A landform created due to the movement of glaciers (flowing ice) is called a glacial landform. Glaciers are huge bodies of ice that flow in water and glacial movements and erosions lead to the formation of various landforms. These huge chunks of ice and meltwater erode striate, and polish rocks, and are potential weathering agents. Fjords, U-shaped valleys, cirque landform, horns, hanging valleys, moraines, glacial erratics, paternoster lakes, glacial till and flour, kettles, are some of the landforms created by glaciers.
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Glacial landforms are common in higher mountain ranges, regions of Greenland, and Antarctica. Nearly 30 percent of our planet was covered with ice in the ice-ages. Due to climate change, the ice started to melt and glaciers led to the formation of various landforms. The regions that were once glaciated, now house most of the glacial landforms. Periglacial landforms are also formed in the cold climate.
Types of Glaciers
The common types of glaciers are ice sheets or ice caps, continental glaciers, mountains or valleys, outlet glaciers. The ice sheets are way bigger as compared to the valley glaciers. The ice sheets bury their underlying landscape and are formed continuously over extensive areas. The flow of the continental ice sheets is from the center to the outward direction. Valley glaciers can be characterized by ice chunks flowing like a river, and are usually found in the mountains. The flow of these valley glaciers is entirely dependent upon the regional relief features. The glacial landforms formed due to the actions of valley glacier systems and continental ice sheets are often similar by their outward appearance but differ in magnitude.
The term periglacial means ‘near glacial’, and the periglacial regions are either adjacent to or very close to the glacial regions. However, these regions are beyond the glacial limit and lie in a zone of the freeze-thaw cycle. These regions are characterized by periglacial landforms.
The freeze-thaw cycle is caused due to the unique anomalous expansion of water. On freezing water changes its state from liquid to solid and expands in volume by nearly 9 percent. In periglacial regions, freezing of water into ice is often accompanied by differential ice growth. In this process, the air is trapped along with water, leading to a greater increase in the volume of ice. Such air and water mixture can exert a pressure of about 200,000 kilopascals, which is sufficient to break the rock, enclosing it. Hence, when water is enclosed in a crack or pore of a rock, it can be a potential agent of weathering. Multiple such cycles of freeze and thaw lead to the formation of ice crystal fractures. In this process, weathering is propelled by frost-shattering and frost-heaving. Furthermore, perennial ice landforms can be observed in the permafrost areas, wherein the ground is frozen throughout the year.
Glacial Erosion Features
The temperature changes in glacial ice determine the rate of glacial erosion. Basal sliding and internal deformation are the two processes that bring about the movement of glaciers under the influence of the earth’s gravitational force. To understand the glacial erosion features, let us go through the classification of glaciers into temperate glaciers and polar glaciers.
Temperate Glaciers: These are also referred to as isothermal glaciers. These glaciers have a constant temperature throughout their masses. This temperature is referred to as the pressure-melting point. It is equivalent to the melting point of ice at a given pressure.
Polar Glaciers: On the other hand, the polar glaciers have a temperature that is lower than the pressure-melting point.
Subpolar Glaciers: The subpolar glaciers are slightly different from the temperate glaciers, they exhibit an intermediate thermal nature. The temperature at the margins of these glaciers is very low whereas their interior parts exhibit temperate nature.
Internal Deformation: The two factors that bring about internal deformation of glaciers are shear stress due to the glacial ice and the slope of the glacial surface. When the weight of ice in a glacier becomes too large to be withstood by its shape while moving in the water, it results in shear stress. The steeper the slope of the glaciers, the greater the strain is created due to its weight. The movement of the ice crystals within the glaciers and the brittle fracture caused under tension affect the rate of internal deformation. Also, the temperature of the glacial ice determines the amount of strain and the subsequent deformation in the glacier. Hence, these brittle fractures are more evident in polar ice than in temperate ice.
Basal sliding is one of the main factors that lead to glacial erosion. The temperature at a glacier’s base plays an important role in determining the rate of erosion of its bed. Basal sliding is the ability of a glacier to slide upon its bed. It is restrained when the temperature of the glacial base is lower than the pressure-melting point. The low basal temperature propels the adhesion of the glacier’s base to the frozen bed.
The rate of basal sliding is quite low in polar glaciers because they are more rigid. This, in turn, diminishes the ability of the polar ice glaciers to creep, and erode by basal sliding. Consequently, polar glaciers have a very slow rate of erosion and mostly they erode due to internal deformation. Hence, less volume of meltwater, sediments, and less number of glacial erosional landforms are formed from polar glaciers.
On the other hand, in temperate glaciers erosion occurs mostly by basal sliding. The temperate glaciers have a layer of rock debris at their base. This layer may have a thickness of some centimeters to few meters. Since the ice in the temperate glaciers is not as rigid as that in the polar glaciers, basal sliding is more likely to occur in them. The base of temperate glaciers is like a sheet of sandpaper, wherein ice and rock debris form the base layers. The capacity of bearing shear strain is low in these glaciers. The rock debris and ice base make it more conducive for temperate glaciers to erode and form various glacial erosion landforms.
Quarrying is another important glacial erosion feature or process. In this process, the big rock particles are eroded away from the base of glaciers, when they are subjected to differential stress, high-stress gradients, or temperature fluctuations.
Glacial Depositional Landforms
The rock debris carried along with the glacial ice is deposited at the glacial margins. It is also deposited at the sites where the velocity of the glacial ice is the lowest. A meltwater deposit is referred to as outwash and is likely to be formed in front of the glacial margins, in channels under the glaciers, or in lakes beneath the glaciers. An outwash is usually bedded, and the layers of rocks in an outwash are well sorted as per the grain size. On the other hand, when the debris is deposited by the glacial ice directly, the glacial depositional landforms so formed are called tills. The common types of tills are melt-out till, lodgement till, ablation till.
Glacial Erosion Landforms
There are two main processes that cause glacial erosion, plucking, and abrasion. When the rock debris at the base of the glacier ice is subjected to constant grinding, a fine abrasive agent called rock flour is obtained. These particles polish rock surfaces by making fine scratches and often renders the rock surfaces highly lustrous. Striations, p-forms, and grooves are among the other features of glacial erosion. These are the tools of abrasions and the scratches they make on the rock surfaces shape the glacial erosional landforms. The common types of glacial erosion landforms of valley glaciers are discussed below.
A cirque resembles the shape of an amphitheater. Cirques are bowl-shaped depressions that are carved out due to the glacial movements. The hollow-end of a cirque is faced towards the down-valley. The back of the cirque landform is called the headwall and is formed by a curved or bow-shaped cliff. Mostly the headwall makes a semi-circular back in the cirques that are cut into flat-topped plateaus. Otherwise, due to height irregularities along the perimeters of a cirque landform, the headwalls are formed in an angular shape. Often, cirques are characterized by shallow basins leading to lakes at their bases. By closely examining the shallow basin of a cirque and the headwall of an adjoining cirque, signs of substantial glacial abrasion and plucking can be observed.
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Cirques are formed when the headwall above the glaciers is subjected to ice wedging and frost shattering. The debris is deposited either on the surface of the glacier or into the cracks between the cirque headwall and the glacier top. The debris deposited on the surface of the glacier gets buried in ice and is eventually moved to the glacial base. Gradually, this rock debris aids in glacial erosion and polishes the headwall’s base and the cirque bottom.
The lakes contained in the over-deepened basins of cirques are called tarns. Often, at the initial and final stages of growth and retreat of valley glaciers, there is no ice beyond the cirques. These types of glaciers mainly lead to the formation of cirque basins and bedrock bottoms of cirques.
U-Shaped Valleys, and Horns
Meltwater stream valleys are unlike valley glaciers. In the stream valleys, the eroded debris falling off the headwalls and sidewalls of the glaciated valleys are washed off with the glacial ice. However, the glaciers move along quite a large area of cross-section of the valleys as compared to the streams. Therefore, the meltwater streams erode a slimmer area of valleys, unlike the glaciers that erode the valley bases. Usually, the meltwater streams erode strips of rock at the lowest part of valleys and carve out V-shaped valleys. When the glaciers come in contact with the V-shaped valleys, the loose debris from the base and the sidewalls are removed. Also, glaciers further erode the sidewalls and base of the valleys, forming U-shaped valleys. The U-shaped valleys are characterized by flatter bases and steeper sidewalls.
Aretes and Horns
Aretes are glacial formations that are often found between two circles facing opposite each other. These are knife-edged ridges formed by the glacial erosion of the U-shaped valleys. The bedrock bottoms of these valleys are further eroded by the glaciers, due to which the upper reaches of the parallel valleys are reduced to aretes. In aretes between two cirques, there is often a low spot, referred to as a col.
Often on the flanks of higher mountain ranges like the Alps, several cirques are arranged radially. When the upper parts of these mountain ranges are eroded by glaciers, a sharp peak is formed. These sharp peaks are called horns and are surrounded by steep headwall cliffs separated by aretes.
Glacial Depositional Landforms
The common types of glacial depositional landforms are moraines and flutes.
Loose rock debris is collected and transported along glacial ice when a glacier moves by the walls and bed of a valley. As a glacier moves down its path, the collected rock debris gets accumulated along its lower margins. The rock debris eventually gets deposited in front of the glacier, as the glacial ice melts away. Also, the loose debris brought down by other tributary valleys that are non-glaciated gets mixed up with the debris deposited by the glaciers. These deposits of debris form a snout at the mouth of the glacier. These debris snouts or tills melted out from glaciers pile up into ridges are termed moraines. The moraines that are formed at the lower ends of glaciers or right in front of the glaciers are termed as end moraines. The moraines that are formed beside the margins of glaciers along the slopes of valleys are termed lateral moraines. Terminal moraines and recessional moraines are some of the other types of moraines formed by glaciers.
Some glaciers have so much debris accumulated beneath the ice, that the glaciers slide upon the bed of a muddy till. Cavities are carved out when a glacier moves along, and the basal ice flows around a debris knob at the bed of the glacier. The muddy paste of loose rock debris flowing under a glacier can be deposited in these cavities in the shape of a tail. These tail-like elongated deposits of till are called flutes. These glacial depositional landforms may vary from several centimeters to meters in height, and from centimeters to kilometers in length. Mostly, continental ice sheet glaciers tend to form large flutes.