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Translations of Encyclopedia about Geology

 

Glaciers and Ice

Glaciers are great streams of ice which, in keeping with the earth's law of gravity, descend into valleys. Conditions for the formation of glaciers is a great amount of precipitation in the form of snow, which solidifies above the snow boundary, turning into large masses of ice.

Glaciers form in cold regions, such as polar regions (98 percent), or high mountain ranges. We classify glaciers depending on where they are: mountain glaciers, ice sheets (Iceland, Norway), icebergs in coastal waters (Antarctica, Greenland), and ice caps (Spitsbergen).

When a given area receives annually more snowfall than it can melt, it creates a so-called accumulation zone on the slopes of the mountains. As a result of repeated melting and freezing, the new snow changes first into old snow and then into firn. The pressure of the weight of new snowfalls as well as the repeated melting and freezing create airy, grainy, light firn ice. When ice grains continue growing and the air originally filling the gaps is pushed out, they turn bluish and glittering. The weight of new precipitation, combined with the weight of the glacier, continues compacting the firn until it becomes a single glacier slab.

Due to pressure, the basis of the glacier is deformed and obeying the earth's gravity it begins to descend to valleys (glacial movement, advance). This stream is called a glacier tongue. It can be quite long. While its upper part, which is continuously supplied with new snow, is in the accumulation zone, its lower part may be already in the stage of diminishing (ablation), where the processes of melting and evaporation prevail over the amount of precipitation.

The end of the glacier tongue is called terminus or snout. Here, melting and glacier ice are on an even foot (equilibrium zone). Water from the melting glacier runs as a glacier stream through glacier gate. These formations may be as tall as 40 metres.

The speed of the glacial movement of glaciers is given by the thickness of the ice and how steep the slopes are. The movement of the glaciers in the Alps ranges between 30 and 150 metres per year, in the case of Himalayan glaciers the speed goes up to 1000 metres, while in Greenland it can reach 7000 metres per year.

As it descends, the glacier carries with it all kinds of loose rocks as well as large boulders. This material, called till, is deposited as moraines. Simultaneously, rocks and pebbles carried by the glacier cut into the ground and slopes, changing the appearance of the landscape. They create channelled glaciated valleys. When the glacier has to overcome a larger obstacle, or when it is compressed against the rocky walls, it may fracture, creating a fissure in the ice called crevasse. Crevasses may form also in the curves of the glacier valley. If the glacier breaks apart on a rocky slope and bends down, forming a hanging glacier.

The movement of the ice masses depends on the shape and steepness of the underlying ground, on their thickness, and the temperature. An important factor is also the supply from the firn region. The speed of the glacial movement is faster on the top and in the middle than in their interior and edges. When a glacier moves through a meandering valley, the speed of the movement behaves analogously to a river stream, that is to say, it is always faster on the outside of the curve. The state of the glaciers is an indication of climatic changes. At the present, the earth is experiencing more pronounced melting of glaciers, which is one of the consequences of the so-called greenhouse effect.

During the last ice age, the ice covered almost 30 percent of the entire continental mass. At that time, the ice covered the earth from the poles to middle latitudes, which means a major part of Europe and Canada were covered by ice. Today, the ice covers only about 10 percent of the earth, mainly Antarctica and Greenland.

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Moraines

As the glacier descends, it reaches regions where the temperatures are above freezing. That's where the ice starts to melt. On its journey to these places, the glacier carried with it large amounts of rocks, breaking it into small pieces. Moraine is a general name for all these rocks, till, sediments, and their forms, which were transported and deposited by the glacier. It includes both sharp and rounded parts of rocks which were formed by the passage of the glacier.

We distinguish several kinds of moraines: ground moraines, on the underside of the glacier, form from fragmented and frozen upper surfaces of the rocks from the bed of the glacier, but also from the surface of the glacier, when loose material penetrates through fissures to the bottom of the glacier. This sediment consists of fine grains and includes small, rounded pebbles, called till. Quite often large boulders, loosened by avalanches, fall on the surface of the glacier. They remain there, untouched by the forces of the glacier. These rough rocks form the upper moraine.

Medial moraine form where the tip of the earlier or existing glacier tongues created fortifications. Lateral moraines deposit on the edges of the glacier material which fell on the surface of the glacier, and this material also travels down to the valley. The confluence of two moving glaciers (joining of their lateral moraines) creates a medial moraine. It descends as a wall of rubble together with the joined glaciers to the valley.

Push moraines form at the lower end of the glacier. They create a half-moon formation around the tongue of the glacier. These moraines may form only if the end of the glacier remains in place for an extended period of time. Individual large boulders deposited in what used to be a glacier area are called erratic boulders.

If or when a glacier completely disappears, it leaves behind a changed landscape. The result of constant erosion (loosening and weathering of soil and rocks) are corries or cirques (bowl-shaped depressions) and rocky crests. These forces also carved U-shaped valleys, and where glaciers met the sea, they formed fjords.

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Landscape Shapes

Similarly to the way the streaming water changes the ground and coastal areas, moving ice of the glaciers impacts its surroundings. The ice, in the form of a glacier, starts in high mountain firn corries and descends as a valley glacier, and its sometimes very long valley tongue melts, when it reaches warmer air.

Valley glaciers, as well as inland ice covering in sheets entire mountain ranges and landscapes, are in motion as long as they are supplied with continuous precipitation in the form of snow. That is what makes them move. Glacial movement acts as a carrier for till, taking it from mountain cliffs to valleys or to the edges of the glacier. Various glacial material is deposited there in the form of moraines. Simultaneously, these transported masses of till carve the ground under the glacier, mainly rocky outcrops, and the rushing streams of melting water erode the rocks and change them. Both these processes are called glacial erosion.

The shaping of the landscape caused by the glacier takes place underneath the glacier and is visible only when the glacier melts and the underlying ground is free. In particular, we see special forms of glacial erosion in Middle Mountain Ranges, which was covered by ice during the ice age, but today is below the snow line These glacial forms include glacial valleys (trogs), flooded valleys (originally river valleys), kars or corries, moraines, and similar features.

Glacial valleys (trog), also called U-shaped valley, is a characteristic feature of a region which is covered by ice or was at one time covered by ice. This previously existing valley (for example, a river valley), became deeper and wider as a result of the mechanical impact of the glacier. The bottom of the valley, which is also called foothill, is generally flat, because the glacier, in its later stages, deposited many sediments. The original foothill is often covered up and is situated much lower than the bottom of the valley. The slopes of the valley, too, are not as steep as originally carved by the glacier, because tailings of till were deposited there. In places, where the mountains are close to the sea, sea water might have penetrated into the valleys when the sea water was on the same level. This is how the fjords were formed. They are found not only in Norway, but also in Antarctica and New Zealand.

Water shaped the landscape because the streams of water from the melting interior ice were forced to move against higher terrain. Wide valleys filled with the melting water formed on the edge of the interior ice, which resulted in the formation of "artificial" lakes or prehistoric river valleys. In view of their west-east orientation, they are used as links for canals connecting southeast rivers with the northwest. Sands, which were during the ice age transported and deposited by the melting waters, helped create inland dunes.

Kar is the place of the origin of the glacier. It consists generally of a recessed cavity wedged in the mountain slope. Kars formed in places where snow and ice began to move in some already existing cavity. They may also be where the valley closes in. Given a certain degree of the slope and the girth of the snow cover, the snow and ice will move because of the earth's gravity. These masses grind the slope of the mountain, creating a kettle-shaped depression in the middle of solid rock. The bottom of the kettle is often filled with water, creating a small lake, which may later dry out. The threshold opposite the slope is blocked with deposited till or moraine wall.

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Ice Ages (Glaciation)

There was a period in the history of the earth when, as a result of a global worsening of the climate, major growth and expansion of glaciers occurred. There were also large zones of stationary and moving ice. This is called ice age. Glaciation period is when cool or cold and warm time periods alternate.

The term glaciation is used in particular concerning the last great ice age, which started approximately 2.5 million years ago during the Pleistocene, and ended approximately 12.000 years ago. The climate began to cool down as early as the Oligocene, when a thick ice mass formed over the south pole. During the following Miocene, this mass expanded, until it covered the entire Antarctica. During the Pliocene, great masses of ice covered the northern hemisphere, Antarctica, and the southern portion of South America.

During the Pleistocene,there was a long glaciation period, during which several ice ages alternated with warmer, interglacial periods.

The Alps were also under a thick layer of ice. The Alpine ice ages take their name from the rivers, which the glaciers reached, giving us the following chronology (in thousands of years):

1. Glaciation 600 to 540 Grunz ice age
1. Interglaciation 540 to 480
2. Glaciation 480 to 430 Mindel ice age
2. Interglaciation 430 to 240
3. Glaciation 240 to 180 Riss ice age
3. Interglaciation 180 to 120
4. Glaciation 120 to 10 Wurms ice age

These glaciations were followed by a postglacial change of climate. The same is true for North America and northern Europe. During the last ice age, the ice was always expanding from the north pole in southerly direction, and during interglaciations it melted and retreated back to the pole. Throughout this time the underlying rock was shaped by grinding. Giant boulders were carried by the ice thousands of kilometres, and ultimately deposited as terminal moraines. At that time, approximately 11 percent of the earth's surface was covered by ice.

Today, this figure represents about 3 percent. When the poles were covered by ice, the temperature in central Europe, North America and central Asia was about 10 to 15 degrees lower. Some animals adapted to the increasing cold by developing thick fur. They include woolly rhinoceros and woolly mammoth, as well as cave bears and deer.

In the world of flora, tundra and treeless steppes replaced the coniferous forests. Some distance from the poles, only coniferous forests replaced deciduous forests, and grass grew in the warmer regions of the earth.

The cause of these glaciations is not yet known. Some scientists believe that, similarly to the seasons, the earth goes through some sort of a cycle, which repeats itself.

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