The Dynamics of Glaciers

Glacier formation

Most ice is stationary; why and how do these rivers of ice move? Before we find out how glaciers move, we need to know how they are formed.

Glaciers are born in small hollows above the snow line. This allows the snow to fall without melting in the summer time. The snow line in Antarctica is at sea level, while in Africa at the temperate latitudes it is at 17,000 feet. The snowflakes fall. Their extremities melt or evaporate, i.e. sublimate. They form into roughly spherical shape. If they survive the summer melt they are referred to as firn, German ‘of last year’. Firn are the building blocks of glaciers. They are less than one tenth of an inch in diameter. When packed together by succeeding snowfall, they turn from the white of snow to the blue of ice as they squeeze out the air. Additionally meltwater from the top will refreeze into the crystals creating larger more compact crystals.

“Snow accumulates in small hollows on mountainsides, and over many years it changes into ice by recrystallization—refreezing of the meltwater and compression; eventually the ice may become thick enough to move because of its own weight.” (Glacier p48)

 

How long does this process take? It depends upon temperature and rate of snowfall. In Iceland with plenty of snow and warmer summers to produce the necessary meltwater, glaciers can be formed in 10 years. In Antarctica where there is little snow and little warmth to produce the necessary meltwater, the glaciers can be centuries in forming.

Glacial Motion

To be called a glacier, the ice must move under its own weight. When does it begin to move? When does it cease being common ice pack and commence being a glacier? It must reach a critical thickness when it can overcome the internal strength of the ice and the friction between the ground and the ice. That critical thickness is about 60 feet. Now that it has reached the critical thickness, how does it move?

Let us think a little about the huge ice mass of a glacier. In the winter it snows. This fluffy snow turns into condensed ice, the firn, of the glacier. When it freezes, it expands into the surrounding area. In the summer when it thaws, the interior of the glacier is too thick to be thawed but the exterior begins to melt. Now at each exterior spot a 9% reduction by volume occurs, shrinking this huge block of ice ever so infinitesimally. But this is occurring all over the glacier. So now the entire surface of the glacier is covered with just a thin layer of water. If you’ve ever held onto a wet baby, a melting ice cube, driven on an ice covered road with a little water on it, paid attention to the signs that said ‘Slippery When Wet’, you will know that our glacier has just become very slippery. Further more, in the lower latitudes especially, most glaciers form on mountain-tops. They are formed very high up. Hence when gravity pulls them down they have somewhere to go. Thus you have this slippery thing, which weighs many tons, being pulled down hill. This method of moving is called basal sliding.

Basal sliding will not occur unless the surface melting extends to the bottom of the glacier. If the ground is too cold and the bottom won’t melt, then the glacier won’t slide along. The thin layer of water acts a lubricant, as we would expect. For one it releases pressure on the sides by reducing volume. And two, the liquid form of water readily flows, pulling everything with it as does a stream. Finally the flowing of water and ice generates a heat, which melts more of the ice, shrinking the volume more, adding more lubricant, adding to the cycle. But if the bottom is frozen there can be no basal sliding. Try sliding down a hot slide in your bare skin without sending water down first and you will understand. No matter how much water there is going down the sides it will in no way help you slide. The water has to be under you, as every slider knows.

At the polar regions, glaciers don’t move by sliding. They move, however, by internal deformation. Basal sliding and internal deformation are the two ways by which glaciers move. Most glaciers do a combination of the two, but the polar ground never gets warm enough to melt the bottom of the glacier, hence they only travel by internal deformation.

Another name for internal deformation is creep. Initially it was thought that glaciers moved like a liquid, but there were a few discrepancies. Then it was found that metals will move, when close to their melting point. They will also shatter with a sharp blow, while liquids won’t shatter. Glaciers also shatter, as we shall see. The solid approaches dissolution into liquid form, weakening its bonds, but not quite enough to liquefy. They called this movement by pre-liquid metals creep. Hence glaciers move by sliding and creeping or a combination of the two.

Creeping or internal deformation occurs when the glacier becomes so heavy that it flattens its molecules out in order to deal with the weight.

“The weight of the ice itself provides the stress that causes glacier ice to creep. Under stupendous pressure, the crystals of the ice rearrange themselves in layers of atoms more or less parallel to the surface of the glacier. These layers then begin gliding over one another. The incremental movement of the layers of atoms within each crystal—plus some slippage between crystals, a phenomenon not yet fully understood— constitutes internal deformation.” (Glacier p49)

 

Hence the ice crystals are hanging out without any purpose to their puny existence. Then the orders come down from above. “Time to organize guys.” The crystals organize their atoms. They flatten out. Then called by the force of gravity, they begin sliding over each other inexorably downhill. What began as an innocent, light, airy snowflake, once organized, becomes one of the most devastating influences upon the earth.

These moving Glaciers grab up debris, when they partially liquefy and then freeze. The water sinks into the cracks and then freezes, further expanding the cracks and also seizing onto what it had frozen into. When the block of ice begins moving the debris is pulled along. This thawing and freezing, which allows it to grab onto things also allows it to move around obstacles, always seeking the lowest ground. When an obstacle is encountered, the area of the glacier partially melts because of the friction, slides around and then reforms on the other side. The scientists call this regelation. This process further reduces the friction. Thus when glaciers are sliding, it will not pick up what it is sliding over, but will slide right over small enough obstacles. But when it stops sliding, perhaps because of the winter freeze, and the ground freezes up, it will grab up the same debris that it passed over when it was sliding. This is the fine sand paper of the glacier.

Glacial Velocity

Most glaciers move quite slowly. There are three elements that influence glacier speed: 1) the thickness of the glacier, 2) the slope, and 3) the temperature of the glacier. The thicker the glacier, the steeper the slope, and the warmer the glacial ice, the quicker it moves. If the glacier is too thin, on too shallow of a slope, or too cold, it will stop. An increase of only a few percent in thickness can increase speed up to 20 percent.

The glaciers in the warmer regions, relying primarily on basal sliding, move more quickly than the glaciers in the frozen regions, which mainly creep because it never gets warm enough to melt the ice. One warm climate glacier moves a yard a day or a fifth of a mile per year. A cold climate glacier in Antarctica moves only a quarter inch a day, a yard every three months, or a fifth of a mile in 90 years. In this case the warm glacier moves 100 times faster than the cold glacier.

The last factor influencing the speed of the glacier is the slope of the geography it is based upon. If there is a moderate slope, it will be pulled downhill gradually like a river. If it is too steep, then it will crack and break off. This was one of the major pieces of evidence that convinced scientists that glaciers are solids that move rather than an extremely viscous fluid. Fluids never crack.

The Eastern Sierras have an extremely sharp slope. Hence the glacier would crack off before it could get to the Owens Valley. On the Western slope, which is much more shallow, the glaciers were able to move much farther down the side of the mountain into the valleys. On the west we see evidence of glacial rivers. On the east we mainly see evidence of glacial erosion.

The glacier cracking is an example of getting too far ahead of yourself. The glaciers that move a little more slowly keep together and generate more continuous power, while the glaciers that move too quickly dissipate their energy because their forward energy disconnects. The Tai Chi metaphor or life metaphor is obvious. Moving too quickly dis-integrates. Don’t get ahead of yourself.

If there is no slope, glaciers can still move, but it is because they create their own altitude. The ice sheet will keep moving as long as some part of the glacier is taller than other parts.

Glacier speed is not constant across the glacier. The top half of the glacier moves faster than the bottom, presumably because of friction. The sides also flow slower than the middle, also because of friction. The middle of the glacier at the equilibrium line flows fastest, the head and terminus flow slower. Above the equilibrium line the glacier grows faster than it melts; this is called the accumulation zone. Below the equilibrium line, the glacier melts faster than it grows; this is called the ablation zone. The season of the year also has an effect on the glacier speed. In summer the glacier might flow up to 20% faster than in winter, because of sliding.

Ice in the accumulation zone flows downward relative to the surface of the glacier, from accumulated snowfall, while ice in the ablation zone flows upward. Hence the ice at the top of the snout, the terminus, is the oldest ice in the glacier. Remember even when a glacier is retreating, the ice is still flowing downhill. It’s just melting faster than it can replenish itself.

Because of all these diverse velocities within the glacier itself, stress fractures set up to equalize pressure. These are invisible until they suddenly open up as a crevasse. These instant crevasses are why glaciers are so dangerous to climb. Because the stresses are minute and constantly forming, even a sharp sound can trigger the creation of a crevasse. If the land under a glacier is very steep then a transverse crevasse forms. The crevasse only forms in the top 100 feet of the glacier. Underneath the ice is too heavily weighted down to crack. Very steep glacial inclines that cause crevasses are call ice falls. These are the most dangerous because at any time the ice can separate from the glacier by melting and fall. Many glacier crevasses have a crisscross pattern because the different speeds of the flowing glacier form cracks that are in the direction of the glacier flow, while cracks perpendicular to the glacier occur when it is accelerating downhill.

Glacial Surges

Glaciers are normally quite well behaved. However they occasionally break out of their normal everyday pattern and cut loose at speeds up to 100 times their average. These are called glacial surges. Our warm glacier traveled a yard a day or a fifth of a mile per year. Muldrow Glacier in 1964 was clocked at 16 yards an hour or 400 yards per day. Most warm glaciers travel at a fraction of a mile per year, while surging glaciers will travel miles in months. One glacier surge traveled 12 miles in 3 years; this was the longest. Another glacial surge went 8 miles in less than 3 months; this was the quickest on record.

The difference between surges and normal glacier movement besides speed is that the surges might last between 3 years at the longest and only a few months at the shortest. Glacial surges are as unpredictable as earthquakes. No scientist knows when they will start or stop. Neither do they know which glaciers will surge and which will not. Certain glaciers do seem to go through periodic surges.

Glaciers that surge build up an ‘ice reservoir’ between the head and the middle of the glacier. This contrasts with a normal glacier, which is continuously redistributing its annual snowfall. Because of the buildup many glaciers are actually retreating just before they surge. It seems that the ice reservoir has been blocked by some kind of obstruction. When it reaches a critical mass, it bursts through the friction with a magnified velocity and power. Once it has spent its wad, it goes into retreat again, and begins building up another reservoir of ice for the next surge.

One scientist even suggested that a surge in Antarctica could set off another Ice Age. Once the surge was over then ice pack would begin to retreat and the Ice Age would begin to abate.

There are two lessons to be learned. One is that instead of letting your anger build up and exploding outward, it might be better to let a little out at a time to avoid the reservoir buildup. The other is that instead of dissipating your energy in small chunks, it might be better to buildup a reservoir of energy, to release all at once for greater power.

Glaciers maintain constant thickness

The reason that surges are so unusual for a glacier is that the one constant for glaciers is thickness. They try to maintain a constant thickness throughout their length.

“With the obvious and dramatic exception of surges, the overall result of variations in a glacier’s flow is the maintenance of a more or less constant thickness of ice.” (Glacier p59)

When it snows at the head, the glacier thickens causing the glacier to speed up. This speed attenuates, stretches out the glacier, and its snout increases. The glacier grows in length, but stays the same thickness. When it doesn’t snow, the glacier loses thickness, causing it to slow down, and then to shrink in length. The glacier is retreating. This stretching of the glacier causes the center to go fastest and the ends the slowest, just as glacier research has shown.

Using glacier terminology borrowed from economics: when the glacier has a positive balance, i.e. an annual surplus, then it will grow in length. A glacier has a negative balance when the net income, i.e. snowfall, is less than the net outgo, i.e. melting and cracking off. When the glacier has a negative balance, i.e. an annual deficit, then the glacier will shrink in length. The net effect is that the glacier thickness will remain constant while the length will grow and shrink.

Ice that doesn’t move will just increase in thickness with the snowfall. When this ice reaches a critical thickness then it compresses itself so much that it cannot maintain its own weight. It begins to press down upon itself. The pressure is so great that the ice on the bottom squeezes out and up the other side, to escape this excruciating pressure. A glacier a half-mile thick exerts a pressure of 62 tons per square foot on any surface it touches.

“The thicker the ice the more effective the glacier as an engine of erosion. A glacier only half a mile thick exerts pressure of 62 tons per square foot. This pressure crushes all the material beneath and adjacent to it and pushes the glacier’s rasp like tools—the jagged rock fragments frozen into ice—deeper into the underlying bedrock.” (Glacier p29)

This same pressure that crushes everything under it, also causes the ice to squish out, thereby moving forward and down. These tons of downward pressure cause glaciers to deform and move. It also scours the land. What does it do to the land? This leads us into our next section.

 

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