Post by Drs. Richard Soare, Susan Conway and Peter Grindrod
Closed-system (hydrostatic) pingos (CSPs) are perenially ice-cored (non-glacial) mounds formed primarily by the near-surface injection of pore water. Their shape ranges from circular and sub-circular to elongate and their size varies from a few to hundreds of metres in diameter. Some of them reach tens of metres in height (see Figs. 1a,b).
They evolve and persist only in permafrost – “ground that is frozen for at least two years” – that is continuous and ice-rich – where “the volume of ice in the ground exceeds the total pore-volume that the ground would have under natural unfrozen conditions” (Harris et al., 1988). Most often, they occur where “ice-rich” permafrost has undergone thermal destabilisation and thawed, forming a thermokarst lake. If and when a thermokarst lake loses its water during warm periods, by drainage or evaporation, then during subsequent periods of cold the previously unexposed lake-basin will begin to freeze. As permafrost aggrades from the lake margins in and from the lake-floor down, pore water migrates away from the freezing front (under hydrostatic pressure) and begins to deform the basin floor upwardly at the basin centre. This is where the aggrading permafrost that overlies the pore water is particularly thin and weak. Eventually, the pore-water freezes, creating an ice-cored mound. Often, polygonised terrain, formed by thermal contraction and underlain at polygon margins by ice-wedges, cross-cuts the permafrost that surrounds and overlies the pingos.
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