PINGO

dome-shaped hill formed in a permafrost area when the hydrostatic pressure of freezing groundwater causes the upheaval of a layer of frozen ground. Pingos may be up to 90 metres (300 feet) high and over 800 metres (1/2 mile) across and are usually circular or oval. The core, which may be only slightly smaller than the pingo itself, consists of a lens of clear, injected ice. Modern pingos occur in the continental tundras and are generally restricted to latitudes of 65 to 75 N. Rupture of the overlying material at the top of the pingo exposes the ice to melting and may create a smaller crater and lake. Two types are recognized, the open-system pingo and the closed-system pingo. The open-system pingo forms in regions of discontinuous or thin permafrost. Artesian pressure builds up under the permafrost layer, and as the water rises, pushing up the overlying material, it freezes in a lens shape. This variety of pingo is most frequently found in the alluvial material of a mountainous or hilly area. The closed-system pingo forms in a shallow lake when advancing permafrost generates hydrostatic pressure under the lake basin. The confined mass of saturated soil freezes, pushing the overlying material upward as it expands. Scars of former pingos have been found in areas near the edges of former Pleistocene ice sheets. Because pingos form under specific conditions, they serve as good indicators of climatic change.