Permafrost is a permanently frozen layer on or under Earth's surface. It consists of soil, gravel, and sand, usually bound together by ice. Permafrost usually remains at or below 0°C (32ºF) for at least two years.

Permafrost can be found on land and below the ocean floor. It is found in areas where temperatures rarely rise above freezing. This means permafrost is often found in Arctic regions such as Greenland, the U.S. state of Alaska, Russia, China, and Eastern Europe.

Permafrost thickness can range from one meter (about three feet) to more than 1,000 meters (about 3,281 feet). Permafrost covers approximately 22.8 million square kilometers (about 8.8 million square miles) in Earth's Northern Hemisphere. Frozen ground is not always the same as permafrost. A layer of soil that freezes for more than 15 days per year is called "seasonally frozen ground." A layer of soil that freezes between one and 15 days a year is called "intermittently frozen ground." Permafrost is frozen for two years or more.

Permafrost does not always form in one solid sheet. There are two major ways to describe its distribution: continuous and discontinuous.

Continuous permafrost is just what it sounds like: a continuous sheet of frozen material. Continuous permafrost extends under all surfaces except large bodies of water in the area. The part of Russia known as Siberia has continuous permafrost.

Discontinuous permafrost is broken up into separate areas. Some permafrost, in the shadow of a mountain or thick vegetation, stays all year. In other areas of discontinuous permafrost, the summer sun thaws the permafrost for several weeks or months. The land near the southern shore of Hudson Bay, Canada, has discontinuous permafrost.

Scientists who study permafrost are able to understand changes in Earth's climate by observing changes in permafrost. Studies show Earth's permafrost warmed by 6ºC during the 20th century. Scientists predict widespread thawing of permafrost by 2100.

When the temperature of permafrost rises above 0ºC, it may thaw, in which case any ice it contains will undergo a phase change from solid to liquid (i.e., it will melt). Generally, the mineral and organic components of the former permafrost will remain solid. Therefore, in regions with more ground ice, thawing permafrost may result in more of a change to the land surface, and vice versa.

Thawing permafrost can raise water levels in Earth's oceans and increase erosion. Erosion happens when permafrost thaws because soil and sediment are easily washed away without ice binding them together.

Permafrost

Lakes and marshes in the Arctic are often shallow due to permafrost.

Permafrost Architecture
Though people do live in permafrost regions, such as Siberia, Canada, and Alaska, building on top of permafrost can be risky. Buildings raise the temperature of the ground beneath them, which can thaw permafrost and cause the building to sink into the mud. Engineers have found ways to build on top of permafrost without raising the ground temperature. These methods include building on top of wood piles and thick gravel pads.

approximately

Adjective

generally or near an exact figure.

Noun

region at Earth's extreme north, encompassed by the Arctic Circle.

climate

Noun

all weather conditions for a given location over a period of time.

continuous permafrost

Noun

layer of soil that is permanently frozen.

discontinuous permafrost

Noun

frozen layer of soil that thaws during the summer.

Noun

act in which earth is worn away, often by water, wind, or ice.

freezing

Adjective

at or below 0 degrees Celsius (32 degrees Fahrenheit).

gravel

Noun

small stones or pebbles.

Noun

water in its solid form.

intermittently frozen ground

Noun

earth that freezes between one and fifteen days a year.

mountain

Noun

landmass that forms as tectonic plates interact with each other.

Northern Hemisphere

Noun

half of the Earth between the North Pole and the Equator.

Noun

permanently frozen layer of the Earth's surface.

sand

Noun

small, loose grains of disintegrated rocks.

seasonally frozen ground

Noun

earth that freezes more than fifteen days a year, but less than two continuous years.

Noun

solid material transported and deposited by water, ice, and wind.

shadow

Noun

dark area where an object prevents light from reaching a surface.

Siberia

Noun

region of land stretching across Russia from the Ural Mountains to the Pacific Ocean.

soil

Noun

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

Noun

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

vegetation

Noun

all the plant life of a specific place.

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    Writers

    Kim Rutledge
    Tara Ramroop
    Diane Boudreau
    Melissa McDaniel
    Santani Teng
    Erin Sprout
    Hilary Costa
    Hilary Hall
    Jeff Hunt

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    Mary Crooks, National Geographic Society
    Tim Gunther

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    Jeannie Evers, Emdash Editing
    Kara West

    Educator Reviewer

    Nancy Wynne

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    Caryl-Sue, National Geographic Society

    Sources

    Dunn, Margery G. (Editor). (1989, 1993). "Exploring Your World: The Adventure of Geography." Washington, D.C.: National Geographic Society.

    Last Updated

    Feb. 15, 2011

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