Introduction to Glaciers

Within this section you will be able to learn what glaciers are, where they are found, how they form, and how they move. Don't get cold feet now... select one of the topics on the left and see just what these icy masses are all about!


What Glaciers Are

There are three main criteria for being a glacier. All of the different types of glaciers meet these criteria, though they may vary in other characteristics.

Made of Ice

Glaciers consist of ice crystals, air, water, and rock debris. Of course ice crystals are obviously the fundamental component. Two characteristics of ice crystals are particularly significant. They are the structure and density of ice crystals.


Ice crystals are weak and can easily be made to slip on planes parallel to the basal plane. Fundamentally, it is this weakness which allows glaciers to deform readily under their own weight and thus to flow. Other processes are also involved in glacier movement. See "How Glaciers Move"


A very unusual characteristic is that water is a substance that is less dense in its solid form than in its liquid form. This means that water may exist at the bottom of suitable glaciers and that in certain situations glaciers may float. This may not seem significant at first, but imagine a world in which glacier ice is denser than water! Things would be very different indeed!

Form on Land

All glaciers must originally form on land, although subsequently they may go to sea. Glaciers may extend out into the water, such as the glaciers of Glacier Bay National Park and Preserve, but they cannot initially form over water. Hence, the vast expanses of sea ice covering the Arctic ocean are not considered glaciers.

Glaciers Move

The final criterion that needs to be met is that a glacier must move. Glaciers move by internal deformation and basal sliding. Ice masses flow across flat terrain as well as down slopes. Only after an ice mass has attained a size, thickness, and configuration that produces enough stress to cause it to flow as a solid or to slip over its base is it considered a glacier. Patches of perennial snow and ice that show no signs of motion are not glaciers.


Where Glaciers Are Found

Glaciers are located wherever topographic and climatic factors are suitable for snow to collect and survive. Some of the variables are precipitation, temperature, latitude, altitude, relief, and aspect.


Precipitation is a potent control over glaciation. Total annual precipitation is a poor guide to the occurrence of glaciers; it is the kind of precipitation that is important. A ready supply of snow is needed.

Many locations on Earth receive huge amounts of snow each year but have no glaciers; other places with light snowfalls have massive glaciers. The amount of snow that falls on the surface is not as critical as the conservation of snow. Some snow must be saved, year after year after year, if a glacier is to form and survive.


A glacier's surface receives heat from a variety of sources, but solar radiation has the most significant influence on ablation. Today's glaciers exist in areas that receive widely differing amounts of solar energy, so from a glacier's point of view, the mean summer temperature is of the greatest importance. In short, the snow that falls needs to stick around, and chances are improved if summers are cool.


High latitudes receive less annual solar radiation and experience prolonged winters at sub-zero temperatures. It is no surprise, then, that glaciers prefer polar regions. In our present plate-tectonic configuration, the South Pole landmass is ideal for glaciers. There isn't currently an ice cap at the North Pole-although it is covered with sea ice-because there isn't a landmass to host one.


Glaciers are found at sea level at the South Pole, but in the mid and low latitudes, glaciers are related to highlands. Alpine regions are prime candidates for glacier formation.


Topography exerts a powerful influence over glaciation. Is there enough land surface to support a glacier? Is the land surface too steep? Is the land surface jagged or do hollows exist that can trap snow?


The orientation of the ground surface with respect to incoming solar radiation is important for the existence of glaciers, particularly at a local scale. In an area that is at the margin of glaciation, slope orientation can mean the difference between a glacier's presence or not. The proper slope aspect lowers snowline. Steep north-facing slopes receive the least direct radiation in the northern hemisphere, but slopes facing east of north are the coolest. This, combined with the fact that northeast facing slopes are the leeside slopes in areas of prevailing southwesterly winds, explains why the cirque glaciers in many upland areas are preferentially orientated towards the northeast.


Types of Glaciers

There are many different types of glaciers and many highly detailed classification systems, but glaciers can most easily be differentiated on the basis of topography and temperature. Glaciers are either unconstrained by topography, or the topography actually constrains the glacier. Select a type of glacier below to learn more about it.


These glaciers cover vast areas. Topography does not play a major role in the extent of these glaciers.

Ice sheets and ice caps

Ice sheets and ice caps fall into the same category. The difference between them is one of scale. Ice sheets are larger. Typically the dividing line is around 50,000 km2. The glaciers that cover Antarctica and Greenland are ice sheets, and the glacier that covers Iceland is an ice cap. Two main components of ice sheets and ice caps are ice domes and outlet glaciers.

Ice shelves

An ice shelf is a very thick sheet of ice that has been shoved out over the sea floor from a land-based glacier. It is still attached to land on one side but most of it is afloat. Massive icebergs (glossary) may calve off of ice shelves.


These are the glaciers that are found in rugged topography and are typically bound within a valley or depression.


An icefield is an extensive area of land ice covering a mountain region; its surface is approximately level and can be distinguished from an ice cap because it does not achieve the characteristic domelike shape, and because flow is strongly influenced by the underlying topography.

Valley glaciers

A valley glacier flows between the walls of a mountain valley in all or part of its length. It may originate in an icefield or a cirque.

Cirque glaciers

A cirque glacier is a small ice mass generally wide in relation to its length and characteristically occupying an armchair-shaped bedrock hollow. It is the most common type of glacier in the mountains of the western United States.

Other small glaciers

This category includes a wide variety of glaciers whose forms are closely controlled by the underlying topography. The permutations are almost limitless. Typically small glaciers are found in hollows or slight depressions in mountainous terrain or bordering coastlines.


How Glaciers Form

Newly fallen snow is porous and not very dense. Air easily penetrates the pore spaces, and the delicate points of each snowflake gradually evaporate. The resulting water vapor condenses, mainly in constricted places near a snowflake’s center. In this way, the fragile ice crystals slowly become smaller, rounder, and denser, and the pore spaces between them disappear.

Snow that survives a year or more gradually becomes denser and denser. The transitional phase between snow and glacier ice is a loose, porous aggregate of small ice grains called firn. When firn is no longer permeable to air, it becomes glacier ice. Eventually glacier ice will grow in grain size under increasing pressure at the base of a glacier.


Parts of a Glacier

The parts of a glacier are tied to its glacial budget. Yes, glaciers believe wholeheartedly in balanced budgets, unlike many modern institutions. In the case of a glacier, income is snow, and being "in the red" is contrary to survival. Expenditures equate to the loss of snow (and the ice made from snow) which are disposed of through ablation. Select a yellow dot below to learn about the different parts that make up a glacier.

The Terminus

The terminus is the end or lowest part of a glacier. It is also know as the foot, the nose, and even the snout of a glacier. Substantial melting occurs here, as well as being the breaking-off point for icebergs.

Ablation Zone

That part of a glacier's surface over which ablation (wastage) exceeds accumulation each year.

Firn Line

The highest level to which the winter snow cover retreats on a glacier. For some temperate/warm glaciers, it is nearly coincident with the equilibrium line.

Accumulation Zone

The part of a glacier's surface over which more snow is deposited than ablated each year.

Equilibrium Line

The boundary between areas of gain and loss on a glacier's surface during one year. It is where accumulation equals ablation, and the net balance is zero.


The ground upon which a glacier rests, or, perhaps more appropriately, the ground over which a glacier flows is called the bed.

A glacier’s bed is referred to throughout this knowledge center. Although it is not part of a glacier per se, the direct interaction between a glacier and its bed warrants mention.


How Glaciers Move

Most glaciers move too slowly for us to see, and different parts move at different rates. Studies and monitoring have revealed, however, that surging glaciers are fairly common. Some 204 glaciers have been identified in western North America alone.

Typically, a glacier’s surface is far from being a featureless white expanse of snow, although this is sometimes the case. There are many surface features related to the movement of glaciers. Select one of the experiments, features, or behaviors below to learn more about how glaciers move.


Run three different experiments to learn how the mass of a glacier actually flows.

A View from Above

Suppose stones are placed in a straight line across the surface of a valley glacier. In a year or two, the line would no longer be straight; it would be displaced and bent down-valley. This elementary study demonstrates that the center of a glacier, where the ice is the thickest, moves more rapidly than at the edges.

A Sliced View

Suppose a bendable pipe is inserted into a deep, narrow hole that was drilled vertically through a glacier from top to bottom. In a year or two, the inclination and position of the pipe would change. The pipe would be bent into a curve by greater movement at the top of the glacier. In addition to the pipe bending, it would also have moved down-valley, revealing that the glacier slipped over its bed. This experiment demonstrates the two dominant types of glacial movement:

Mysterious Movement

Suppose a child broke his or her mother’s favorite vase while playing baseball indoors and foolishly wished to dispose of the pieces in a glacier. He or she would eventually realize his/her error of not taking into consideration the vertical component of glacial movement. If the vase is buried in the accumulation zone, it will ultimately emerge in the ablation zone. Eventually their mother would find the evidence. The basic pattern of flow in the accumulation zone is downward, but changes to upward in the ablation zone. Such variations in ice movement are associated with two different types of flow regimes within a glacier:

Extending Flow
A glacier is extended and thinned where flow is accelerating.

Compressive Flow
A glacier is compressed and thickened where flow is decreasing in velocity.


Explore some of the different features associated with the flowing of glacial mass.


A bergschrund is a deep and often wide gap or crevasse, or series of closely spaced crevasses, in ice or firn at or near the head of a valley glacier. A bergschrund separates the moving ice and snow from the relatively immobile ice and snow adhering to the headwall of a valley (or cirque).


A crevasse is a deep (some as much as 100 meters deep), nearly vertical fissure, crack, or rift in a glacier. Crevasses are caused by stresses resulting from differential movement over the uneven surface underlying a glacier. Crevasses may be concealed by snowbridges, and are, therefore, hazards when traversing a glacier.


One of the striking features of many glaciers is the banding in ice that occurs in the ablation zone. Foliation consists of alternating layers of white bubbly ice and bluish ice. Layers vary from several millimeters to several meters in width. It is generally accepted that foliation is related to glacier movement, and that it is best displayed on those glaciers that have undergone considerable deformation, as for example at the foot of an icefall.

Ice Falls

Icefalls occur on the parts of a glacier that flow over convex bedrock surfaces, and are thereby very steep. Icefalls are highly crevassed. Extending flow predominates over icefalls.

Ice Streams

Ice streams are currents of ice in an ice sheet or ice cap that flow more rapidly than the surrounding ice. Ice streams are usually flowing to an ocean or an ice shelf and not constrained by exposed rock.


Ogives are broad banded surface patterns that generally curve down-glacier as a result of faster ice movement toward the center of a glacier. Ogives are common below icefalls.


Take a look at the different ways in which a glacier is classified based on its movement.


A stable glacier is still a moving body of ice; it just isn’t increasing in size of changing location. A stable glacier occupies about the same position year-after-year, except for minor seasonal fluctuations. In a stable state, the rate of a glacier’s forward movement is balanced by recession of the ice edge through melting. It is neither advancing nor retreating. In short, ablation equals accumulation.


In order for a glacier to be considered a glacier, it must be moving. “Moving” and “advancing” are not the same when used to describe glaciers. By definition, a glacier is always moving, but it may not be advancing. Advancement in this situation has more to do with a glacier’s mass balance than a glacier’s motion. In short, accumulation exceeds ablation.

Retreating / Receding

During extended warm periods of a year or more, glaciers will retreat/recede if the amount of snow and ice that is lost through melting—and to a lesser extent through evaporation, sublimation, wind erosion, and calving—is greater than the amount of snow that is gained through precipitation or wind deposition. In short, ablation exceeds accumulation.


During the last century and a half, many coastal Alaskan glaciers have receded at rates far in excess of rates of retreat on land. Their dramatic recession is due to frontal calving, in which icebergs progressively break off from the front of glaciers that terminate in deep water. Although the base of such a glacier may lie far below sea level along much of its length, its terminus can remain stable as long as it is grounded, say against a shallow submarine ridge, such as a moraine shoal. Once the glacier retreats off the shoal, however, water will replace the space that had been occupied by ice. With the glacier now terminating in water, conditions are ripe for calving.


When a glacier surges, you can actually see it move or build up at its terminus. A surge of ice may locally increase velocities by as much as 10 to 100 times the normal velocity. Rates of five meters per hour have been recorded. Surging is initiated when a threshold of instability is reached and ice in the upper ablation zone begins to move rapidly down-glacier.

Surging is still one of the least understood aspects of glacier movement; it is necessary to explain both the mechanism that triggers a surge and the high flow velocity subsequently attained.



Caves and Karst Index



Threats to Caves and Karst


Caves and Karst in National Parks


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