On a cold day, bare earth crunches underfoot. This is because ice needles have formed, pushing the soil upwards, and the pressure of a footstep crushes them down again. In frozen regions, a similar effect on a grand scale produces unusual hillocks known as pingos.
Pingos can be as much as 50 metres high. They typically start in old lake beds. Freezing water forms into a lens shape and expands as it becomes ice. As more water is added and the ice mass becomes larger, the ground above is gradually forced upwards into a mound. This can grow at the rate of a few centimetres a year.
Pingos are conspicuous landmarks in otherwise flat landscapes, and useful vantage points for spotting caribou herds or whale offshore; the name comes from an Inuit language. A quarter of the world’s pingos are in Canada’s Northwest Territories, where there is a National Landmark dedicated to preserving them.
Pingos meet their end when they expand so much that the crust above them breaks open. Sunlight melts the ice, water flows out, and the pingo collapses. This leaves what looks like a miniature volcanic crater known as a pingo scar.
A rash of craters appearing in Siberia in 2014 were initially thought to be the result of meteorite strikes or underground gas explosions. Pingo collapse turned out to be a more likely explanation.
Geologists have identified ancient pingo scars in Ireland and in Norfolk, remnants of Britain’s glacial adolescent pimples.
