Glacier.
The Baltoro Glacier in
Icebergs breaking off glaciers at Cape York,
A glacier is a large, slow-moving mass of ice, formed from compacted layers of snow that slowly deforms and flows in response to gravity and high pressure. The word glacier comes from French via the Vulgar Latin glacia, and ultimately from Latin glaciesmeaning ice.
Glacier ice is the largest reservoir of fresh water on Earth, and second only to oceans as the largest reservoir of total water. Glaciers cover vast areas of the Polar Regions and are found in mountain of every continent except
Types of Glacier.
Main article: Glacier morphology
Mouth of the Schlatenkees Glacier near
There are two main types of glaciers: Alpine glaciers, which are a highland glacier that flow slowly down a valley in a mountainous region like a river of ice, and continental glaciers-ice sheets, which can cover larger areas. Most of the concepts in this article apply equally to alpine glaciers and continental glaciers. Glaciers are also categorized by thermal characteristics, climate setting, and behavior.
Ice sheets are the largest glaciers, enormous masses of ice that are not visibly affected by the landscape and that cover the entire surface beneath them, except possibly on the margins where they are thinnest. Antarctica and
The smallest, alpine glaciers form high on the mountain slopes and are niche, slope or cirque glaciers. As a mountain glacier increases in size it can begin to flow down valley, and are referred to as valley glaciers. Larger glaciers can cover an entire mountain, mountain chain or even a volcano; this type is known as an ice cap or ice field, such as the
Tidewater glaciers are glaciers that flow into the sea. As the ice reaches the sea pieces break off, orcalve, forming icebergs. Most tidewater glaciers calve above sea level, which often results in a tremendous splash as the iceberg strikes the water. If the water is deep, glaciers can calve underwater, causing the iceberg to suddenly explode up out of the water. The Hubbard Glacier is the longest tidewater glacier in
Anatomy.
The Upper Grindelwald Glacierand the Schreckhorn, in the Swiss Alps, showing accumulation and ablation zones
On the opposite end of the glacier, at its foot or terminal, is the deposition or ablation zone, where more ice is lost through melting than gained from snowfall and sediment is deposited. The place where the glacier thins to nothing is called the terminus. In terms of thermal characteristics a temperate glacier is at melting point throughout the year, from its surface to its base. The ice of a polar glacier is always below freezing point from the surface to its base, though the surface snow pack may seasonally experience melting. Sub-polar glaciers have both temperate and polar ice, depending on the depth beneath the surface and position along the length of the glacier.
Glaciers are broken into zones as well based on surface snow pack and melt conditions. The ablation zone is the region where there is a net loss in glacier mass, and bare glacier ice is exposed. The equilibrium line separates the ablation zone and the accumulation zone. At this altitude, the amount of new snow gained by accumulation is equal to the amount of ice lost through ablation. The accumulation zone is a region where snow pack or superimposed ice accumulation persists. A further zonation of the accumulation zone distinguishes the melt conditions that exist. The dry snow zone is a region where no melt occurs, even in the summer and the snow pack remains dry. The percolation zone is an area with some surface melt, and melt water percolating into the snow pack, often this zone is marked by refrozen ice lenses, glands, and layers. In this zone the snow pack does not reach the melting point throughout. Near the equilibrium line on some glaciers this leads to development of a superimposed ice zone is a zone where melt water refreezes as a cold layer in the glacier forming a continuous mass of ice. The wet snow zone is the region where all of the snow deposited since the end of the previous summer has been raised to 0°C.The upper part of a glacier that receives most of the snowfall is called the accumulation zone. In general, the glacier accumulation zone accounts for 60-70% of the glacier's surface area, more if the glacier calves icebergs. The depth of ice in the accumulation zone exerts a downward force sufficient to cause deep erosion of the rock in this area. After the glacier is gone, this often leaves a bowl or amphitheater-shaped isostatic depression ranging from large lake basins, the Great Lakes or
The "health" of a glacier is usually assessed by determining the glacier mass balance or observing terminus behavior. Healthy glaciers have large accumulation zones, more than 60% of their area snow-covered at the end of the melt season, and a terminus with vigorous flow.
In the aftermath of the Little Ice Age, around 1850, the glaciers of the Earth have retreated substantially through the 1940s (see Retreat of glaciers since 1850). A slight cooling led to the advance of many alpine glaciers from 1950-1985. However, since 1985 glacier retreat and mass balance loss has become increasingly ubiquitous and large.
Motion.
Nadelhorn Glacier above Saas-Fee,
Main article: Ice sheet dynamics
Glaciers have a tendency to move, or "flow", downhill. While the bulk of a glacier flows in the direction of lower elevation, every point of the glacier can move at a different rate, and in a different direction. The general motion is due to the force of gravity, and the rate of flow at each point on the glacier is affected by many factors.
Ice behaves like an easily breaking solid until its thickness exceeds about 50 meters (160 ft). The pressure on ice deeper than that depth causes plastic flow. The glacial ice is made up of layers of molecules stacked on top of each other, with relatively weak bonds between the layers. When the stress of the layer above exceeds the inter-layer binding strength, it moves faster than the layer below.
Another type of movement is basal sliding. In this process, the whole glacier moves over the terrain on which it sits, lubricated by melt water. As the pressure increases toward the base of the glacier, the melting point of water decreases, and the ice melts. Friction between ice and rock and geothermal heat from the Earth's interior also contribute to thawing. This type of movement is dominant in temperate glaciers. The geothermal heat flux becomes more important the thicker a glacier becomes.
Fracture zone and cracks
Ice cracks in the Titlis Glacier
Signs warning of the hazards of a glacier in
The top 50 meters of the glacier, being under less pressure, are more rigid; this section is known as the fracture zone, and mostly moves as a single unit, over the plastic-like flow of the lower section. When the glacier moves through irregular terrain, cracks up to 50 meters deep form in the fracture zone. The lower layers of glacial ice flow and deform plastically under the pressure, allowing the glacier as a whole to move slowly like a viscous fluid. Glaciers flow down slope, usually this reflects the slope of their base, but it may reflect the surface slope instead. Thus, a glacier can flow rises in terrain at their base. The upper layers of glaciers are more brittle, and often form deep cracks known as crevasses. The presence of crevasses is a sure sign of a glacier. Moving ice-snow of a glacier is often separated from a mountain side or snow-ice that is stationary and clinging to that mountain side by a bergshrund. This looks like a crevasse but is at the margin of the glacier and is a singular feature.
Crevasses form due to differences in glacier velocity. As the parts move at different speeds and directions, shear forces cause the two sections to break apart opening the crack of a crevasse all along the disconnecting faces. Hence, the distance between the two separated parts while touching and rubbing deep down, frequently widens significantly towards the surface layers, many times creating a wide chasm. Crevasses seldom are more than 150 feet deep. Beneath this point the plastic deformation of the ice under pressure is too great for the differential motion to generate cracks. Transverse crevasses are transverse to flow, as a glacier accelerates where the slope steepens. Longitudinal crevasses form semi-parallel to flow where a glacier expands laterally. Marginal crevasses form from the edge of the glacier, due to the reduction in speed caused by friction of the valley walls. Marginal crevasses are usually largely transverse to flow.
Crossing a crevasse on the
Crevasses make travel over glaciers hazardous. Subsequent heavy snow may form fragile snow bridges, increasing the danger by hiding their presence at the surface. Below the equilibrium line glacier melt water is concentrated in stream channels. The melt water can pool in a proglacial lake, a lake on top of the glacier, or can descend into the depths of the glacier via moulins). Within or beneath the glacier the stream will flow in an englacial or sub-glacial tunnel. Sometimes these tunnels reemerge at the surface of the glacier.
Speed
The speed of glacial displacement is partly determined by friction. Friction makes the ice at the bottom of the glacier move more slowly than the upper portion. In alpine glaciers, friction is also generated at the valley's side walls, which slows the edges relative to the center. This was confirmed by experiments in the 19th century, in which stakes were planted in a line across an alpine glacier, and as time passed, those in the center moved farther.
Mean speeds vary from stagnant areas with no motion where trees can establish themselves on surface sediment deposits such as in
A few glaciers have periods of very rapid advancement called surges. These glaciers exhibit normal movement until suddenly they accelerate then return to their previous state. During these surges, the glacier may reach velocities far greater than normal speed. These surges may be caused by failure of the underlying bedrock, the ponding of melt water at the base of the glacier — perhaps delivered from a supraglacial lake — or the simple accumulation of mass beyond a critical "tipping point".
Ogives
Ogives are alternating dark and light bands of ice occurring as narrow wave crests and wave valleys on glacier surfaces. They only occur below icefalls but not all icefalls have ogives below them. Once formed, they bend progressively down glacier due to the increased velocity toward the glacier's centerline. Ogives are linked to seasonal motion of the glacier as the width of one dark and one light band generally equals the annual movement of the glacier. The ridges and valleys are formed because ice from an icefall is severely broken up thereby increasing ablation surface area during the summertime creating a swale and creating space for snow accumulation in the winter creating a ridge. Sometimes ogives are described as either wave ogives or band ogives in which they are solely undulations or varying color bands respectively.
Occurrence.
Black glacier in Aconcagua vicinity,
Glaciers occur on every continent and in approximately 47 of the world's countries. Extensive glaciers are found in Antarctica,
Permanent snow cover is affected by factors such as the degree of slope on the land, amount of snowfall and the force and nature of the winds. As temperature decreases with altitude, high mountains — even those near the Equator — have permanent snow cover on their upper portions, above the snow line. Examples include Mount Kilimanjaro and the Tropical Andes in South America; however, the only snow to occur exactly on the Equator is at 4,690 m (15,387 ft) on the southern slope of Volcán Cayambe in
Conversely, areas of the Arctic such as Banks Island and Antarctic,
Glaciers on Mars
Northern polar icecap on Mars
Elsewhere in the solar system, the vast polar ice caps of Mars rival those of the Earth and show glacial features. Especially the south polar cap is compared to glaciers on Earth. Other glacial features on Mars are glacial debris aprons and the lineated valley fills of thefretted terrain in northern Arabia Terra. Topographical features and computer models indicate the existence of more glaciers in Mars' past.
Glacial Geology.
Rocks and sediments are added to glaciers through various processes. Glaciers erode the terrain principally through two methods: abrasion and plucking.
Diagram of glacial plucking and abrasion
As the glacier flows over the bedrock's fractured surface, it softens and lifts blocks of rock that are brought into the ice. This process is known as plucking, and it is produced when sub glacial water penetrates the fractures and the subsequent freezing expansion separates them from the bedrock. When the ice expands, it acts as a lever that loosens the rock by lifting it. This way, sediments of all sizes become part of the glacier's load. The rocks frozen into the bottom of the ice then act like grit in sandpaper.
Abrasion occurs when the ice and the load of rock fragments slide over the bedrock and function as sandpaper that smooths and polishes the surface situated below. This pulverized rock is called rock flour. This flour is formed by rock grains of a size between 0.002 and 0.00625 mm. Sometimes the amount of rock flour produced is so high that currents of melt waters acquire a grayish color. These processes of erosion lead to steeper valley walls and mountains slopes in alpine settings, which then can cause avalanches and rock slides which further add material to the glacier.
A visible characteristics of glacial abrasion are glacial striations. These are produced when the bottom's ice contains large chunks of rock that mark scratches in the bedrock. By mapping the direction of the flutes the direction of the glacier's movement can be determined. Chatter marks are seen as lines of roughly crescent shape depressions in the rock underlying a glacier caused by the abrasion where a boulder in the ice catches and is then released repetitively as the glacier drags it over the underlying basal rock.
The rate of glacier erosion is variable. The differential erosion undertaken by the ice is controlled by six important factors:
§ Velocity of glacial movement
§ Thickness of the ice
§ Shape, abundance and hardness of rock fragments contained in the ice at the bottom of the glacier
§ Relative ease of erosion of the surface under the glacier.
§ Thermal conditions at the glacier base.
§ Permeability and water pressure at the glacier base.
Material that becomes incorporated in a glacier is typically carried as far as the zone of ablation before being deposited. Glacial deposits are of two distinct types:
§ Glacial till: material directly deposited from glacial ice. Till includes a mixture of undifferentiated material ranging from clay size to boulders, the usual composition of a moraine.
§ Fluvial and outwash: sediments deposited by water. These deposits are stratified through various processes, such as boulders being separated from finer particles.
The larger pieces of rock which are encrusted in till or deposited on the surface are called glacial erratics. They may range in size from pebbles to boulders, but as they may be moved great distances they may be of drastically different type than the material upon which they are found. Patterns of glacial erratics provide clues of past glacial motions.
Moraines
Glacial moraines are formed by the deposition of material from a glacier and are exposed after the glacier has retreated. These features usually appear as linear mounds of till, a non-sorted mixture of rock, gravel and boulders within a matrix of a fine powdery material. Terminal or end moraines are formed at the foot or terminal end of a glacier. Lateral moraines are formed on the sides of the glacier. Medial moraines are formed when two different glaciers, flowing in the same direction, coalesce and the lateral moraines of each combine to form a moraine in the middle of the merged glacier. Less apparent is the ground moraine, also called glacial drift, which often blankets the surface underneath much of the glacier downslope from the equilibrium line. Glacial melt waters contain rock flour, an extremely fine powder ground from the underlying rock by the glacier's movement. Other features formed by glacial deposition include long snake-like ridges formed by streambeds under glaciers, known as eskers, and distinctive streamlined hills, known as drumlins.
Stoss-and-lee erosional features are formed by glaciers and show the direction of their movement. Long linear rock scratches (that follow the glacier's direction of movement) are called glacial striations, and divots in the rock are called chatter marks. Both of these features are left on the surfaces of stationary rock that were once under a glacier and were formed when loose rocks and boulders in the ice were transported over the rock surface. Transport of fine-grained material within a glacier can smooth or polish the surface of rocks, leading to glacial polish. Glacial erratics are rounded boulders that were left by a melting glacier and are often seen perched precariously on exposed rock faces after glacial retreat.
The term moraine is of French origin, and it was coined by peasants to describe alluvial embankments and rims found near the margins of glaciers in the French Alps. In modern geology, the term is used more broadly, and is applied to a series of formations, all of which are composed of till.
Drumlins
A drumlin field forms after a glacier has modified the landscape. The teardrop-shaped formations denote the direction of the ice flow.
Drumlins are asymmetrical, canoe shaped hills with aerodynamic profiles made mainly of till. Their heights vary from 15 to 50 meters and they can reach a kilometer in length. The tilted side of the hill looks toward the direction from which the ice advanced (stoss), while the longer slope follows the ice's direction of movement (lee).
Drumlins are found in groups called drumlin fields ordrumlin camps. An example of these fields is found east of
Although the process that forms drumlins is not fully understood, it can be inferred from their shape that they are products of the plastic deformation zone of ancient glaciers. It is believed that many drumlins were formed when glaciers advanced over and altered the deposits of earlier glaciers.
Glacial valleys
A glaciated valley in the Mount Baker-Snoqualmie National Forest showing the characteristic U-shape and flat bottom.
This image shows the termini of the glaciers in the Bhutan Himalaya. Glacial lakes have been rapidly forming on the surface of the debris-covered glaciers in this region during the last few decades.
Before glaciation, mountain valleys have a characteristic "V" shape, produced by downward erosion by water. However, during glaciation, these valleys widen and deepen, forming a "U"-shaped glacial valley. Besides the deepening and widening of the valley, the glacier also smooths the valley due to erosion. In this way, it eliminates the spurs of earth that extend across the valley. Because of this interaction, triangular cliffs called truncated spurs are formed.
Many glaciers deepen their valleys more than their smaller tributaries. Therefore, when the glaciers recede from the region, the valleys of the tributary glaciers remain above the main glacier's depression, and these are called hanging valleys.
In parts of the soil that were affected by abrasion and plucking, the depressions left can be filled by lakes, called paternoster lakes.
At the 'start' of a classic valley glacier is the cirque, which has a bowl shape with escarped walls on three sides, but open on the side that descends into the valley. In the cirque, an accumulation of ice is formed. These begin as irregularities on the side of the mountain, which are later augmented in size by the coining of the ice. Once the glacier melts, these corries are usually occupied by small mountain lakes called tarns.
There may be two glacial cirques 'back to back' which erode deep into their back walls until only a narrow ridge, called an arête is left. This structure may result in a mountain pass.
Glaciers are also responsible for the creation of fjords (deep coves or inlets) and escarpments that are found at high latitudes.
Features of a glacial landscape
Arêtes and horns (pyramid peak)
An arête is a narrow crest with a sharp edge. The meeting of three or more arêtes creates pointed pyramidal peaks and in extremely steep-sided forms these are called horns.
Both features may have the same process behind their formation: the enlargement of cirques from glacial plucking and the action of the ice. Horns are formed by cirques that encircle a single mountain.
Arêtes emerge in a similar manner; the only difference is that the cirques are not located in a circle, but rather on opposite sides along a divide. Arêtes can also be produced by the collision of two parallel glaciers. In this case, the glacial tongues cut the divides down to size through erosion, and polish the adjacent valleys.
Roche moutonnée
Some rock formations in the path of a glacier are sculpted into small hills with a shape known as roche moutonnée or sheepback rock. An elongated, rounded, asymmetrical, bedrock knob can be produced by glacier erosion. It has a gentle slope on its up-glacier side and a steep to vertical face on the down-glacier side. The glacier abrades the smooth slope that it flows along, while rock is torn loose from the downstream side and carried away in ice, a process known as 'plucking'. Rock on this side is fractured by a combination of various forces, such as water, ice in rock cracks, and structural stresses.
Alluvial stratification
The water that rises from the ablation zone moves away from the glacier and carries with it fine eroded sediments. As the speed of the water decreases, so does its capacity to carry objects in suspension. The water then gradually deposits the sediment as it runs, creating an alluvial plain. When this phenomenon occurs in a valley, it is called a valley train. When the deposition is to an estuary, the sediments are known as "bay mud".
Landscape produced by a receding glacier
Outwash plains and valley trains are usually accompanied by basins known as kettles. These are glacial depressions which are produced when large ice blocks are stuck in the glacial alluvium and after melting, they leave holes in the sediment. The diameter of these depressions ranges from 5 m to 13 km, with depths of up to 45 meters. Most are circular in shape due to the melting blocks of ice becoming rounded. Lakes often form in these depressions and these are known as kettle lakes.
Deposits in contact with ice
When a glacier reduces in size to a critical point, its flow stops, and the ice becomes stationary. Meanwhile, melt water flows over, within, and beneath the ice leave stratified alluvial deposits. Because of this, as the ice melts, it leaves stratified deposits in the form of columns, terraces and clusters. These types of deposits are known as deposits in contact with ice.
When those deposits take the form of columns of tipped sides or mounds, which are called kames. Some kames form when melt water deposits sediments through openings in the interior of the ice. In other cases, they are just the result of fans or deltas towards the exterior of the ice produced by melt water.
When the glacial ice occupies a valley it can form terraces or kame along the sides of the valley.
A third type of deposit formed in contact with the ice is characterized by long, narrow sinuous crests composed fundamentally of sand and gravel deposited by streams of melt water flowing within, or beneath the glacier. After the ice has melted these linear ridges or eskers remain as landscape features. Some of these crests have heights exceeding 100 meters and their lengths surpass 100 km.
Loess deposits
Very fine glacial sediments or rock flour is often picked up by wind blowing over the bare surface and may be deposited great distances from the original fluvial deposition site. These eolian loess deposits may be very deep, even hundreds of meters, as in areas of
Transportation & Erosion.
§ Entrainment is the picking up of loose material by the glacier from along the bed and valley sides. Entrainment can happen by regelation or by the ice simply picking up the debris.
§ Basal Ice Freezing is thought to be to be made by glaciohydraulic super cooling, though some studies show that even where physical conditions allow it to occur, the process may not be responsible for observed sequences of basal ice.
§ Plucking is the process involves the glacier freezing onto the valley sides and subsequent ice movement pulling away masses of rock. As the bedrock is greater in strength than the glacier, only previously loosened material can be removed. It can be loosened by local pressure and temperature, water and pressure release of the rock itself.
§ Supraglacial debris is carried on the surface of the glacier as lateral and medial moraines. In summer ablation, surface melt water carries a small load and this often disappears down crevasses.
§ Englacial debris is moraine carried within the body of the glacier.
§ Subglacial debris is moved along the floor of the valley either by the ice as ground moraine or by meltwater streams formed by pressure melting.
Deposition.
§ Lodgement till is identical to ground moraine. It is material that is smeared on to the valley floor when its weight becomes too great to be moved by the glacier.
§ Ablation till is a combination of englacial and supraglacial moraine It is released as a stationary glacier begins to melt and material is dropped in situ.
§ Dumping is when a glacier moves material to its outermost or lowermost end and dumps it.
§ Deformation flow is the change of shape of the rock and land due to the glacier.
Isotatic Rebound.
Main article: Isostatic rebound
Isostatic pressure by a glacier on the Earth's crust
This rise of a part of the crustis due to anisostatic adjustment. A large mass, such as an ice sheet/glacier, depresses the crust of the Earth and displaces the mantle below. The depression is about a third the thickness of the ice sheet. After the glacier melts the mantle begins to flow back to its original position pushing the crust back to its original position. This post-glacial rebound, which lags melting of the ice sheet/glacier, is currently occurring in measurable amounts in Scandinavia and the Great Lakes region of
An interesting geomorphological feature created by the same process, but on a smaller scale, is known as dilation-faulting. It occurs within rock where previously compressed rock is allowed to return to its original shape, but more rapidly than can be maintained without faulting, leading to an effect similar to that which would be seen if the rock were hit by a large hammer. This can be observed in recently de-glaciated parts of
