Write a brief note on Permafrost

When the temperature of a layer below ground surface is less than 0°C throughout the year, for several years, the earth material there will be called permafrost.

About one-fifth of the land regions of the Northern Hemisphere is believed to be under permafrost. Permafrost is believed to cover about half of the areal extent of Canada, Alaska and Russia, most of Antarctica and Greenland. Permafrost may be continuous nearer the polar areas and discontinuous away from them. According to Mackay, continuous permafrost covers Baffin Island, most of Greenland, extreme northern parts and islands of Canada, west of the Hudson Bay and northern parts of Alaska and Northern Siberia extending from the Gulf of Anadir in the east to the Gulf of Ob in the west.

Discontinuous permafrost occurring south of the zones of continuous permafrost occupies the rest of Alaska save the southern maritime coastal belt, and an east- west middle Canadian belt extending up to Labrador coast approximately north of 51 degree north.

In Asia, the zone of discontinuous permafrost extends north of the neck of Kamchatka Peninsula and extends farthest south in eastern Siberia up to the Baikal lake whence westwards it contracts northwards reaching the Arctic coast near Novaya Zemlya.

The thickness, according to Muller, of permafrost generally decreases with latitude. It is, according to Muller, 200-400 meters thick along latitude 70 degree north. Decreasing southward, the thickness falls to about 50 meters about the latitude 51 degree north. According to Brown, the thickness of permafrost is the maximum (over 500 meters) in Taimir Peninsula.

We have mentioned above two types of permafrost—continuous and discontinuous. According to Ray and others there may be third type—sporadic permafrost. Continuous permafrost has no interruptions except under deep-water bodies of lakes, rivers or inlets of the sea. Discontinuous permafrost is marked by small unfrozen patches of ground. Sporadic permafrost occurs in the regions of extensively unfrozen ground as relatively small isolated patches.

What has been shown as zone of discontinuous permafrost in Canada by Mackay includes the zone of sporadic permafrost shown by Brown. In other words, the southern part of discontinuous permafrost in Canada has been recognized as a zone of sporadic permafrost. According to Black, the zone of sporadic permafrost in North America is somewhat different from that shown by Brown. It extends more south, up to 50 degree north near the northern shore of Lake Superior and extends a lobe along the Rockies up to about 38 degree north. In Eurasia, the zone of sporadic permafrost extends up to about 50 degree north between 120 degree east and 90 degree east. Further, west it narrows down towards the coast west of Novaya Zemlya. Eastward its southern limit crosses the coast near the Amur delta.

According to Mackay, there is a fourth type of permafrost. It is sea bottom permafrost and occurs in the southern parts of the East Siberian Sea and Laptev Sea. This is the largest zone of sea bottom permafrost in the Northern Hemisphere. In North America, there is a small patch of sea bottom permafrost in the Mackenzie Bay.

As to the origin of permafrost, it is believed to have come into existence during the Pleistocene Ice Age. Whether it continued through the warmer Quaternary interglacial intervals is not certain. It is, however, believed that permafrost is compatible with the present climate, which can support or form it.

It is estimated that about four-fifths of the volume of permafrost is ice. The ice may be in the form of interstitial ice but it may also take die form of large masses of pure ice as much as 30 meters or more across.