ABSTRACT
Continuous and discontinuous zones of per - manently frozen ground, known as permafrost, currently underlie some 25 per cent of the Earth’s land surface. Permafrost is soil or rock that remains frozen for two or more consecutive years. It is not the same as frozen ground, as depressed freezing points allow some materials to stay unfrozen below 0°C and considerable amounts of liquid water may exist in frozen ground. Permafrost underlies large areas of the Northern Hemisphere Arctic and subarctic. It ranges from thin layers that have stayed frozen between two successive winters to frozen ground hundreds of metres thick and thousands of years old. It develops where the depth of winter freezing is greater than the depth of summer thawing, so creating a zone of permanently frozen ground.
