Glacier is a large mass of ice that flows slowly under the influence of gravity. Glaciers consist of packed snow that has built up over many years. The snow’s weight eventually compresses its lower layers into ice. Glaciers scrape the ground as they move over it, eroding old landforms and creating new ones. They range in thickness from several feet or meters to 10,000 feet (3,000 meters) or more.
Glaciers form in the colder regions near the North and South poles and in mountainous areas. During periods called ice ages, glaciers can grow to cover large portions of Earth’s surface.
Loading the player...Glacier
Kinds of glaciers.
Scientists classify glaciers according to their size and shape. An ice sheet is a dome-shaped glacier covering an area greater than 19,300 square miles (50,000 square kilometers). The ice in an ice sheet flows slowly outward from one or more central domes. Faster moving glaciers called outlet glaciers flow outward from the edge of an ice sheet. Ice sheets can reach a thickness of more than 2 miles (3 kilometers). They often conceal the entire landscape beneath them except for the tallest mountain peaks, called nunataks. Huge ice sheets cover most of Antarctica and Greenland.
An icecap is a dome-shaped glacier that covers an area of 19,300 square miles (50,000 square kilometers) or less. Icecaps resemble ice sheets, with outlet glaciers flowing outward from a central ice dome. Icecaps occur in Iceland and Norway and on several Arctic islands.
In some high mountain areas, glaciers form in a cirque, a bowl-shaped hollow with steep walls. A relatively small glacier confined to a cirque is called a cirque glacier. A valley glacier is a long, narrow stream of ice that flows down a mountain valley. Valley glaciers occur in mountainous regions worldwide.
Different valley glaciers flowing downhill sometimes come together, much as the tributaries of a river merge. Unlike tributaries, however, each of the glaciers remains a separate mass and continues to flow on its own. Rock debris carried by the glaciers accumulates at the boundaries between them. Some valley glaciers flow out of the mountains and onto flatter ground. No longer confined by valley walls, the ice spreads out to form a rounded lump called a piedmont glacier.
Scientists also classify glaciers by comparing the temperature of a glacier’s ice with the ice’s melting point, the temperature at which it turns to water. Within a temperate glacier, the ice remains at or near its melting point throughout most of the year. Temperate glaciers contain ample meltwater (water from melted ice). They are not frozen firmly to the ground. The ice in a polar glacier remains well below its melting point throughout the year. Polar glaciers contain little or no meltwater. Most polar glaciers are frozen to the ground and cannot slide over it. A single large glacier can stretch across different climate zones or elevations that have different temperatures. One portion of such a glacier might be temperate, and another portion might be polar.
How glaciers form.
Glaciers form in areas where some snow remains on the ground throughout the year. This snow accumulates in layers over hundreds or thousands of years. Eventually, the weight of the upper snow layers compresses the lower layers into tiny pellets of ice called firn. At greater depths, the weight further compresses firn into solid ice. As snow turns into firn and ice, the frozen mass becomes more dense. With enough pressure, the ice can begin to flow, and the mass becomes a glacier.
Meltwater plays an important role in the formation of most glaciers. As ice on the glacier’s surface melts, meltwater seeps deep into the ice mass, filling open spaces between ice crystals and particles and then refreezing. Meltwater also reduces the friction between ice particles, speeding the internal movement of the glacier as well as the rate at which the glacier slips over the ground. An abundance of meltwater makes temperate glaciers move faster than polar glaciers.
Glaciers grow and shrink with seasonal variations in temperature and snowfall. Typically, lower air temperatures in winter and spring prevent a glacier from melting, and heavy snowfall causes it to grow. In summer and autumn, higher air temperatures can melt portions of a glacier, causing it to shrink.
In frigid regions, glaciers can shrink as they enter the sea. The ice on a glacier’s leading edge, which floats, is pushed upward by seawater. Huge chunks of ice break off in a process known as calving. The chunks, called icebergs, are carried away by wind and ocean currents.
Glaciers also grow and shrink with changes in Earth’s climate. During an ice age, lower air temperatures and increased snowfall cause glaciers to thicken and expand. During an interglacial period, higher air temperatures and lower snowfall cause glaciers to thin and retreat.
Glaciers covered vast areas of Asia, Europe, and North America during the Pleistocene Epoch, a period in Earth’s history from about 2.6 million to 11,500 years ago. Many ice ages occurred during that time. During the Pleistocene Epoch, the huge Laurentide Ice Sheet covered much of what are now Canada and the northern United States. Today, many valley glaciers around the world are retreating and thinning in response to global warming, an increase in the average temperature of Earth’s surface since the mid-1800’s.
How glaciers move.
Gravity causes the ice in a glacier to move. From the surface down to a depth of about 130 feet (40 meters), the ice is brittle. This area can become stretched or compressed as the glacier changes its speed or moves over uneven or steep terrain. The ice often breaks, forming deep cracks called crevasses. At greater depths, the ice in a glacier flows like a thick plastic without breaking. At these depths, the pressure of the overlying layers causes ice crystals to change shape and regroup. These small changes in the individual crystals cause the entire ice mass to move internally.
Glaciers move over the ground at different speeds. Heat from the ground and friction causes some melting at the bottom of a temperate glacier. The meltwater lubricates the bedrock, causing the glacier to slide over the ground more quickly. Temperate glaciers typically advance only 1 inch (2.5 centimeters) or so a day, but, in steep terrain, such glaciers can move as much as 20 feet (6 meters) per day. Pressure from an icecap or ice sheet can push some outlet glaciers from about 30 to 100 feet (9 to 30 meters) per day. The various parts of a glacier can also move at different speeds. For example, the central and uppermost portions of a valley glacier flow the fastest. Friction with the valley walls and floor causes the sides and bottom of the glacier to move more slowly.
How glaciers shape the land.
The impact of prehistoric glaciers can be seen in many modern landscapes. For example, the rolling terrain of the northern United States was shaped by glaciers that melted more than 10,000 years ago. Geologists believe ancient glaciers also carved long, narrow inlets called fiords in Norway and similar features found in Alaska, British Columbia, Maine, Newfoundland and Labrador, Greenland, and New Zealand.
Advancing glaciers create a variety of landforms by eroding, transporting, and depositing rock debris. Meltwater from glaciers can enter cracks in the bedrock. This water then freezes and expands, breaking away pieces of rock that are carried off by the glacier. Glaciers also scoop up rock debris and drag it along at their bases. This debris polishes, scratches, or grooves exposed bedrock as the glacier scrapes over it. Valley glaciers can carve sharp-bottomed, V-shaped river valleys into gently curving, U-shaped valleys. An advancing glacier can shape a rocky knob into a formation called a stoss-and-lee feature. The side that faced the approaching glacier, called the stoss side, is smooth and gently sloping. The opposite side, called the lee side, is rough and steep.
A moving glacier transports and deposits till, angular rock fragments that range in size from gravel to boulders. Glaciers deposit till in uneven ridges known as moraines. A ground moraine forms under the glacier and features an irregular surface. Ridges called lateral moraines develop along the sides of a valley glacier. Where two valley glaciers come together, their lateral moraines merge to form a medial moraine where the two streams of ice meet. A terminal moraine or end moraine marks the farthest point of advance of a valley or piedmont glacier. A recessional moraine forms when a glacier that was retreating begins to advance or temporarily halts. Advancing glaciers can also deposit till in an oval-shaped hill called a drumlin.
Loading the player...Parts of a glacier falls into the sea
Glaciers also shape the land as they thin and retreat. Meltwater flows over, under, and through the ice of a retreating glacier. Streams of meltwater carry rock fragments freed from the ice by melting. This process polishes and rounds the fragments and separates them by size, leaving fragments of different sizes in different deposits. An esker is a narrow, winding ridge of sand and gravel deposited by a stream of water flowing through a tunnel or crevasse in a melting glacier. Sometimes, large chunks of melting ice become buried by sediment. These chunks melt slowly, leaving behind a kettle, a circular or oval depression that fills with water. Streams draining a melting glacier transport sediment over a broad flat area known as an outwash plain.
Famous glaciers.
The French and Swiss Alps feature some of the world’s best-known glaciers, including the Mer de Glace on Mont Blanc and the Aletsch Glacier near the Jungfrau. Europe’s largest glacier, Vatnajskull in Iceland, covers 3,130 square miles (8,100 square kilometers). Jostedalsbreen in Norway, the largest glacier on the European continent, covers about 190 square miles (490 square kilometers). The largest North American glacier outside Greenland is the 1,930-square-mile (5000 square-kilometer) Malaspina Glacier near Yakutat Bay in Alaska’s Saint Elias Mountains. Other North American glaciers include those in Banff National Park in Alberta, Glacier National Park in Montana, and on Mount Rainier in Washington. Some other well-known glaciers are in the Andes Mountains of South America, the Himalaya of southern Asia, and the Southern Alps/Kā Tiritiri o te Moana of New Zealand.
