Wednesday, January 18, 2012

THE STUDY OF GLACIERS

Glaciers are large, slow moving, masses of ice, that deform and move down slope under their own weight. Glacial landscapes are distinctive due to glaciers being powerful agents of both erosion and deposition. Approximately 10% of the earth's surface is covered by glaciers. Large areas of glacial ice are found in high latitude areas such as Greenland and Antarctica, however glaciers are also found in areas of high altitude (e.g. the Rockies).  

The distribution of glaciers
The distribution of glaciers is affected by both latitude and altitude. Glaciers are found in areas of high latitude and also areas of high altitude. Latitude is fundamental in determining the climate of an area. High latitude, polar regions have a very low average annual temperature due to the low angle of the suns rays which reduces the amount of solar energy received. 

These regions also have high albedo surfaces (snow reflects large amounts of solar radiation back into space). In contrast equatorial regions receive much greater amounts of solar radiation due to the greater angle of the sun and much less heat is absorbed or reflected, resulting in much greater temperatures. Conditions are therefore ideal for glacier formation in polar, high latitude regions (e.g. Antarctica and Greenland), where large amounts of snow fall. Close to the equator, temperatures are too warm for snow. However, some glaciers do exist in equatorial regions as altitude is also an important contributing factor.

Glaciers are found in areas of high-altitude (mountainous regions), such as the Alps, Himalayas and the Andes, due to very low temperatures. As altitude increases, there is a fall in atmospheric pressure; this results in an expansion in the volume of air which causes a reduction in temperature. Temperatures decrease by 1oC for every 100m increase in altitude.

Local scale factors such as relief and slope aspect (direction faced by the slope) also affect the distribution of glaciers. For example, in the Northern Hemisphere, north-facing slopes are shadier and therefore more conducive to snow accumulation and the subsequent formation of glacial ice.
Glaciers are mainly classified by size:
  • Niche Glaciers - these are the smallest glaciers occupying small hollows on North-facing slopes in the Northern hemisphere.
  • Cirque / Corrie Glaciers - these are small glaciers occupying areas between 0.5 - 10km2. They occupy small shady / sheltered depressions. As snow continues to accumulate in these hollows, eventually glacier ice is formed and the depression is scoured and deepened by glacial erosion.
  • Valley Glaciers - these form where cirque/corrie glaciers begin to extend down into a valley, bounded by steep valley sides. These large masses of ice are often fed by a number of smaller tributary glaciers and they may cover areas up to 1000s of km2.
  • Highland Ice Fields - Where valley glaciers combine to form large scale coverage of highland areas with glacial ice. Numerous outlet glaciers may exist. Ice may cover large areas with only the highest mountain summits exposed, these are called nunataks.
  • Piedmont Glaciers - These are formed where valley glaciers extend into the surrounding lowland areas e.g. Wilson Piedmont (Antarctica)
  • Ice caps / Ice sheets - These are huge areas of glacial ice spreading outwards. Ice sheets are the largest accumulations of ice and they are defined as areas where there is continuous ice cover over more than 50,000km2. Although there are only two ice sheets in existence today (Greenland and Antarctica), during the last glacial period, vast ice sheets also covered areas of Europe and North America. Today the Antarctica ice sheet is said to contain 85% of all the earth's glacial ice.
The Movement of Glaciers
Whilst ice appears to be solid it can take on the behaviour of a fluid, indeed, glaciers are sometimes referred to as 'rivers of ice' - but how do they move?

The movement of ice is what gives the glacier its power as an agent of erosion and deposition. As the ice moves it picks up material eroded at its base and sides and transports it until it is later deposited. Whilst Ice has the power to erode and transport large amount of material, if it is stationary it also has the ability to some extent to protect the landscape below. 

Variations in Glacier Movement
Glaciers do not move in a uniform way. Experiments, such as that by James Forbes (1842) have been undertaken to investigate the movement of glaciers, using poles positioned in a line across the glacier. Observations have shown that glacier movement is fastest in the middle and declines from the surface to the base (see diagram below) - this can be explained in part by a reduction in friction away from the sides and base of a glacier.


Basal Temperatures and the movement of Glaciers.
Essentially, glaciers move due to the influence of gravity where there is an imbalance between accumulation and ablation resulting in an increased down-slope force as mass builds up in the accumulation zone. As accumulation continues the slope reaches a point at which the sheer stress overcomes the resisting forces of friction and the glacier ice deforms and begins to move in a down slope direction. The type of movement that actually takes place varies however according to the type of glacier, whether it is warm or cold glacier. 

The essential difference between these two types of glacier is their basal temperature which may be below or at/above the Pressure Melting Point; this determines which mechanisms of movement are able to take place. The pressure melting point is the temperature at which ice is on the verge of melting, as pressure increases this temperature becomes lower. Ice which is at the pressure melting point is able to deform easier than that below it. At this point, the presence of melt water facilities the movement of a glacier. 

COLD BASED GLACIERS 
  • High latitude locations with low relief
  • Ablation and accumulation limited
  • Basal temperatures below pressure melting point  - therefore no basal sliding as no meltwater
  • Glacier is frozen at the bed and therefore there is very little movement and little erosion or deposition.
  • Slow rates of movement - temperatures throughout are below 0'C 

Main Method of movement: 
  • Internal Flow / Internal Deformation

WARM BASED GLACIERS 
  • High altitude locations with steep relief
  • High accumulation and ablation
  • Basal temperatures at / near Pressure Melting Point (may be above freezing)
  • Melt water is created as temperatures are more often at or near Pressure Melting Point throughout the ice.
  • Rapid rates of movement - mobile glacier able to carry out erosion. 

Main Method of movement: 
  • Basal Slippage - involving a combination of mechanisms (regelation slip; creep etc.) 


Methods of Movement

1. INTERNAL DEFORMATION
In cold-based glaciers where there is no melt water and thus no basal sliding, movement is by international flow (internal deformation), where the ice deforms and acts plastically due to movement between or within individual grains of ice. It may include:

  • Inter-granular flow - this is where individual grains of ice slip move in relation to each other re-orientating themselves.

  • Laminar flow - this is where individual layers of ice (e.g. annual accumulations) within a glacier move in relation to each other.

In internal deformation, there are different rates of movement within the ice mass, with the ice in the middle of the glacier moving faster than that at the sides and the base. Velocity of ice movement through internal deformation is strongly related to the gradient. As ice moves over a steep slope, internal deformation may not be able to deform quickly enough. This results in the formation of crevasses as the ice fractures. Where the gradient increases, the fractured ice accelerates forward becoming thinner (this is known as extending flow). When there is then a reduction in the gradient, the ice slows and becomes thicker (compressional flow) 

2. BASAL SLIPPAGE
Basal Slippage is the sliding of the glacier over the underlying rock and this can represent up to 80% of the glacier movement. Friction and pressure between the glacier and the bed can raise basal temperatures above the pressure melting point. When this occurs, a layer of melt water between the ice and valley bed helps to reduce friction and acts as a lubricant facilitating the sliding process.

  • Basal Slippage may include regelation slip and creep:
  • Regelation slip is where as a glacier moves over an obstacle, pressures build up on the up-glacier side, causing localised pressure-melting. The melt water flows around the obstacle and then refreezes on the 'down glacier' side where normal pressure conditions exist.
  • Creep - this is where the stress in ice increases as it encounters an obstacle causing it to deform under the pressure, spreading around the obstacle 'plastically' and re-freezing on the other side where the stress is reduced.

3. GLACIAL SURGE
Glacial surges are where the gradient of the ice has been so steep in the accumulation zone that it has become unstable causing a sudden forward movement. This type of movement is rare but very significant as velocities may reach 10-100 times the normal rate.

Rates of Movement
The rate of movement of glaciers generally ranges between 3-300m yr-1. Rates of movement are affected by a number of factors (see diagram below). Gradient of the valley floor is an important control on rates of glacial movement and rates of 1000-2000m yr-1 may be reached where there are steep gradients! The Thickness of ice is also important as it impacts upon the pressure melting point which can result in large amounts of melt water which help to increase speeds of flow.




Glacial Erosion
As glaciers move over the landscape, they are able to change it through the processes of weathering and erosion. The rock that is removed by these processes is then transported by the glacier and deposited elsewhere. We do not understand precisely how the processes of glacial erosion take place due to; (i) difficulties in observing these erosion processes at work beneath the glacier and (ii) difficulties in evaluating how much change has actually taken place due to erosion through lack of knowledge of what the pre-glacial landscape was like. 

However, glacial landscapes and their features have provided valuable evidence to support our understanding of glacial erosion processes. 

There are 3 main processes of glacial erosion.


1. ABRASION
This is where the bedrock underlying the glacier is eroded by debris embedded in the base and sides of the glacier. As the glacier moves over the bedrock, this material scrapes away at the rock like sandpaper wearing it away. As it does so it leaves behind scratches and grooves in the rock, known as striations. Where these grooves are discontinuous but regular in occurrence they are known as chatter marks. The depth of the striations will be dependent on factors such as resistance of the bedrock, as well as the fragments that are undertaking the erosion. 


As the bedrock is eroded by abrasion, further material may become entrained in the ice increasing the amount of abrasion that is able to take place. Where fine material is embedded in the base of the glacier it will act to 'polish' and smooth the bedrock below. Indeed as abrasion takes place, rock material is ground down to produce a very fine 'rock flour'. The characteristic blue-green color of glacial lakes and streams (opposite) is due to the high concentrations of this rock flour in suspension.




Rates of abrasion are greatest where:
  • basal sliding can occur (cold-based glaciers do not abrade as they are frozen to the bed) (however where there is excess meltwater under pressure, this may reduce the contact between debris and bedrock reducing abrasion)
  • there is plenty of rock debris to act as 'cutting' agents in the abrasion process;
  • the rock debris is more resistant than the bedrock it is abrading;
  • there is a plentiful supply of debris at the base of the glacier
  • the glacier is travelling at a greater velocity across the bedrock;
  • the ice is thick, enabling the embedded debris to exert greater pressure on the bedrock below


2. PLUCKING
The process of plucking (also known as quarrying), results in the removal of much larger fragments of bedrock than that undertaken by abrasion. The process is most effective on well jointed rock and that which has been pre-weakened by weathering processes such asfreeze-thaw and pressure release. Plucking occurs where ice is at pressure melting point. As the melt water produced refreezes (e.g. on the 'lee' side of a rock obstacle) it en trains material in the base of the glacier. As the glacier continues to advance, the newly entrained material is prised out of the bedrock. This material is then able to be used in the process of abrasion.




Rates of plucking are greatest where: 
  • the bedrock is well jointed;
  • melt water is present enabling entrainment during refreezing
  • the ice is thick, creating greater frictional drag as the ice moves over the bed.



3. SUB-GLACIAL MELT WATER EROSION
Melt water under the glacier is able to erode both chemically and physically. Melt water under the glacier is often travelling under pressure and may fluvially abrade the underlying bedrock using the sediment that it is carrying. This process is most effective where the suspended sediment is coarse. Melt water may also be able to erode through the process of solution, this is particular effective in areas where the bedrock is chalk or limestone, and minerals in the rock becomes solutes, dissolved by the melt water. 

Weathering Processes
The weathering processes of freeze-thaw and pressure release (dilation) are important in the glacial erosion system as they help to 'prepare' the bedrock for erosion processes, by loosening and fracturing the rock providing weakness that can be exploited through processes such as plucking.
  • Freeze-thaw: this is where rocks are weakened as water entering joints, freezes and expands exerting pressures which prise the joints apart. Material which is loosened by this process may then be entrained to form basal debris used in the process of abrasion.
  • Pressure-release - As the glacier begins to melt, the pressure exerted by the glacier is reduced, for example on the 'down glacier' side of a roche moutonn√©e, and the underlying rocks fracture as dilation of the rock occurs. 

Rates of glacial erosion
Stationary glaciers (cold-based glaciers) are much less erosive than warm based-glaciers. It is the mobility of warm-based glaciers, due to the presence of melt water, and the material that they transport that facilitates greater amounts of erosion. There are a number of factors affecting rates of glacial erosion:






Glacial Transport
Glaciers are capable of transporting very large volumes of material over very large distances. The rate of transport will be dependent on both the supply of material and the velocity of the glacier. The rock material carried by a glacier is known as moraine. As well as material added to the glacier as it erodes the bedrock at its base and sides, material may come from a variety of sources, these include amongst others:
  • rockfalls from weathering of the surrounding slopes;
  • wind-blown materials, including dust and volcanic ash
  • avalanches 
The material carried by a glacier can be classified as either supraglacial, englacial or subglacial moraine.

1. Supraglacial - this is material carried on top of the ice e.g. that falling on to the ice from weathering of surrounding slopes or that is wind blown. If this material is covered by further accumulation of snow or falls down crevasses in the surface of the ice, it may eventually become englacial.

2. Englacial - this is material carried within the glacier itself

3. Subglacial - this is material carried along in the base of the glacier. Much of this is likely to have been derived from glacial erosion, however some may have been englacial material that has gradually worked its way down through the ice. Surface melt water streams that flow down crevasses may also provide material which becomes subglacial. Where compressional flow occurs, this material may however be thrust upwards to become englacial or even supraglacial.

Linear accumulations of moraine along the edges of the glacier surface are known as linear moraine whereas as an accumulation of material towards the center of the glacier surface, away from the edges is known as medial moraine (see diagram below). Medial moraines form at the confluence of two glaciers where a tributary glacier joins up with the main glacier.




Direction and extent of glacial transport.
Material left behind when glaciers retreat helps to give an indication of where the glacier has travelled from as well as the method by which the material was transported. Glacialerratics can give a good indication of the capacity of a glacier to transport large blocks of material over long distances. An erratic is a rock that is found on rocks of different lithology having been transported glacially and stranded when the glacier retreated. These may vary in size from small rocks to large boulders. Many of these rocks can be traced back to their source giving an indication of the extent of the glacial transport.


Glacial Deposition
When does glacial deposition occur?
Deposition of material carried by the glacier occurs either as partial deposition as a result of a reduction in the velocity of the glacier or complete deposition as the glacier retreats and melts. Glacial sediment is collectively known as drift. Material that is deposited directly by ice is known as till (or boulder clay), however, material may also be deposited by glacial melt water, these deposits are known as outwash deposits. There is an important distinction between the characteristics of glacial and fluvio-glacial deposits as fluvio-glacial are well sorted due to the action of water (as water looses its energy its drops the largest material first), whereas glacial deposits are both unsorted and unstratified (not layered). There is a third group of deposits known as ice-contact stratified drift, these are deposited close to melting ice and they are partly sorted by melt water and roughly stratified.

Differences in characteristics between glacial and fluvio-glacial deposits

Glacial Deposits 
  • Unstratified (difficult to identify layers)
  • Material is angular, from physical weathering and erosion (unaffected by water) and various shapes and sizes (boulders - rock flour) 
  • Unsorted (random sorting as ice melts and deposits material regardless of size) 
Fluvio-glacial Deposits 
  • Stratified (vertical layering due to seasonal / annual layers of sediment accumulation) 
  • Material is smooth and rounded (due to attrition), it is sorted and graded. 
  • Sorted - larger rocks and boulders are deposited first as the melt water looses energy. 


1. GLACIAL DEPOSITION
Glacial Till deposited directly by a glacier, can be classified as either lodgement till or ablation till. 
  • Lodgement till is where material beneath the glacier has become lodged in the bed as the glacier is advancing or retreating, for example where a glacier is so overloaded with debris that it smears it onto the valley floor. An increase in ice thickness can also increase friction beneath the ice, causing lodgement. 
  • Ablation till, this is where material is deposited as the ice around it melts away. This may because of solar radiation causing melting along the margins of a glaciers, or it may even be due to the melting of basal ice due to geothermal heating. As a glacier retreats due to ablation at the snout, material is left behind.
2. FLUVIOGLACIAL DEPOSITION
Melt water is abundant in warm-based glaciers and the term 'fluvioglacial' is used to describe the processes and effects of this melt water. 

If deposition by melt water occurs within subglacial, englacial or supraglacial streams, this is known as ice-contact stratified drift. Where meltwater streams flowing beyond the margins of the glacier carry and deposit material, it is known as out wash.

Where does the melt water come from?
  • surface melting (this is the most important source) - e.g. effects of solar radiation
  • precipitation - entering the glacial system as runoff from the surrounding slopes
  • geothermal heating - causing basal melting as temperatures rise
  • mechanical heating - e.g. as a result of increased pressure (reaching pressure melting point) as a glacier passes over / round an obstacle.


There are two main drainage systems in glaciers:
i. supraglacial streams / englacial water near the surface of a glacier, these may feed subglacial streams where they flow down crevasses;

ii. subglacial water at the base (this is often important as it may be flowing at high pressures under the glacier, cutting its own channel in the bedrock of the valley floor). It is this water that helps to act as a lubricant facilitating the movement of a glacier.

Streams known as proglacial streams are also created in front of a glacier carrying meltwater away. Melt water discharge has a diurnal pattern, highest in the day when solar radiation increases melting and low at night. At times the sudden release of melt water stored within/on a glacier causes a glacial outburst. These are frequent in places such as Iceland where geothermal heat creates large amounts of melt water. The resulting large outbursts are known as j√∂kulhlaups. Landforms of fluvio-glacial deposition may be formed in ice-contact situations under or within the glacier or as a result of out wash ahead of the glacier snout.

GLACIAL & FLUVIOGLACIAL DEPOSITS AND THEIR RESULTING LANDFORMS









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