- A glacier is a large mass of ice that is persistently moving under its own weight over the land or as linear flows down the slopes of mountains in broad trough-like valleys
- Glaciers are formed in the areas where the accumulation of snow exceeds its ablation (melting and sublimation) over many years, often centuries.
- Glaciers move under the influence of the force of gravity.
- The movement of glaciers is slow, unlike water flow. Glaciers flow like very slow rivers.
- Their movement could be a few centimetres to a few metres a day or even less or more.
Types of Glaciers
Glaciers are categorized by their morphology, thermal characteristics, and behaviour. Glaciers are mainly of four types - continental glaciers, ice caps, piedmont glaciers and valley glaciers.
- Continental Glaciers - Continental glaciers are continuous masses of ice that are much larger than alpine glaciers. By definition, they have areas larger than 50,000 km2, some examples of Continental Glaciers are Antartica, Iceland, Greenland etc.
- Ice caps - Ice caps are the covers of snow and ice on the mountain ranges from which the valley or mountain glaciers originate. Though they can also be found at the lower altitudes. Ice caps have an area of less than 50,000 km2.
- Piedmont Glaciers - The piedmont glaciers form a continuous ice sheet at the base of mountains. The Malaspina Glacier in Alaska is one of the most famous examples of this type of glacier
- Valley Glaciers - A glacier that fills a valley is called a valley glacier. The valley glaciers are commonly known as Alpine Glacier and are found in the valleys created by lofty mountains such as Himalaya in India.
Mechanism of erosion and deposition
- Erosion by glaciers is tremendous because of friction caused by sheer weight of the ice.
- The rate of erosion is determined by several factors such as the velocity of flow, gradient of the slope, the weight of the glacier, the temperature of the ice and the geological structure of the valley
- A glacier erodes its valley through two processes plucking and abrasion.
- Plucking - By Plucking , the glacier freezes the joints and beds of the underlying rocks tears out individual blocks and drags them away
- Abrasion - By abrasion , the glacier scratches, scraps, polishes and scours the valley floor with the debris frozen into it. These fragments are powerful agents of denudation
- As glaciers move over bedrock, large blocks and fragments of rocks are plucked from the land by glaciers. This mass of rocks and debris creates huge erosion potential and erodes the bed and sides of the valley through which glaciers flow.
- The movement of glaciers continuously erodes the bedrock and levels of the plain. Eventually, the slope is so much reduced that no further movement is possible and so glacier stops and deposits the debris in the vast outwash plain.
Glaciation generally gives rise to erosional features in the highlands and depositional features on the lowlands, though these processes are not mutually exclusive because a glacier plays a combined role of erosion, transportation and deposition throughout its course
- Cirques are horseshoe shaped, deep, long and wide troughs or basins with very steep to vertically dropping high walls at its head as well as sides.
- Cirques are often found along the head of Glacial Valley
- The accumulated ice cuts these cirques while moving down the mountain tops.
- After the glacier melts, water fills these cirques, and they are known as cirque lake.
- Horns form through head-ward erosion of the cirque walls.
- If three or more radiating glaciers cut headward until their cirques meet, high, sharp pointed and steep-sided peaks called horns form.
- Arete is a narrow ridge of rock which separates two valleys.
- Aretes are typically formed when two glacial cirques erode head-wards towards one another
- The divides between Cirque side walls or head walls get narrow because of progressive erosion and turn into serrated or saw-toothed ridges referred to as aretes with very sharp crest and a zig-zag outline.
- Glaciated valleys are trough-like and U-shaped with wide, flat floors and relatively smooth, and steep sides.
- When the glacier disappears, and water fills the deep narrow sections of the valley, a ribbon lake is formed.
- A fjord or fiord is a long, narrow and steep-sided inlet created by a glacier
- They are formed where the lower end of a very deep glacial trough is filled with sea water
- Fjords are common in Norway, Chile, and New Zealand etc.
- A hanging valley is a tributary valley that is higher than the main valley. Hanging valleys are common along glaciated fjords and U-shaped valleys.
- The main valley is eroded much more rapidly than the tributary valleys as it contains a much larger glacier
- After the ice has melted tributary valley, therefore, hangs above the main valley
- The faces of divides or spurs of such hanging valleys opening into main glacial valleys are quite often truncated to give them an appearance like triangular facets.
- Often, waterfalls form at or near the outlet of the upper valley
- Thus, the hanging valley may form a natural head of water for generating hydroelectric power
- An outwash plain is a plain at the foot of the glacial mountain
- They are made up of fluvioglacial sediments, washed out from the terminal moraines by the streams and channels of the stagnant ice mass.
- As it flows, the glacier grinds the underlying rock surface and carries the debris along.
- When the glacier reaches its lowest point and melts, it leaves behind a stratified deposition material, consisting of rock debris, clay, sand, gravel etc. with larger boulders being deposited near the terminal moraine, and smaller particles travelling further before being deposited.
- The stratified surface thus formed is called as an outwash plain and a downward extension of the deposited finer particles and clay material is called valley train.
- The unassorted coarse and fine debris dropped by the melting glaciers is called glacial till.
- The long ridges of deposits of these glacial till is called as Moraines
- Depending on its position, moraines are classified into be ground, lateral, medial and terminal moraine.
- Terminal Moraines - Terminal moraines are long ridges of debris deposited at the end (toe) of the glaciers.
- Lateral Moraines - Lateral moraines form along the sides parallel to the glacial valleys. These moraines partly or fully owe their origin to glaciofluvial waters pushing up materials to the sides of glaciers.
- There can be many lateral moraines on either side in a glacial valley. The lateral moraines may join a terminal moraine forming a horse-shoe shaped ridge
- Ground Moraines - Many valley glaciers retreating rapidly leave an irregular sheet of till over their valley floors. Such deposits varying greatly in thickness and in surface topography are called ground moraines.
- Medial Moraines - The moraine in the centre of the glacial valley flanked by lateral moraines is called medial moraine. They are imperfectly formed as compared to lateral moraines.
- Sometimes medial moraines are indistinguishable from ground moraines.
Types of Moraine
- An esker is a long, winding sinuous ridge of stratified sand and gravel
- Eskers are frequently several kilometres long and, because of their peculiar uniform shape, are somewhat like railway embankments
- When glaciers melt in summer, the water flows on the surface of the ice or seeps down along the margins or even moves through holes in the ice.
- These waters accumulate beneath the glacier and flow like streams in a channel beneath the ice.
- Such streams flow over the ground with ice forming its banks.
- The stream underneath carries coarse materials such as boulders, blocks which gets deposited in the bed and when the glacier melts the deposits forms a sinuous ridge called eskers.
- Drumlins are smooth oval shaped ridge-like features composed mainly of glacial till with some masses of gravel and sand.
- The drumlins form due to the dumping of rock debris beneath heavily loaded ice through fissures in the glacier.
- The long axes of drumlins are parallel to the direction of ice movement.
- They may measure up to 1000m in length and 30-35 m or so in height.
- One end of the drumlins facing the glacier called the stoss end is blunter and steeper than the other end called the tail.