Earth’s Frozen Water
After oceans, the largest quantity of water on Earth is frozen. In the North and South pole large masses of compressed snow and ice, known as ice sheets, account for about 10 percent of the Earth’s Land area. In these sheets are about three quarters of the planet’s freshwater supply.
What is a Glacier?
Glaciers are generally formed in year long cold climates. Snow piles up and eventually becomes so massive that it applies enough pressure that the bottom snow partially melts. The mass of snow and ice begins to move downhill and is now referred to as a glacier.
Likewise, you may have heard the term “polar ice cap.” An ice cap is a glacier that formed on relatively flat lands and expanded out from its center. In Canada, the high basins of the Coast, Columbia, and the Rocky Mountains are cold enough to form glaciers and icefields. An icefield is an upland area of ice that feeds two or more glaciers.
A valley glacier forms in a mountain range. High in the mountains the average temperature is low enough to prevent snow from melting during the summer. Over time more snow accumulates, pushing the glacier further down the mountain. The Athabasca Glacier, which is currently shrinking each year, is the only glacier accessible by road.
A continental glacier is formed when a huge mass of snow and ice accumulates. They are almost exclusively found around the North and South poles. These glaciers can become so thick they can almost bury mountain ranges.
Ice, unlike other solid states, has some unique characteristics, because of these unique characteristics glaciers develop shapes and flows that are not seen elsewhere. For example, when a glacier forms over a steep cliff and breaks up, an icefall occurs. Likewise, a crevasse is formed when a large fissure, or crack, occurs in the glacier’s ice.
Advancing or Retreating
Glaciers can move because of their immense mass. The bottom ice of a glacier is flexible and flows like a river, downhill, following the path of least resistance. Like liquid water, it squeezes its way into narrow areas, where it becomes thicker, likewise, it thins in wide areas. Most glaciers tend to spread out at the toe where the ice thins.
Still, what happens when ice melts faster than it flows. It looks like the glacier is retreating back up the hill, but it is slowly melting away. This is referred to as a retreating glacier. On the other hand, when a glacier flows faster than it melts it is referred to as an advancing glacier. No matter if a glacier retreats or advances it leaves a pile of sediment near the toe, giving us a good landmark of the distance it retreated or advanced from year to year.
Pack Ice and Icebergs
For most of the year, the surface of the Arctic Ocean is solid with pack ice. Pack ice is a sheet of ice rarely more than 5 m thick that breaks easily. It is formed from freezing sea water. Large pieces sometimes break off and melt as they move toward warmer waters. Icebergs are large chunks of ice that break loose, or calve, from continental glaciers as the glaciers flow downslope into the ocean (see video). Each year, about 16 000 icebergs break loose from Greenland’s glaciers. Some icebergs are several kilometres long and rise more than 100 m above the surface of the ocean. As these icebergs float, sun and wind melt the tops. The bottoms of the icebergs, submerged in the ocean, melt more slowly.
How Glaciers Shape the Land
As they move over land, glaciers act like bulldozers. They push aside and forward any loose materials they encounter. Eroded sediments pile up along a glacier’s sides, as shown in the image below. Glaciers also weather and erode rock. When glacial ice melts, water flows into cracks in rocks. Later, the water refreezes in these cracks, expands, and splits the rock into pieces. These rock pieces are then lifted out by the glacier. This process results in boulders, gravel, and sand being caught by the bottom and sides of a glacier.
As the glacier moves forward, rock fragments and sand at its base scrape the soil and bedrock. These rock fragments erode even more material than ice could alone. When bedrock is gouged by dragged rock fragments, marks are left behind. These marks, called striations, are parallel scars or scratches (see image below). Striations show in which direction the glacier moved.
Evidence of Valley Glaciers
If you visit the mountains, you can tell whether valley glaciers existed there. Valley glaciers erode bowl-shaped basins, called cirques, in the sides of mountains. Sometimes two or more glaciers erode a mountain summit from several directions. This can form a ridge called an arête, or a sharpened peak called a horn. The image below shows all these land- forms. They are evidence of valley glaciers.
When glaciers begin to melt, they no longer have enough energy to carry much sediment. The sediment is deposited on the land. When the glacier slows down, a jumble of boulders, sand, clay, and silt drops from its base. This mixture of different-sized sediments is called till.
Till is also deposited in front of a glacier when it stops moving forward. Unlike the till that drops from a glacier’s base, this second type of deposit does not cover a wide area. Because it is made of the rocks and soil that the glacier has been pushing along, it looks like a big ridge of material left behind by a bulldozer. Such a ridge is called a moraine.
When more snow melts than accumulates, the glacier starts to melt and retreat. Material deposited by the meltwater from a glacier is called outwash. Outwash is shown in the image below. The meltwater carries sediments and deposits them in layers, sometimes in the shape of a fan. Another type of outwash deposit looks like a long, winding ridge. This deposit forms beneath a melting glacier when meltwater forms a river within the ice. This river carries sand and gravel and deposits them within its channel. When the glacier melts, a winding ridge of sand and gravel is left behind. This winding ridge is called an esker (see image below).
Meltwater is water formed in the summer from melting snow and ice, this water carves channels through glaciers and some can form streams that flow to the surface of the ice. Likewise, sometimes a rounded drain is formed in the ice from these waters, these are referred to as millwells.
The Importance of Glaciers
One of the most important features of glaciers is that they act like natural reservoirs. Glaciers collect snow and ice and gradually release these waters as meltwater back into the river systems. These meltwaters are rather consistent and allow us and other animals a constant supply of water.
Glaciers also exert direct influence over the water cycle by slowing the passage of water through the cycle. By storing vast quantities of water and releasing this water when it is needed most, during hot, dry summer months. Glaciers also give us important queues of past events. Many scientists use evidence from glaciers to determine the Earth’s climate in the past.
An ice age is a long period of cooling in the global temperature of the Earth. During this time, glaciers advance from the poles. During the last ice age, which started ~120 000 years ago and ended ~11 000 years ago, Canada was completely frozen. We still have evidence of this today, many striations, moraines, U-shaped valleys, and erratics are all clues to the presence of glaciers. We have evidence of at least seven previous ice ages.
You may be wondering what causes an ice age, currently, the leading hypotheses are listed below:
- There may be occasional reductions in the thermal energy given off by the Sun.
- An increase in volcanic activity may add large volumes of dust to the atmosphere. This might reduce the amount of the Sun’s energy reaching Earth.
- Periods of mountain formation would increase the area of high mountain ranges on Earth. The extra snow remaining on these cold peaks through the summer reflects sunlight and may reduce the temperature.
- The movement of Earth’s tectonic plates alters the shape of the oceans. This change affects the flow of ocean currents. With less mixing between hot and cold waters, some regions might become cold enough to start an ice age.
- Changes in the tilt of Earth’s axis, or in its orbit around the Sun, may produce colder climates.
Today, many newspaper and magazine headlines warn us about the greenhouse effect and global warming. These two events are related but are not the same.
As you learned in previous science studies, the greenhouse effect is natural warming caused by gases in our atmosphere trapping heat (see image below). Carbon dioxide is the main greenhouse gas. It is important to realize that the greenhouse effect is not bad in itself. Without the greenhouse effect, life as we know it would not be possible on Earth. Like Mars, Earth would be too cold to support life.
Global warming means that global temperatures are rising. Many scientists hypothesize that one reason for global warming is the increase of greenhouse gases in our atmosphere. An increase in greenhouse gases increases the greenhouse effect. In the last 100 years, the mean surface temperature on Earth has increased 0.5°C. This increase may very well be due to global warming.
If Earth’s mean surface temperature continues to rise, ice caps will melt. Low-lying areas might experience increased flooding. Already some ice caps are beginning to break apart and sea level is rising in certain areas. Some people believe that these events are related to Earth’s increased temperature.
The information on this page and it associated figures are adapted from the Science Focus 8.Science Focus 8, pp. 375-387