Pingos

Question 1

What is a pingo?

Answer 1

A pingo is a mound 100 to 500 meters in diameter which may rise to around 50 meters above the generally flat tundra landscape.

Question 2

Why is proximity to water important to the formation of pingos?

Answer 2

Proximity to water is important to the formation of pingos as at the centre of each pingo there is usually an ice core.

Two theories have been suggested for their formation based on the mechanism by which water is supplied to the growing core.

Question 3

Where are closed-system pingos generally found?

Answer 3

Closed-system pingos are generally found in lowland areas where permafrost is continuous.

Question 4

Describe the formation of a closed-system pingo?

Answer 4

• A large, deep lake insulates, even in winter, the ground below, creating an unfrozen layer, a talik.
• Gradual infilling of the lake by organic matter and sediments reduces its size and depth, enabling freezing to penetrate from the surface and inward growth of permafrost from the sides and below
• This advancing permafrost traps the body of water and puts it under pressure, forcing the overlying sediments upwards when water eventually freezes to form a massive body of ground ice (ice core) which deforms the overlying soil and rock into a pingo

Question 5

Where do open-system pingos occur?

Answer 5

Open-system pingos occur in valley bottoms and in areas of thin or discontinuous permafrost.

Question 6

Describe the size of open-system pingos?

Answer 6

Open-system pingos are generally smaller than the largest closed-system forms.

Question 7

Describe the formation of an open-system pingo?

Answer 7

• Groundwater travels beneath the permafrost from an underlying aquifer (water-bearing rock)
• In places, groundwater is forced through a gap in the permafrost due to pressure from a steady supply of groundwater. As the water rises, it becomes colder and freezes, causing a doming at the surface.
• Freely available groundwater continues to feed the growing ice mass, so the pingo grows from below the ground

Question 8

When will a pingo collapse?

Answer 8

When freeze-thaw cycles cease during warmer climates a threshold is crossed. The ice will melt and the pingo will collapse.

Question 9

What happens once the ice core has melted and the pingo has collapsed?

Answer 9

When the core has melted, a doughnut-shaped mound will remain, possibly containing a small lake in the centre.