Glaciation


Nature Gallery (Earth - Climate)
Glaciation
At present, relatively few parts of the Earth's surface are covered in glaciers. Only the North and South Pole, and high mountain belts such as the Andes, Rockies, Himalayas, and Alps can be said to be truly glaciated. This was not always the case, however. Over the last few million years, several phases of glaciation, or Ice Ages, have taken place. During these periods, the average temperature was between 3°C and 5°C lower than it is at present. As a result, large regions of northern Europe, Russia, and North America were covered in hundreds of metres of ice and snow. The temperature of the Earth is constantly fluctuating, with small increases and decreases over periods of a few decades, and much greater changes over longer stretches of time. It is impossible to determine exactly how many Ice Ages have occurred, since it depends on the definition of the climate characteristics suitable for such a period. It also becomes increasingly difficult to look for evidence of older glaciations, because each successive glaciation tends to obliterate the evidence of the previous one. The last Ice Age ended more than 10,000 years ago, but there have been periods of very cold weather since, the most notable being the "little Ice Age" in the 16th century, when it was said that during most winters the inhabitants of London could skate on the River Thames. These Ice Ages, or epochs of glaciation, have probably had the greatest influence upon the shaping of the landscape in areas where glaciation took place. The effects are threefold: cold weather causes an increase in certain types of weathering; glaciers cause a vast amount of erosion, creating a very specific environment; and in the areas adjacent to those which are glaciated, or in areas where glaciers are retreating, processes of deposition occur which also result in specific landforms.

Cold Climate Weathering
Certain climatic conditions cause rocks to weather much more quickly than they would do otherwise. If the temperature is regularly fluctuating around freezing point (0°C), a weathering process known as freeze-thaw occurs. Water enters rocks through cracks, joints, or surface pores, and as the temperature decreases to below freezing point, the water turns to ice. As ice has a much larger volume than water, a high pressure builds up in the cracks and joints. As the temperature rises again, the ice will melt into water. Many freeze-thaw cycles will weaken the rocks, and they will eventually break away from their surrounding parent rock. If a rock falls onto a glacier, it may be transported many miles before it is deposited. Rocks from Norway, for example, have been found as far away as southern Britain, Belgium, France, and the Netherlands. These blocks of rock, which have been transported hundreds of miles and then deposited, are called glacial erratics. Upland Glaciers and Erosion
Glaciers always form in upland areas where temperatures are cooler, usually on east- and north-facing slopes which receive the least sun and therefore the least ablation, or melting. If the temperature decreases, snow in the mountains will remain from one winter season to the next. The snow gradually builds up and the pressure of successive layers, combined with the continual melting and re-freezing of the water, turns it into ice. Over time, a great thickness of ice will accumulate, and it will start to move over the ground. This movement is aided by meltwater at the base of the glacier, which is caused by the pressure of the ice above it. Ice moves from the high mountain plateaux down the valleys and eventually spreads out over the lowland plains. The high mountain plateaux are the major snow accumulation zones, the valleys represent the areas where most erosion and transport take place, and the lowland plains are mainly areas of ablation and deposition. The speed at which glaciers move depends mainly on the temperature of the glacier. Polar glaciers, found today in northern Greenland, stick to the surface and there is little or no movement. Alpine glaciers, in regions where the temperature is above 0°C for considerable parts of the year, produce a great deal of meltwater, and can move between 30 and 70 metres (100 and 230 feet) in one season. Some glaciers may even surge forwards hundreds of metres in several days, but this is rare and the mechanisms are not fully understood. Glacial erosion takes place at the margins of the ice, usually at the base and sides of the glacier. One of the main processes of erosion is known as plucking. If you clutch an ice cube which has come straight out of the freezer, you will notice that it sticks to your hand, and there will be a small resistance as you peel it from your skin. In the same way, pressured ice sticks to the ground surface at the base of the glacier. As the glacier moves down a valley, it has to tear itself away from the rock surface, often plucking some of the rock with it. Rocks which are carried by the glacier, whether eroded by the glacier or dropped onto the glacier through weathering, are called moraine. Moraine at the base of the glacier (basal moraine) drags along the ground surface, scraping it in a process known as abrasion. Evidence of abrasion can sometimes be seen on large slabs of rock with lines or etch marks on the surface. These marks, called striations, indicate where the glacier has scraped the surface of the rock, and can be used to determine the direction of glacier flow. The two types of glaciers responsible for the greatest erosion are cirque glaciers and valley glaciers. Cirque glaciers tend to develop at high altitudes just below mountain peaks. They are usually circular in shape and, after thousands of years of ridge at the sides and base, develop very steep side and back walls. When two cirques form adjacent to one another, the rock between them becomes progressively eroded until there is only a narrow ridge, called an arête, separating them. If three or more cirques develop around a peak, a pyramidal peak forms where three or four arêtes join up. The Matterhorn, in the Swiss Alps, is a pyramidal peak. Valley glaciers are glaciers that move through valleys. Over many thousands of years, valley glaciers have eroded the valleys in which they travelled into very deep, wide U-shaped features. Many of these valleys today contain long, thin ribbon lakes that meander across their wide, flat valley floors. U-shaped valleys that formed close to the coast became flooded when the sea level rose after the last Ice Age, and are now fjords. Norway's fjords, including the Strynsvatn Fjord, were formed in this way. Lowland Glaciers and Deposition
Glaciers deposit material in their lower reaches in the same way as rivers. The further ice moves from its source of accumulation into warmer, lower altitude regions, the more likely it is to ablate, or melt, and therefore start to deposit the material which it has eroded in the mountains. As glaciers retreat, material is left behind. Much fine material is released from the glaciers in the form of suspended sediment in meltwater streams. A large quantity of the rock which has been plucked and abraded will have been ground into a fine powder, which can be carried for miles by meltwater streams before being deposited. Summer meltwater can have very high levels of discharge. At the end of a glacial period, when large areas of ice melt simultaneously, an incredible amount of water is available to carry material away from the glaciated region. This material is usually referred to as till, and makes up much of the surface geological deposits in lowland Europe and much of the United States. Coarser material, which is often unsorted, may be deposited at the furthest extent of the glacier and is called an annual or terminal moraine. It is possible to trace the history of glacial retreat from terminal moraine deposits. The largest single depositional features are drumlins. These are smooth, elongated mounds with their long axes parallel to the direction of ice movement. They can be over 50 metres (160 feet) in height, more than 1 kilometre (0.6 miles) in length, and look like small hills in the base of valleys. They are composed mainly of till and unsorted boulder material which were deposited by glaciers as they lost energy. There are a large number of drumlins on the Isle of Man.