• glacier
    A lake is the mouth of this "river of ice."

    Photograph by James P. Blair

    Paleoclimatology
    Paleoclimatology is the study of the Earth's atmosphere in prehistoric times. Paleoclimatology depends on ice and bubbles in glaciers and ice sheets. Scientists extract long tubes of ice, called ice cores, from thick ice sheets, usually in the Antarctic. Ice cores are layered, with the deepest ice having the oldest information. Wide bands indicate a heavy snowfall. Darkly colored bands indicate smoke or other chemicals in the atmosphere. Ice cores can measure the state of the atmosphere as far back as 80,000 years.

    For instance, cores from ice sheets from the year 1883 contained chemicals from the massive eruption of Krakatoa, a volcanic island in Indonesia. Ice cores showed those chemicals drifted from the South Pacific to Antarctica and Greenland and stayed in the atmosphere for many years afterward.

    Icefall
    Glaciers are called "rivers of ice." Just like rivers, glaciers have fall lines where the bed of the glacier gets narrow or descends rapidly. Ice flows down the icefall just like water falls down a waterfall.

    The Khumbu Icefall is one of the most difficult terrains on Mount Everest.

    Third Pole
    Siachen Glacier, a huge glacier in the Himalayan Mountains, is sometimes called the Third Pole. Siachen Glacier is the worlds highest area of conflict. Although India controls Siachen, both India and Pakistan claim the area as part of their country. Siachen Glacier is the site of the worlds highest helicopter landing pad, which India built for military and emergency use.

    Why So Blue?
    Some glaciers and icebergs are blue, for the same reason water is blue. The chemical bond between oxygen and hydrogen in water absorbs light in the red end of the visible light spectrum.

    Blue glaciers and icebergs are not blue for the same reason the sky is blue. The sky is blue due to atmospheric scattering of light (Raleigh scattering), a different phenomenon.

    A glacier is a huge mass of ice that moves slowly over land. The term “glacier” comes from the French word glace (glah-SAY), which means ice. Glaciers are often called “rivers of ice.”

    Glaciers fall into two groups: alpine glaciers and ice sheets.

    Alpine glaciers form on mountainsides and move downward through valleys. Sometimes, alpine glaciers create or deepen valleys by pushing dirt, soil, and other materials out of their way. Alpine glaciers are found in high mountains of every continent except Australia (although there are many in New Zealand). The Gorner Glacier in Switzerland and the Furtwangler Glacier in Tanzania are both typical alpine glaciers. Alpine glaciers are also called valley glaciers or mountain glaciers.

    Ice sheets, unlike alpine glaciers, are not limited to mountainous areas. They form broad domes and spread out from their centers in all directions. As ice sheets spread, they cover everything around them with a thick blanket of ice, including valleys, plains, and even entire mountains. The largest ice sheets, called continental glaciers, spread over vast areas. Today, continental glaciers cover most of Antarctica and the island of Greenland.

    Massive ice sheets covered much of North America and Europe during the Pleistocene time period. This was the last glacial period, also known as the Ice Age. Ice sheets reached their greatest size about 18,000 years ago. As the ancient glaciers spread, they carved and changed the Earth’s surface, creating many of the landscapes that exist today. During the Pleistocene Ice Age, nearly one-third of the Earth’s land was covered by glaciers. Today, about one-tenth of the Earth’s land is covered by glacial ice.

    How Glaciers Form

    Glaciers begin forming in places where more snow piles up each year than melts. Soon after falling, the snow begins to compress, or become denser and tightly packed. It slowly changes from light, fluffy crystals to hard, round ice pellets. New snow falls and buries this granular snow. The hard snow becomes even more compressed. It becomes a dense, grainy ice called firn. The process of snow compacting into glacial firn is called firnification.

    As years go by, layers of firn build on top of each other. When the ice grows thick enough—about 50 meters (160 feet)—the firn grains fuse into a huge mass of solid ice. The glacier begins to move under its own weight. It does this through a process called compression melting. The glacier is so heavy and exerts so much pressure that the firn and snow melt without any increase in temperature. (Most substances do not melt under pressure. Water is somewhat unusual.) The meltwater makes the bottom of the heavy glacier slicker and more able to spread across the landscape.

    Pulled by gravity, an alpine glacier moves slowly down a valley. Some glaciers, called hanging glaciers, don't flow the entire length of a mountain. Avalanches and icefalls transfer glacial ice from hanging glaciers to a larger glacier beneath them, or directly to the valley below.

    An ice sheet spreads out from its center. The great mass of ice in a glacier behaves plastically, or like a liquid. It flows, oozes, and slides over uneven surfaces until it covers everything in its path.

    Different parts of a glacier move at different speeds. The flowing ice in the middle of the glacier moves faster than the base, which grinds slowly along its rocky bed.

    The different speeds at which the glacier moves causes tension to build within the brittle, upper part of the ice. The top of the glacier fractures, forming cracks called crevasses. Crevasses are in the top 50 meters (160 feet) of the glacier. Crevasses can be very dangerous for mountaineers. They can open quickly and be very deep.

    Moulins are another formation that carve into glaciers. A moulin is a deep, nearly-vertical pipeline in the glacier formed by meltwater on top of the glacier falling through a crack in the ice. Moulins are often much deeper than crevasses, going all the way to the bottom of the glacier.

    Most glaciers move very slowly—only a few centimeters a day. Some, though, can move 50 meters (160 feet) a day. These fast-moving rivers of ice are called galloping glaciers.

    Where a glacier meets the coast, it becomes a tidewater glacier. Its leading edge lifts and floats in the water, forming cliffs of ice that may be 60 meters (200 feet) high. Chunks of ice at the edge of the tidewater glacier break away into the water—a process called calving. Calving is a violent process. It results in large waves and loud crashes. Floating chunks of glacial ice, broken off during calving, are called icebergs.

    Glacial Features

    Although glaciers move slowly, they are extremely powerful. Like huge bulldozers, they plow ahead year after year, crushing, grinding, and toppling almost everything in their paths. Forests, hills, and mountainsides are no match for glaciers.

    Sometimes, glaciers form on volcanoes. When these volcanoes erupt, they are especially dangerous. They send floods of water, ice, and rocks over the land and into the atmosphere.

    Alpine glaciers begin to flow downhill from bowl-shaped mountain hollows called cirques. As the glaciers overflow the cirque, they move downward. They dig deep into the terrain, forming rugged, dramatic landscapes.

    As they move, glaciers erode or wear away the land beneath and around them. Glaciers carry great amounts of soil, rock, and clay. Some of the boulders they carry are as big as houses.

    Rocks carried hundreds and even thousands of kilometers by glaciers are called glacial erratics. Glacial erratics differ significantly from the landscape in which they were deposited. The Big Rock, for instance, is a 15,000-ton quartzite boulder near Okotoks, Alberta, Canada. The Big Rock was deposited from what is now northern Alberta, about 1,640 kilometers (500 miles) away, during the last ice age.

    Embedded, or stuck, in a glacier’s base, these large rocks grind against the ground like the prongs of a rake. They dig long grooves, called striations, in the surface of the Earth. Geologists can tell in what direction an ancient glacier moved by studying striations left in rock.

    Glaciers eventually deposit their loads of rock, dirt, and gravel. These materials are called moraine. Piles of moraine dumped at a glacier’s end, or snout, are called terminal moraines.

    Lateral moraine forms along the side of a glacier. Medial moraine appears as dark lines near the center of the glacier. Supraglacial moraine appears on the surface of the glacier—dirt, dust, leaves, and anything else that falls onto a glacier and sticks. Ogives are frozen “waves,” or ridges, on the surface of a glacier.

    When glaciers began their final retreat 10,000 years ago, they left behind many landscape features, such as lakes, valleys, and mountains.

    Many hollowed-out areas carved by glaciers became lakes. Bowl-shaped cirques, where most alpine glaciers form, became mountain lakes. These alpine lakes are called tarns.

    Glaciers can also create lakes by leaving depressions in the earth. The Finger Lakes in the western part of the U.S. state of New York were excavated during the last Ice Age. The lakes were once stream valleys. Along the streams, the glacier scooped out troughs that now contain deep lakes.

    Glacial retreat created other features of the landscape. Materials deposited by a glacier as it retreats are called ground moraines. The jumble of rock, gravel, and dirt making up ground moraines is called till. Much of the fertile soil in the Great Plains of North America was formed from layers of till left by ancient ice sheets.

    Glacial valleys exist on almost every continent. These valleys are scooped out as a glacier scrapes through them. There are no glaciers in Australia, but Mount Kosciuszko still has glacial valleys from the last Ice Age.

    Distinctive mountain formations called aretes and horns are the result of glacial activity. An arête is a sharp ridge of rock that forms when two glaciers collide. Each glacier erodes a glacial valley on either side of the arête. Glacier National Park in the U.S. state of Montana is filled with deep glacial valleys and sharp arêtes.

    An arête where three or more glaciers meet to form a peak is called a horn. These tall, singular landforms are also called pyramidal peaks. The Matterhorn in Switzerland and Italy (and its copy in Disneyland, California) is a glacial horn.

    Roche moutonnee is a smooth, rounded rock formation created as a glacier crushes and rearranges rocks in its path. Roche moutonnee is visible in many hilly areas as outcroppings of flat rock.

    In contrast to alpine glaciers, ice sheets do not create landscape features as they spread. They tend to smooth out the land beneath them.

    People and Glaciers

    Glaciers provide people with many useful resources. Glacial till provides fertile soil for growing crops. Deposits of sand and gravel are used to make concrete and asphalt.

    The most important resource provided by glaciers is freshwater. Many rivers are fed by the melting ice of glaciers. The Gangotri Glacier, one of the largest glaciers in the Himalayan Mountains, is the source of the Ganges River. The Ganges is the most important source of freshwater and electricity in India and Bangladesh. (Electricity is created by dams and hydroelectric power plants along the Ganges.)

    Many companies link glacial water to clean, fresh taste. Because water has been trapped in the glacier for so long, many people believe it has not been exposed to pollutants that liquid water is exposed to.

    Glaciers dug basins for most of the world’s lakes and carved much of the Earth’s most spectacular mountain scenery. The dramatic, diverse landscape of Yosemite Valley, California, was sculpted entirely by glaciers during the last Ice Age.

    Threats to Glaciers

    The processes that remove snow, ice, and moraine from a glacier or ice sheet are called ablation. Ablation includes melting, evaporation, erosion, and calving.

    Glaciers melt when ice melts more quickly than firn can accumulate. Earth’s average temperature has been increasing dramatically for more than a century. Glaciers are important indicators of global warming and climate change in several ways.

    Melting ice sheets contribute to rising sea levels. As ice sheets in Antarctica and Greenland melt, they raise the level of the ocean. Tons of fresh water are added to the ocean every day. In March 2009, a 160-square-mile piece of the Wilkins Ice Shelf broke off of the Antarctic Peninsula. Large icebergs created by such an event create hazards for shipping.

    Large additions of fresh water also change the ocean ecosystem. Organisms, such as many types of corals, depend on salt water for survival. Some corals may not be able to adjust to a more freshwater habitat.

    The loss of glacial ice also reduces the amount of fresh water available for plants and animals that need fresh water to survive. Glaciers near the Equator, such as those on the tropical island of Papua or in South America, are especially at risk.

    The residents below Chacaltaya Glacier in Bolivia, for instance, depended on the glacier for almost all of their fresh water and electricity. Chacaltaya Glacier provided these resources to La Paz, Bolivia’s largest city. Chacaltaya Glacier was also the world’s highest ski resort. In 2009, Chacaltaya Glacier melted entirely.

    A few glaciers may actually be benefiting from global warming. Although winter temperatures are rising, so is the amount of snowfall in areas like Pakistan’s Upper Indus River Basin. Glaciers are growing quickly there.

    Glaciers in Scandinavia also grew because of increased snowfall in the 1990s. But now the glaciers are retreating, or melting, at a very fast pace.

    Less precipitation also affects some glaciers. In 1912, the glaciers on Tanzania’s Mount Kilimanjaro covered 12 square kilometers (4.6 square miles). In 2009, Kilimanjaro’s alpine glaciers had shrunk to two square kilometers (0.8 square miles). This reduction is the result of few heavy snowfalls.

  • Term Part of Speech Definition Encyclopedic Entry
    ablation Noun

    removal of material from the surface of an object, including melting, evaporation, or erosion.

    accumulate Verb

    to gather or collect.

    alpine glacier Noun

    mass of ice that moves downward from a mountain.

    arete Noun

    sharp mountain ridge created by the collision of two glaciers.

    asphalt Noun

    chemical compound made of dark, solid rocks and minerals often used in paving roads.

    atmosphere Noun

    layers of gases surrounding a planet or other celestial body.

    Encyclopedic Entry: atmosphere
    benefit Verb

    to be helpful or useful.

    boulder Noun

    large rock.

    brittle Adjective

    fragile or easily broken.

    broad Adjective

    wide or expansive.

    bulldozer Noun

    vehicle used for moving large obstacles, such as boulders or trees.

    calving Noun

    process where a glacier cracks and breaks apart.

    century Noun

    100 years.

    cirque Noun

    half-open, amphitheater-like hollow area near the head of a valley or mountainside resulting from glacial erosion.

    city Noun

    large settlement with a high population density.

    clay Noun

    type of sedimentary rock that is able to be shaped when wet.

    cliff Noun

    steep wall of rock, earth, or ice.

    Encyclopedic Entry: cliff
    climate change Noun

    gradual changes in all the interconnected weather elements on our planet.

    Encyclopedic Entry: climate change
    coast Noun

    edge of land along the sea or other large body of water.

    Encyclopedic Entry: coast
    collide Verb

    to crash into.

    compress Verb

    to press together in a small space.

    compression melting Noun

    process by which ice or snow melts under extreme pressure.

    concrete Noun

    hard building material made from mixing cement with rock and water.

    continent Noun

    one of the seven main land masses on Earth.

    Encyclopedic Entry: continent
    continental glacier Noun

    ice sheet that covers an enormous area.

    coral Noun

    tiny ocean animal, some of which secrete calcium carbonate to form reefs.

    country Noun

    geographic territory with a distinct name, flag, population, boundaries, and government.

    crevasse Noun

    deep crack, especially in a glacier.

    Encyclopedic Entry: crevasse
    crop Noun

    agricultural produce.

    Encyclopedic Entry: crop
    crystal Noun

    type of mineral that is clear and, when viewed under a microscope, has a repeating pattern of atoms and molecules.

    dam Noun

    structure built across a river or other waterway to control the flow of water.

    dense Adjective

    having parts or molecules that are packed closely together.

    deposit Verb

    to place or deliver an item in a different area than it originated.

    dirt Noun

    dry earth or soil.

    dome Noun

    shape that is half of a sphere.

    Encyclopedic Entry: dome
    dramatic Adjective

    very expressive or emotional.

    dust Noun

    tiny, dry particles of material solid enough for wind to carry.

    Encyclopedic Entry: dust
    ecosystem Noun

    community and interactions of living and nonliving things in an area.

    Encyclopedic Entry: ecosystem
    electricity Noun

    set of physical phenomena associated with the presence and flow of electric charge.

    embed Verb

    to attach firmly to a surrounding substance.

    Equator Noun

    imaginary line around the Earth, another planet, or star running east-west, 0 degrees latitude.

    Encyclopedic Entry: equator
    erode Verb

    to wear away.

    eruption Noun

    release of material from an opening in the Earth's crust.

    excavate Verb

    to expose by digging.

    extract Verb

    to pull out.

    fall line Noun

    imaginary line along which parallel rivers plunge, or fall.

    Encyclopedic Entry: fall line
    fertile Adjective

    able to produce crops or sustain agriculture.

    firn Noun

    grainy ice that forms glaciers. Also called neve.

    firnification Noun

    process of compacting snow and ice into firn and glaciers.

    flood Noun

    overflow of a body of water onto land.

    Encyclopedic Entry: flood
    forest Noun

    ecosystem filled with trees and underbrush.

    fracture Verb

    to break.

    freshwater Noun

    water that is not salty.

    fuse Verb

    to combine or meld together.

    galloping glacier Noun

    glacier that moves several feet an hour. Also called a surge glacier.

    Ganges River Noun

    (2,495 kilometers/1,550 miles) river in Southeast Asia emptying into the Bay of Bengal. Also called the Ganga.

    Gangotri Glacier Noun

    large glacier in the Himalaya Mountains, the source of the Ganges River.

    geologist Noun

    person who studies the physical formations of the Earth.

    glacial erratic Noun

    rock, deposited by a glacier, that differs from the geology and landscape in which it is found.

    glacial period Noun

    time of long-term lowering of temperatures on Earth. Also known as an ice age.

    glacial retreat Noun

    process by which glaciers melt faster than precipitation can replace the ice.

    glacial valley Noun

    depression in the earth created by a moving glacier.

    glacier Noun

    mass of ice that moves slowly over land.

    Encyclopedic Entry: glacier
    global warming Noun

    increase in the average temperature of the Earth's air and oceans.

    Encyclopedic Entry: global warming
    granular Adjective

    made up of small, rounded pellets or grains.

    gravel Noun

    small stones or pebbles.

    gravity Noun

    physical force by which objects attract, or pull toward, each other.

    ground moraine Noun

    materials such as earth and gravel deposited by a glacier as it retreats.

    hanging glacier Noun

    glacier that forms at the top of a mountain or valley but doesn't descend to the main glacier or valley below, instead calving in avalanches and icefalls to the valley or main glacier below.

    hazard Noun

    danger or risk.

    helipad Noun

    take-off and landing area for helicopters.

    hill Noun

    land that rises above its surroundings and has a rounded summit, usually less than 300 meters (1,000 feet).

    Encyclopedic Entry: hill
    Himalaya Mountains Noun

    mountain range between India and Nepal.

    horn Noun

    mountain formation where three or more glaciers meet to form a peak. Also known as a pyramidal peak.

    hydroelectric power Noun

    usable energy generated by moving water converted to electricity.

    ice Noun

    water in its solid form.

    Encyclopedic Entry: ice
    ice age Noun

    long period of cold climate where glaciers cover large parts of the Earth. The last ice age peaked about 20,000 years ago. Also called glacial age.

    iceberg Noun

    large chunks of ice that break off from glaciers and float in the ocean.

    Encyclopedic Entry: iceberg
    ice core Noun

    sample of ice taken to demonstrate changes in climate over many years.

    ice pellet Noun

    rain that freezes as it falls to Earth. Also called sleet.

    ice sheet Noun

    thick layer of glacial ice that covers a large area of land.

    Encyclopedic Entry: ice sheet
    Krakatoa Noun

    island in Indonesia, site of major volcanic eruption in 1883. Also called Krakatau.

    lake Noun

    body of water surrounded by land.

    landscape Noun

    the geographic features of a region.

    Encyclopedic Entry: landscape
    lateral moraine Noun

    material deposited at the edges of a glacier.

    Matterhorn Noun

    (4,478 meters/14,691 feet) mountain in the Alps, on the border between Switzerland and Italy.

    medial moraine Noun

    material that is built up where two glaciers meet to form a new glacier.

    military Noun

    armed forces.

    moraine Noun

    material, such as earth, sand, and gravel, transported by a glacier.

    Encyclopedic Entry: moraine
    moulin Noun

    vertical shaft in a glacier created by surface water falling through a crack in the ice.

    mountain Noun

    landmass that forms as tectonic plates interact with each other.

    mountaineer Noun

    someone who climbs mountains.

    ocean Noun

    large body of salt water that covers most of the Earth.

    Encyclopedic Entry: ocean
    ogive Noun

    small ridges on top of a glacier that resemble waves.

    outcropping Noun

    layer of bedrock visible above the surface of the Earth.

    paleoclimatology Noun

    study of the atmosphere of prehistoric Earth.

    plain Noun

    flat, smooth area at a low elevation.

    Encyclopedic Entry: plain
    plastically Adverb

    in a moldable, alterable manner.

    Pleistocene Noun

    epoch lasting from about 2 million years ago to 12,000 years ago.

    plow noun, verb

    tool used for cutting, lifting, and turning the soil in preparation for planting.

    pollutant Noun

    chemical or other substance that harms a natural resource.

    precipitation Noun

    all forms in which water falls to Earth from the atmosphere.

    Encyclopedic Entry: precipitation
    prong Noun

    pointed end of a fork, antler, or other tool.

    pyramidal peak Noun

    mountain formation where three or more glaciers meet to form a peak. Also known as a horn.

    rake Noun

    tool with pointed prongs for loosening soil.

    river Noun

    large stream of flowing fresh water.

    Encyclopedic Entry: river
    roche moutonnee Noun

    smooth, rounded rock formation created by glaciation.

    rock Noun

    natural substance composed of solid mineral matter.

    Scandinavia Noun

    region and name for some countries in Northern Europe: Iceland, Norway, Sweden, Finland, and Denmark.

    sea level Noun

    base level for measuring elevations. Sea level is determined by measurements taken over a 19-year cycle.

    Encyclopedic Entry: sea level
    shipping Noun

    transportation of goods, usually by large boat.

    ski resort Noun

    facility where people can ski for recreation or sport.

    smoke Noun

    gases given off by a burning substance.

    snout Noun

    end of a glacier.

    snowfall Noun

    amount of snow at a specific place over a specific period of time.

    soil Noun

    top layer of the Earth's surface where plants can grow.

    stream Noun

    body of flowing fluid.

    striation Noun

    tiny, straight groove left in rock from sediment trapped in a moving glacier.

    supraglacial moraine Noun

    material that builds up on the surface of a glacier.

    tarn Noun

    mountain lake formed by a melting glacier.

    temperature Noun

    degree of hotness or coldness measured by a thermometer with a numerical scale.

    Encyclopedic Entry: temperature
    terminal moraine Noun

    material deposited at the end of a glacier. Also called an end moraine.

    terrain Noun

    topographic features of an area.

    tidewater glacier Noun

    mass of moving ice that eventually reaches the ocean.

    till Noun

    rock, earth, and gravel left behind by a retreating or melting glacier.

    tropical Adjective

    existing in the tropics, the latitudes between the Tropic of Cancer in the north and the Tropic of Capricorn in the south.

    trough Noun

    a gently sloping depression in the ocean floor.

    valley Noun

    depression in the Earth between hills.

    volcano Noun

    an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.

    Encyclopedic Entry: volcano
    water Noun

    chemical compound that is necessary for all forms of life.

    waterfall Noun

    flow of water descending steeply over a cliff. Also called a cascade.

    Encyclopedic Entry: waterfall
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