carry boulders in a high powered flow.

Supraglacial moraine is moraine that is deposited on the surface of a glacier. The moraine was probably englacial before it became supraglacial. It would have become supraglacial by net melting. It may also have fallen on top of the glacier. Subglacial moraine is moraine that is beneath the glacier, which is not encased in ice and is rolled, ploughed or slides along by traction. Englacial moraine is moraine which is inside the glacier. It has probably frozen on to the ice and been covered by fresh ice.

Medial moraine is moraine which forms when supraglacial material is concentrated into a thin ribbon when two tributaries meet. Lateral moraines are primarily made up of material produced by frost action which has fallen onto the edge of a glacier and been left behind when the glacier retreats. Terminal moraines mark the maximum extent of a glacier, they are usually composed of both supra and englacially transported materials that were left behind in a large ridge when the glacier melted and retreated. An erratic is a boulder of a certain rock type resting on a surface of a different geology. An example of this is at Norber Blocks in Yorkshire, blocks of granite rest on a surface of carboniferous limestone. While the glaciers were moving south through Britain, the erratics were picked up and deposited at the point where the ice melted.

The material deposited by glaciers is different from that deposited by rivers, as the material deposited by glaciers is larger, such as small rocks, pebbles and boulders, whereas a river would be depositing silt, sand and gravel. This is because a glacier is able to carry much heavier materials than a river, as glaciers are solid and rivers are liquid.

A glacier deposits larger items than a river, but it also deposits rocks of different types than the underlying rock, as shown earlier by erratic rocks. Landforms are made by glaciers also give us a clue to whether a material has been deposited by a glacier or a rock.

 

 

Corries are bowl shaped hollows with a steep backwall and a shallow lip at the front. A small round lake or tarn often forms in the corrie bowl. Corries are usually found high in the mountains and are formed by ice erosion.

 

 

 

A kame is a small mound of sand and gravel. It may be formed by material deposited in crevasses by meltwater streams flowing across the surface of the ice. As the ice melts, the material subsides to the ground.

A kame terrace is formed by the build up of sediment in meltwater lakes at the glacier’s edge. After glaciation the kame terrace appears as a ridge running along the valley side. It may be confused with a lateral moraine, but analysis of the material should reveal its true identity.

A kettle hole is a small depression or hollow in a glacial outwash plain, formed when a block of ice embedded in the outwash deposits eventually melts, causing the sediment above to subside.

Outwash is sedimentary material deposited by meltwater issuing from a glacier snout or ice sheet margin.

Outwash sands and gravels differ from glacial moraines in being sorted and subjected to attrition. The coarsest sediments are deposited close to the ice margin; finer material will be transported greater distances before deposition.

An alluvial fan is a cone of sediment deposited at an abrupt change of slope. For example, where a tributary post-glacial stream meets the flat floor of a U-shaped valley. Alluvial fans are also common in arid regions where streams flowing off escarpments may periodically carry large loads of sediment during flash floods.

Some rivers in U-shaped valleys seem very small compared to the size of the valley. This is because the valley has been eroded away by a glacier, and so the valley was made to fit a glacier, not a river. When the glacier melted, the valley was probably proportionate to the size of the river, due to the huge amounts of water generated. Now the river may be small due to the amount of water it gets, and the only source of water for the river will be rainfall. The meltwater plus rainfall is much greater than just rainfall, and so the river is much smaller.

 

 

 

Man uses areas of glaciation for a number of things, such as sheep farming, forestry, quarrying, skiing, hiking, lowland farming and fishing.

Glacial deposits such as gravels, sands, and clays are quarried from glaciated landscapes. The steep glaciated hills are great for skiing, climbing and hill walking, and are also a haven for birds and other animals. Fjords make excellent deepwater ports for tankers, ferries and submarines. There are also a great number of shipyards found in fjords.

 

If the landscape is U-shaped, then it has been formed by glacial processes.

If the land is V-shaped, then it has been formed by fluvial processes.

If there is a corrie, then the landscape has been formed by glacial processes.

If there are moraines, then the landscape has been formed by glacial processes.

If there are erratics, then the landscape has been formed by glacial processes.

When all the ice on the land melted, the water that was locked in the glaciers melted and the sea level rose to near what it is today. During ice ages, Britain ceases to be an island and we have a land link to Europe. The erosive power of the meltwater would be huge, resulting in various landforms which were created by meltwater flows. These may include deepened valleys and estuaries.