One of Alaska’s natural wonders is found underfoot. It is permafrost, ground that remains frozen year after year. Made up of soil and rocks as well as frozen water, permafrost forms when the depth of winter freezing exceeds the depth of summer thawing.
Permafrost is thickest in arctic Alaska north of the Brooks Range, but it is found to some extent beneath nearly 85 percent of Alaska. On the arctic coastal plain it extends as much as 2,000 feet below the surface and is found virtually everywhere. From the Brooks Range southward its thickness gradually decreases and it becomes more and more discontinuous, broken by taliks, pockets of unfrozen ground. Near Anchorage, permafrost is found only in isolated patches, and in Southeast Alaska it is found only high in the mountains.
Much of the permafrost in Alaska is tens of thousands of years old. In arctic and interior Alaska, river erosion and gold mining have revealed the remains of now-extinct animals from the last great ice age 100,000 to 10,000 years ago, when animals such as woolly mammoths, mastodons, lions, and saber-toothed cats roamed what is now Alaska. Pingos and ice wedges are demonstrations of how permafrost has changed the landscape.
Many of the grasses, flowers, and berries of the arctic tundra owe their existence to the presence of permafrost. With only a few inches of precipitation a year, arctic Alaska could well be a barren desert. But permafrost forms a frozen floor beneath seasonally thawed ground, which can be from several inches to a few feet deep. Rainfall and snowmelt cannot percolate or drain off. Instead, water collects at surface, providing moisture to nourish plants and forming innumerable shallow lakes and ponds. Tundra plants, in turn, insulate the permafrost beneath them from thawing. They seal out the warm temperatures and sunlight of summer so the permafrost remains frozen.
Melting of permafrost can pose problems for humans and their activities. If overlying vegetation is removed or disturbed, its insulating qualities are lost and the permafrost begins to melt. Waterlogged ground becomes soft and collapses. Buildings and roads may slump or tilt, and vehicles bog down in mud.
Alaskans have developed innovative techniques for building on permafrost so it will not melt. Houses in permafrost areas are frequently built on pilings so they will not transfer heat to the ground. Floors may be insulated. Water and sewer pipes are installed above ground.
Engineering for the Trans Alaska Pipeline was complicated by the fact that three-fourths of the line, which carries oil at temperatures between 82 - 116 degrees Fahrenheit, would extend across land underlain by permafrost. To prevent melting of the permafrost, more than half the pipeline was built aboveground on elevated supports, some of which are refrigerated. In places where buried pipe extends across unstable permafrost, it is wrapped with insulation, and in a few locations the pipe is insulated and buried in refrigerated ditches.