Category Geography

Almost 10 per cent of Earth’s total landmass is covered by ice. This includes glaciers, Ice caps and ice sheets. Glaciers cover 15 million km2. During the last ice age, 32 per cent of the total land area was covered by ice.

Most of the Earth’s ice that we see is to be found in large masses of “nearly” pure ice: ice-sheets and glaciers of various types, ice shelves and sea ice packs. It is quite easy to calculate the surface of the areas covered with ice: it has been calculated that this amounts to approximately 15 million km2, equal to one tenth of the surface of the Earth’s emersed land. It is more difficult, on the contrary, to calculate the volume of ice because the thickness of the entire covered area must be known: using special techniques it is possible to measure the ice thickness in various points of a glacier and therefore to estimate the volume. For example the average thickness of the Antarctic sheet is 2,100 m, with peaks of 4,800 m in Land of Wilkes, in the Eastern sector: with a surface of little less than 13,600,000 km2, the total volume of the Antarctic ice is 30 million km3.

Credit: Energy & environment

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A rock resting on rocks from which it differs drastically is a glacial erratic. It would have been transported to a location many kilometres away from its place of origin by glacial erosion. An erratic can vary in size from a small rock to a very big boulder. Studying such rocks helps scientists define the path of glacial movement.

A glacial erratic is glacially deposited rock differing from the type of rock native to the area in which it rests. Erratics, which take their name from the Latin word errare (to wander), are carried by glacial ice, often over distances of hundreds of kilometres. Erratics can range in size from pebbles to large boulders such as Big Rock (16,500 tonnes or 18,200 short tons) in Alberta.

Geologists identify erratics by studying the rocks surrounding the position of the erratic and the composition of the erratic itself. Erratics are significant because:

• They can be transported by glaciers, and they are thereby one of a series of indicators which mark the path of prehistoric glacier movement. Their lithographic origin can be traced to the parent bedrock, allowing for confirmation of the ice flow route.
• They can be transported by ice rafting. This allows quantification of the extent of glacial flooding resulting from ice dam failure which release the waters stored in proglacial lakes such as Lake Missoula. Erratics released by ice-rafts that were stranded and subsequently melted, dropping their load, allow characterization of the high-water marks for transient floods in areas like temporary Lake Lewis.
• Erratics dropped by icebergs melting in the ocean can be used to track Antarctic and Arctic-region glacial movements for periods prior to record retention. Also known as dropstones, these can be correlated with ocean temperatures and levels to better understand and calibrate models of the global climate.

Credit: Wikipedia

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A kame, or knob, is a glacial landform, an irregularly shaped hill or mound composed of sand, gravel and till that accumulates in a depression on a retreating glacier, and is then deposited on the land surface with further melting of the glacier. Kames are often associated with kettles, and this is referred to as kame and kettle or knob and kettle topography. The word kame is a variant of comb (kame, or kaim is the Old Scottish word for comb), which has the meaning “crest” among others. The geological term was introduced by Thomas Jamieson in 1874.

According to White, “kames were formed by meltwater which deposited more or less washed material at irregular places in and along melting ice. At places the material is very well washed and stratified; at others it is more poorly washed, with inclusions of till masses that fell from ice but were covered before they were completely washed. Kame gravels thus tend to be variable and range from fine to coarse grained and even to cobbly and boulder.”

With the melting of the glacier, streams carry sediment to glacial lakes, building kame deltas on top of the ice. However, with the continuous melting of the glacier, the kame delta eventually collapses onto the land surface, furthering the “kame and kettle” topography.

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The variety of loose rocks and sediments dumped over landscape give evidence about the type of glacier and glaciation that took place in the area. A moraine ridge is the landform created by the debris left by a glacier after it has moved away. Moraine ridges are given names according to the size of debris and how they were formed. Examples are: lateral moraine, recessional moraine, medial moraine and ground moraine.

A moraine is material left behind by a moving glacier. This material is usually soil and rock. Just as rivers carry along all sorts of debris and silt that eventually builds up to form deltas, glaciers transport all sorts of dirt and boulders that build up to form moraines.

Moraines only show up in places that have, or used to have, glaciers. Glaciers are extremely large, moving rivers of ice. Glaciers shape the landscape in a process called glaciation. Glaciation can affect the land, rocks, and water in an area for thousands of years. That is why moraines are often very old.

Lateral Moraine

A lateral moraine forms along the sides of a glacier. As the glacier scrapes along, it tears off rock and soil from both sides of its path. This material is deposited as lateral moraine at the top of the glacier’s edges. Lateral moraines are usually found in matching ridges on either side of the glacier. The glacier pushes material up the sides of the valley at about the same time, so lateral moraines usually have similar heights.

Medial Moraine

A medial moraine is found on top of and inside an existing glacier. Medial moraines are formed when two glaciers meet. Two lateral moraines from the different glaciers are pushed together. This material forms one line of rocks and dirt in the middle of the new, bigger glacier.

Supraglacial Moraine

A supraglacial moraine is material on the surface of a glacier. Lateral and medial moraines can be supraglacial moraines. Supraglacial moraines are made up of rocks and earth that have fallen on the glacier from the surrounding landscape. Dust and dirt left by wind and rain become part of supraglacial moraines. Sometimes the supraglacial moraine is so heavy; it blocks the view of the ice river underneath.

Ground Moraine

Ground moraines often show up as rolling, strangely shaped land covered in grass or other vegetation. They don’t have the sharp ridges of other moraines. A ground moraine is made of sediment that slowly builds up directly underneath a glacier by tiny streams, or as the result of a glacier meeting hills and valleys in the natural landscape. When a glacier melts, the ground moraine underneath is exposed.

Terminal Moraine

A terminal moraine is also sometimes called an end moraine. It forms at the very end of a glacier, telling scientists today important information about the glacier and how it moved. At a terminal moraine, all the debris that was scooped up and pushed to the front of the glacier is deposited as a large clump of rocks, soil, and sediment.

Credit: National Geographic

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kettle, also called Kettle Hole, in geology, depression in a glacial outwash drift made by the melting of a detached mass of glacial ice that became wholly or partly buried. The occurrence of these stranded ice masses is thought to be the result of gradual accumulation of outwash atop the irregular glacier terminus. Kettles may range in size from 5 m (15 feet) to 13 km (8 miles) in diameter and up to 45 m in depth. When filled with water they are called kettle lakes. Most kettles are circular in shape because melting blocks of ice tend to become rounded; distorted or branching depressions may result from extremely irregular ice masses.

Two types of kettles are recognized: a depression formed from a partially buried ice mass by the sliding of unsupported sediment into the space left by the ice and a depression formed from a completely buried ice mass by the collapse of overlying sediment. By either process, small kettles may be formed from ice blocks that were not left as the glacier retreated but rather were later floated into place by shallow melt water streams. Kettles may occur singly or in groups; when large numbers are found together, the terrain appears as mounds and basins and is called kettle and kame topography.

Credit: Britannica

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