Glaciers are flowing masses of ice, created by years of snowfall and cold temperatures. Approximately one-tenth of Earth is covered by glaciers, including Antarctica and parts of Greenland, Iceland, Canada, Russia, and Alaska. Mountainous regions on every continent except Australia also contain glaciers. Glaciers have enormous powers to reshape the face of Earth. Even today, glaciers are altering how our planet looks, and they hold clues to its past and future.
How glaciers form
Glaciers are created in areas where the air temperature never gets warm enough to completely melt snow. After a snowfall, some or most of the snow may melt when it comes into contact with warmer ground temperatures. As the air temperature drops, the melted snow refreezes, turning into small ice granules called firn or névé (pronounced nay-VAY). As additional layers of snow accumulate on top, the firn underneath is compacted. When the accumulation reaches about 150 feet (46 meters) deep, the weight and pressure cause the lower layers to recrystallize into solid ice. As years pass, snow accumulates and the slab of ice grows steadily thicker. Eventually the mound of ice becomes too massive to sit still, and gravity pulls the ice downhill. Once the ice begins to move, it is considered a glacier.
Types of glaciers
Glaciers that flow down a valley from high mountainous regions usually follow paths originally formed by rivers of snowmelt in the spring and summer. These valley or mountain glaciers end in a valley or ocean, and tend to increase the sharpness and steepness of the surrounding mountains along the way. In the Alps, a mountain system in south-central Europe, there are more than 1,200 valley glaciers.
Piedmont glaciers are large, gently sloping ice mounds. Also known as lakes of ice, piedmont glaciers form when a valley glacier reaches the lowlands or plain at the foot of a mountain and spreads out. These are common in Alaska, Greenland, Iceland, and Antarctica.
Glaciers that form in small valleys on the sides of mountains are called ice caps. Found in Norway, Iceland, Greenland, and Antarctica, ice caps usually do not move out of their basinlike area.
The largest form of glacier is called a continental glacier, a huge ice sheet that moves slowly outward from its center. Ice sheets may cover hundreds of thousands of square miles, and are so heavy that they cause the rock underneath to compress into Earth. The largest continental glacier is found on Antarctica, where the ice is more than 2.5 miles (4 kilometers) thick at its center, and hides entire mountain ranges beneath its surface. It extends more than 5 million square miles (12.9 million square kilometers). The Antarctic ice sheet accounts for 90 percent of all the ice in the world, and contains more water than all of Earth's rivers and lakes put together.
Words to Know
Continental glacier: The largest form of and slowest moving glacier, covering large expanses of a continent.
Glacial till: Rock and soil scoured from Earth and transported by a glacier, then deposited along its sides or at its end.
Greenhouse effect: The warming of Earth's atmosphere due to water vapor, carbon dioxide, and other gases in the atmosphere that trap heat radiated from Earth's surface.
Ice age: Period of glacial advance.
Ice caps: Smaller glaciers that form in basinlike depressions in a mountain.
Kettle lakes: Bowl-shaped lakes created by large ice blocks, which formed depressions in Earth's surface.
Meltwater: Melted ice in the glacier's bottom layer, caused by heat that develops as a result of friction with Earth's surface.
Moraines: Large deposits of glacial till that form hills.
Piedmont glacier: Large, gently sloping glaciers found at the foot of mountains, which are fed by alpine glaciers.
Surging: A sudden increase in a glacier's movement as a result of meltwater underneath decreasing its friction.
Valley glacier: Glacier that forms at a high elevation in a mountain region and flows downhill through valleys originally created by rivers.
Most glaciers that exist today are remnants of the last glacial period from 1,800,000 to 11,000 years ago. As glaciers advance and retreat, they plow through rock, soil, and vegetation like a huge bulldozer, altering everything they come into contact with. Ice acts like an adhesive, scooping up rocks and soil that add to the glacier's tremendous powers of erosion.
Thus, a river valley that was once V-shaped becomes U-shaped; the rocks and soil carried with the glacier, known as glacial till, are deposited in huge mounds along the sides and at the end of the glacier, creating entirely new hills, or moraines. Chunks of ice buried in this till create large depressions that later became what are known as kettle lakes.
Glaciers also scour the land to great depths, creating larger lakes such as the North American Great Lakes. During the last ice age, Earth's surface was depressed due to the weight of the glaciers. As the glaciers retreated, Earth's crust rose upward like a sponge. This crustal rebounding, as it is called, is still occurring at slow rates in parts of North America and Europe.
Most glaciers move fairly slowly, only inches to a few feet per day. When large amounts of ice melt under the glacier as a result of friction with Earth's surface, the meltwater acts like grease to rapidly increase its movement. This sudden increase in speed is called surging.
Clues to Earth's past and future
While the effects of glaciers tell us where they have occurred in the past, present glaciers are providing clues as to variations in climate over time and potential changes in the future. Scientists continue to debate the reasons why ice ages occur, but there seems to be agreement that glaciers increase on the planet when Earth rotates farther away from the Sun. When Earth rotates closer to the Sun, glaciers retreat.
The U.S. National Academy of Sciences has predicted that if global temperatures rise from 1.5 to 5°F (0.75 to 2.5°C) over the twenty-first century as a result of the greenhouse effect, significant portions of Earth's glaciers could melt. (The greenhouse effect is the warming of Earth's atmosphere due to water vapor, carbon dioxide, and other gases in the atmosphere that trap heat radiated from Earth's surface.) Because glaciers hold 75 percent of the world's freshwater supply, such a meltdown would result in massive flooding of every continent's coastlines, drastically altering the shapes of every continent on Earth.
Ice core samples taken from Antarctic ice also have provided evidence of Earth's climate over the last 160,000 years. Data has shown a direct link between warming and cooling trends and the amount of two greenhouse gases, carbon dioxide and methane, in the atmosphere. These same cores show significant increases in both gases in the past 200 years.
Scientific reports issued at the beginning of the twenty-first century indicate that mountain glaciers from Montana to Mount Everest to the Swiss Alps were in a stage of retreat. In the Alps, scientists have estimated that by 2025 glaciers will have lost 90 percent of the ice that was there a century ago. Glacier melting, however, is generally quickest in and near the tropics. Ancient glaciers in the Andes have recently melted at an extraordinary rate. Between 1998 and 2000, one Peruvian glacier pulled back 508 feet (155 meters) a year. And Mount Kilimanjaro in the African country of Tanzania has lost 82 percent of the icecap it had when it was first carefully surveyed in 1912. At the current rate, scientists believe the icecap will disappear before 2015. Scientists theorize that all of these factors indicate that a global warming trend is taking place and that it is at least partly caused by gases released by human activities.
[See also Antarctica; Greenhouse effect; Ice ages; Icebergs ]
A glacier is a mass of ice larger than 247 acres (one tenth of a square kilometer) that starts on land, but can move out over coastal waters in polar regions where calving (breaking apart) produces icebergs. All glaciers move as the result of their weight and gravity. Glaciers require a specific cold climate to exist.
There are three types of glaciers: alpine glaciers in mountain valleys; piedmont glaciers near the base of mountain ranges where glacier masses have combined; and continental glaciers, which are called ice sheets when they are very large. Continental glaciers have covered vast areas of Earth at different times through history.
When glaciers advance they change the landscape. When they retreat, tell-tale signs are left behind. Paleoclimatologists studying the signs left from retreating ancient glaciers have discovered that massive glaciation occurred fairly regularly. Past glaciation cycles appear to be related to subtle changes in solar radiation. Scientists are actively looking for answers to how human activities that are already causing climate changes will influence Earth's glaciation cycles in the future.
Historical Background and Scientific Foundations
Glaciers start where more snow falls in the winter than melts in the summer. First, a snow field collects. Through successive freezing and thawing the snow becomes grain-sized ice called firn. Over time, the snow field becomes an ice field. When the icy mass gets large enough so it begins to move, it is called a glacier.
Alpine glaciers move down the mountain in channels, scouring the walls to form characteristic U-shaped valleys called cirques. A piedmont glacier forms where several glaciers combine at the foot of mountains. These are common in Alaska.
A glacier picks up surface soil and rocks and carries the load (called till) forward with the ice. When the glacier recedes, till is dropped. Till that is dropped in front of a receding glacier is known as a terminal moraine. The overall landscape left behind a retreating continental glacier is irregular. Glaciated landscape features include hollows that are collectively described as kettle drum topography. Boulders are scarred as they are carried for hundreds of miles or kilometers to new locations. They are called erratics because the rock type is not consistent with the area where the boulders are dropped.
In the mid-1800s, Swiss-born geologist Louis Agassiz (1807–1873) first suggested the concept of continental glaciers when he noticed evidence of glacial changes in the landscapes over large areas of Canada that resembled the glaciated alpine landscapes of Switzerland. Subsequently geologists determined there was not one but a series of periods in ancient history where large glaciers covered up to 32% of the total land area of Earth.
Evidence that there have been eight Ice Age cycles over the past 750,000 years has been collected from deposits left behind on land, and sea floor sediments. Evidence also comes from ice core samples taken from Greenland and Antarctica where continental glaciers that date back to the Ice Ages still cover most of the land area.
Impacts and Issues
Each period of vast continental glaciation was separated by a warmer interglacial period that lasted about 10,000 years. Serbian engineer Milutin Milankovitch (1879–1958) developed a mathematical explanation for the Ice Ages based on variations in solar radiation. The Milankovitch theory is in close agreement with 20,000, 40,000, and 100,000 year cycles of glaciation. The most recent Ice Age began about 70,000 years ago, peaked about 20,000 years ago and ended about 10,000 years ago. Since inter-glacial periods were generally around 10,000 years, Earth could be approaching another Ice Age.
There is a big question about Earth's future that was not considered by Milankovitch. Where climate was controlled by natural cyclical variations in solar radiation in the past 750,000 years, it may not be controlled only by natural causes in the future. Human activities have changed the climate through greenhouse gases emitted from the widespread use of fossil fuels. Carbon dioxide and methane levels are higher in the atmosphere in the twenty-first century than they have been in any previous interglacial period.
WORDS TO KNOW
CALVING: Process of iceberg formation when huge chunks of ice break free from glaciers, ice shelves, or ice sheets due to stress, pressure, or the forces of waves and tides.
GLACIATION: The formation, movement, and recession of glaciers or ice sheets.
ICE AGE: Period of glacial advance.
KETTLE DRUM: Usually termed a kettle: depression or sinkhole left in a mass of rocky debris by a retreating glacier. A large, isolated chunk of ice buried in debris melts, causing a collapse of the overlying debris.
MORAINE: Mass of boulders, stones, and other rock debris carried along and deposited by a glacier.
PALEOCLIMATOLOGY: The study of past climates, throughout geological history, and the causes of the variations among.
PIEDMONT: A region of foothills along the base of a mountain range.
Glaciers along the coast of Greenland are melting at record rates as a result of global warming. Since glaciers contain 70% of Earth's freshwater, when they melt the freshwater changes the dynamics of the ocean water around them. Melting glaciers also cause the sea level to rise. Global warming is adding to sea levels rising as the oceans get warmer and expand. Where glaciers need a very specific cold climate to form, there is no easy answer to the question of when the next Ice Age will start.
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glacier, moving mass of ice that survives year to year, formed by the compacting of snow into névé and then into granular ice and set in motion outward and downward by the force of gravity and the stress of its accumulated mass. Glaciers are usually found in high altitudes and latitudes.
Glaciers are of four chief types. Valley, or mountain, glaciers are tongues of moving ice sent out by mountain snowfields following valleys originally formed by streams. In the Alps there are more than 1,200 valley glaciers. Piedmont glaciers, which occur only in high latitudes, are formed by the spreading of valley glaciers where they emerge from their valleys or by the confluence of several valley glaciers. Small ice sheets known as ice caps are flattened, somewhat dome-shaped glaciers spreading out horizontally in all directions and cover mountains and valleys. Continental glaciers are huge ice sheets whose margins may break off to form icebergs (see iceberg). The only existing continental glaciers are the ice sheets of Greenland and Antarctica, but during glacial periods they were far more widespread. Glaciers may be classified as warm or cold depending on whether their temperatures are above or below -10°C (14°F).
Glaciers alter topography, and their work includes erosion, transportation, and deposition. Mountain glaciers carve out amphitheaterlike vertical-walled valley heads, or cirques, at their sources. They transform V-shaped valleys into U-shaped valleys by grinding away the projecting bases of slopes and cliffs and leveling the floors of the valleys; in this process tributary valleys are frequently left "hanging," with their outlets high above the new valley floor. When the tributary valleys contain streams, waterfalls and cascades are formed, such as Bridal Veil Falls of Yosemite National Park. Elevations over which glaciers pass usually are left with gently sloping sides in the direction from which the glacier approached (stoss sides) and rougher lee sides. Humps and bosses of rock so shaped are known as roches moutonnées.
The debris from glacial erosion is carried upon, within, and underneath the ice. The debris frozen into the underside of the glacier acts as a further erosive agent, polishing the underlying rock and leaving scratches, or striae, running in the direction of the movement of the glacier. Glacial deposits are often known as till or drift. The melting of the ice in summer forms glacial streams flowing under the ice, while the retreat of a large glacier sometimes leaves a temporary glacial lake, such as the ice age Lake Agassiz. Fjords generally owe their origin to glaciers.
A glacier moves as a solid rather than as a liquid, as is indicated by the formation of crevasses (see crevasse). The center of a glacier moves more rapidly than the sides and the surface more rapidly than the bottom, because the sides and bottom are held back by friction. The rate of flow depends largely on the volume of ice in movement, the slope of the ground over which it is moving, the slope of the upper surface of the ice, the amount of water the ice contains, the amount of debris it carries, the temperature, and the friction it encounters. Glaciers are always in movement, but the extent of the apparent movement depends on the rate of advance and the rate of melting. If the ice melts at its edge faster than it moves forward, the edge of the glacier retreats; if it moves more rapidly than it melts, the edge advances; it is stationary only if the rate of movement and the rate of melting are the same.
The causes of glacial movement are exceedingly complex and doubtless are not all operative on the same glacier at the same time. Important elements in glacial movement are melting under pressure followed by refreezing, which may push the mass in the direction of least resistance; sliding or shearing of layers of ice one on top of the other; and rearrangement of the granules when pressure causes melting. Sudden, rapid movements of glaciers, called glacier surges, have been observed in Alaskan and other glaciers, with evidence for such abnormal movements as the crumpled lines of surface debris found on them. It is thought that the relatively sudden movement and melting of glaciers may be indicative of climate warming.