Guest essay by Don Easterbrook
A crater in northern Siberia, spotted by a passing helicopter, has received worldwide attention and continues to be a top news story. Since then, two more mysterious holes have been discovered elsewhere in the region. Now the new holes, smaller in diameter but similar in shape – are posing a fresh challenge for Russian scientists, according to the The Siberian Times. Theories range from meteorites to an explosion of methane due to global warming.
Figure 1. Yamal ‘mystery crater.’ (Siberian Times)
Anna Kurchatova of the Sub-Arctic Scientific Research Centre said the crater was formed by a mixture of water, salt, and gas igniting an underground explosion as result of global warming. Kurchatova suggests that global warming may have caused an ‘alarming’ melt in the under-soil ice and released gas, causing an effect like the popping of a champagne bottle cork. ‘The version about melting permafrost due to climate change, causing a release of methane gas, which then forces an eruption is the current favorite, though scientists are reluctant to offer a firm conclusion without more study.’
Scientists with the Russian Academy of Sciences Institute of the Earth Cryosphere, which is leading the investigation, suggested that the holes formed when melting permafrost triggered an explosion of methane gas. That theory was bolstered when an icy lake was found at the bottom of the hole. Andrei Plekhanov from Scientific Research Center of the Arctic said the crater appears to be made up of 80 percent ice, which adds to the theory that it was caused by the effects of global warming.
Dr. Plekhanov said: “I’ve never seen anything like this, even though I have been to Yamal many times.”
WHAT IS REALLY GOING ON HERE?
Actually, these craters are not mysterious at all—there are hundreds of them all over the Yamal Peninsula and their origin has been well known for many years.
Figure 2. Craters of the Yamal Peninsula. The ice cores have completely melted out, leaving lakes. The surrounding ridges are still visible. (Google Earth)
As you can see from the images in Figure 2, there are hundreds of these craters, mostly not as fresh as the recent ones, but showing the same features—a depression surrounded by a ring of raised ground. These are pingos!!
Pingo is an Inuit term for an isolated, dome shaped hill, used to describe large ice-cored mounds found in the permafrost regions of Siberia and various other places in the Arctic. Pingos range in height from a few meters to more than 40 m (130 ft) and from a few meters to 1,000 m (3,300 ft) in diameter. Small pingos typically have rounded tops, but larger ones are commonly broken open at the top where melting of the ice core forms a crater resembling a volcanic cone Where they occur in stratified silt or sand, the beds commonly dip outward from the center, much like those adjacent to an intrusive body. The ice in the core of a pingo is typically massive and of segregation/injection origin. Tension fractures are common at the summit of the mound, but expansion of pingo ice is rare and short-lived. Ice up to 7 m (23 ft) thick has been found in pingos of Sweden. As the ice core melts, a small freshwater lake may occupy the summit crater that forms.
Open system pingos
Open system pingos form where groundwater under artesian pressure beneath thin permafrost forces its way upward and freezes as it approaches the surface where it forms an ice core that heaves the surface upward. Although the initial growth of these types of pingos may occur where ice lenses lie above the water table, their continued growth requires a particular combination of hydrostatic pressure and soil permeability. Thin, discontinuous permafrost and artesian water pressure play important roles in the development of open system pingos. The role of artesian pressure is not to force the overlying sediments upwards but rather to provide a slow, regular supply of groundwater to the growing ice core.
Most open-system pingos are oval or oblong in shape and typically occur as isolated mounds or in small groups developed in either soil or bedrock. Rupturing near their top is common. Concentrations of open-system pingos occur in Siberia, the northern interior of the Yukon, Alaska, Spitsbergen, and Norway.
Closed system pingos
When a lake in a permafrost environment is progressively drained and covered by encroachment of vegetation from the margins, the permafrost table progressively rises to the level of the former lake floor. The rising permafrost table expels pore water ahead of the freezing front, and when the pore water pressure exceeds the overburden strength, upward heaving of the frozen ground occurs as the ice core progressively grows. The size and shape of the resulting pingo typically reflects that of the original body of water.
Closed system pingos vary in height from a few meters to over 60 m (~200 ft) and up to 300 m (~1000 ft) in diameter, ranging from symmetrical conical domes to asymmetric and elongate hills. The top of the pingos are commonly ruptured to form small, star like craters that eventually form shallow-rimmed depressions as the ice core melts.
The mechanism of pingo formation in a closed system starts with a deep, ice-covered lake, surrounded by permafrost. The lake inhibits the development of permafrost beneath it, and the ground remains unfrozen. As the lake is slowly drains or is filled with sediment, at some point the lake ice freezes to the bottom, and the bottom sediments begin to freeze. As the layer of ice and permafrost covers former lake floor, a closed system is set up in the still-unfrozen ground beneath because the permafrost cap prevents the escape of groundwater. As permafrost continues inward growth around the unfrozen core, water pressure increases. Pore water is expelled from the unfrozen sediment by the advancing permafrost, and to relieve the pressure, the surface bulges upward. Eventually, all of the water in the enclosed system groundwater mass becomes frozen and the excess water forms a core of clear ice under the bulge.
Growth rate of pingos:
The birth and growth of a small pingo studied by Mackay (1988) is representative of more than 2,000 closed system pingos of the western Canadian Arctic and Alaska. The pingo appeared on the former floor of a lake that drained suddenly about 1900. Small frost mounds began appearing between 1920 and 1930. The pingo grew steadily until 1976, but the growth rate decreased after that. Mackay also monitored the growth of other small pingos in a lake in the Mackenzie Delta region that drained between 1935 and 1950. The pingos grew rapidly in the initial years, commonly 1.5 m/year (5 ft/yr), then decreased. Mackay suggests that about 15 new pingos per century appear in the Mackenzie Delta region, and only about 50 seem to be actively growing. Similar conclusions have been reached by Russian investigators in Siberia.