Arctic National Wildlife Refuge
Alaska Region   

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Frozen Ground in the Arctic

The arctic tundra contains ground features not found in warmer regions. The arctic is so cold that the ground beneath the tundra surface remains frozen all year. This permanently frozen ground is called permafrost. The soil in the permafrost area remains colder than 32 degrees Fahrenheit (0 degrees Celsius).

If the soil never warmed up, there would be no plants growing in the arctic. When the summer sun warms the tundra surface, however, the top few inches of soil thaw. This melted part is called the active layer. Plant roots grow within the active layer, and insects burrow here.

  • What date do you think the active layer is melted deepest? You may be surprized... Unearth the mysteries of the active layer.

cut-away view of soil beneath tundra, 
showing active layer and ice wedge - USFWSThis is a bluff on the coast of the Arctic Ocean. Waves have eroded the soil, exposing the ground beneath the tundra surface. Frozen soil can contain ice, like this ice wedge, or it can be dry, like the soil around the ice wedge.

ice wedge larger than a man standing nearby - USFWSAnother ice wedge, this one exposed along a river bank.

aerial photo of tundra covered by ponds and polygons - USFWSThe soil buckles and cracks above the ice wedges, causing these polygons to form (in the lower half of the picture).

The long, narrow lake in the center of the picture is a thermokarst lake.

aerial view of polygonized tundra - USFWSA closer view of some arctic polygons. These are about 70 feet (20 meters) across, although polygons may be as small as 10 feet (about 3 meters) across.

Ice wedges form a honeycomb of ice walls beneath the soil surface. Look again at the ice wedge at the beginning of this web page. Notice that on either side of it, other wedges are partially visible in side view.

small pingo on tundra - USFWSPingos form when water moves up under the root mat, and freezes. When water freezes it expands, pushing up the soil. Pingos can be as small as one foot high (one third of a meter) or over 35 feet high (over 10 meters). This one is about 4 feet high (just over one meter).

raised and cracked soil exposing ice layer beneath  - USFWSWhen the soil on a pingo cracks open, and the ice core is exposed, the pingo begins to melt and break up. This is a frost blister, not a true pingo, but it shows what the inner ice and overlapping soil would look like in a pingo.

September 12, 2008