Active Layer at the Soil Surface

The arctic soil is so cold that the ground beneath the tundra surface remains frozen all year. This permanently frozen ground is called permafrost. Each summer, when the sun warms the tundra surface, 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.

Beneath the tundra in the northern part of the Arctic Refuge, the active layer thaws to about 18 inches (about half a meter) below the surface. Farther south, the active layer thaws deeper. The active layer also thaws deeper beneath lakes, rivers, and roads because the water and bare soil absorb and hold more summer heat.

This web page includes:

Steps showing the thawing and freezing cycle:

Under the tundra in the northern part of the Arctic Refuge, the active layer begins to thaw after the winter snows melt in early summer. Snow can fall in any month, but it doesn't usually stay on the ground until September. What date do you think the active layer is melted deepest? You may be surprized...

Here's how the active layer changes throughout the year (These dates are averages. The actual dates on any year may be slightly earlier or later.):

On May 15, the tundra soil is still buried under winter snow, and the active layer is completely frozen.

By June 1, the snow has begun to melt, but the active layer remains frozen.

On June 15, the active layer is already thawed half way (50%) to its maximum depth. It is about 9 inches (23 cm) deep. The sun remains above the horizon for 24 hours each day, and plants are producing new growth.

By July 1, the active layer has thawed to 75% of its total depth. It is about 13.5 inches (34.5 cm) deep.

On July 15, the active layer has thawed to 85% of its total depth. It is about 15 inches (38 cm) deep.

On August 1, the active layer has thawed to 90% of its total depth. It is about 16 inches (40.5 cm) deep.

By August 15, the active layer has thawed to 92% of its total depth. It is about 16.5 inches (42 cm) deep. Autumn has come to the tundra. The air is cooler, and plants are finishing their growth for the year.

On September 1, the active layer has thawed to 94% of its total depth. It is about 16.75 inches (42.5 cm) deep. Air temperatures are now below freezing, and the soil surface begins to refreeze.

On September 15, the active layer has thawed to 96% of its total depth. It is about 17.25 inches (43.25 cm) deep. The soil surface continues to freeze down into the active layer.

By October 1, the active layer has thawed to 98% of its total depth. It is about 17.75 inches (44.5 cm) deep.

On October 15, the active layer has thawed to its maximum depth of 18 inches (about 45.5 cm). The soil above it is refreezing rapidly. Did you guess that the active layer would reach its maximum depth in mid October?

By November 1, there is just a narrow region above the base of the active layer that is not frozen.

By November 15, the active layer has completely frozen, and will remain this way until early summer.

Animation of the cycle:

Active layer in animation (watch the base of the active layer carefully):