The term thermokarst refers to characteristic landforms which result from thawing of ice-rich permafrost or the melting of massive ice. On Earth thermokarst occurs extensively in arctic areas, and, on a smaller scale, also in mountainous areas such as the Himalaya and the Swiss Alps. It is characterised by an irregular hummocky topography, where irregular pits and depressions develop by thaw settlement in an otherwise smooth topography. In areas underlain by permafrost, these depressions are usually occupied by a thermokarst lake (also called thaw lake), as the meltwater cannot drain away. Even small surface irregularities can start thermokarst processes and create thermokarst lakes. Water initially pools in a depression and its presence begins to thaw the permafrost beneath. As thaw continues along lake margins, the lake extends. Thaw extension continues until intervening higher ground is breached, interconnecting different lakes and eventually creating an outlet channel.
As arctic areas are very sensitive to environmental change, even small changes in regional conditions such as climate can be made clearly visible by thermokarst processes. Therefore thermokarst landforms and lakes can be very useful as geo-indicators, especially to chart the effects of global warming which disproportionally affects arctic areas.
But also human activity, such as the presence of a road or drilling activity may initiate the development of thermokarst terrain, either by damaging the surface or by temporarily providing a heat source which initiates local thawing.
Evidence for past thermokarst activity can also be found in the sedimentary record (a hint for those of you still looking for a sexy specialism: you might consider thermokarst sedimentology — the study of the sedimentary processes and facies associated with thermokarst), and provides evidence for abrupt climate warming during glacialinterglacial transitions, which enhances thermokarst activity in areas of ice-rich permafrost. The ensuing thaw-related processes of melt-out, soft-sediment deformation and resedimentation produce typical thermokarst sediments and sedimentary structures. For example, many of the thermokarst sediments and sedimentary structures in the Tuktoyaktuk Coastlands, western Arctic Canada, formed as a result of rapid climate warming during the last glacialinterglacial transition.
Apart from ice-rich permafrost, there are additional environments which may give rise to thermokarst terrain. One of those is a proglacial outwash plain which may lead to the formation of glacial thermokarst, where kettle-holes result from the thawing of fossil ice blocks overlain by moraines. Another environment are floodplains which in cold climates may be covered by large amounts of ice-rich sediment, for example after an ice-dammed lake drains in a catastrophic event. Subsequent melting of the ice contained in the sediments then results in alluvial or fluvial thermokarst. Good examples of the latter can be found in Siberia and of course — as these are the best of times, and the worst of times, depending of the variety of English one speaks — on Mars.
Thermokarst on Mars was first described during the seventies. Ares Vallis, which is part of a compex of outflow channels at the southern margin of Chryse Planitia, has a topography which is very similar to certain areas in Siberia and which is typical of alluvial thermokarst terrain. Almost half of the terrain in both cases consists of alas valleys (or alases), which are steep-sided depressions with flat floors developed by the preferential thawing of icy beds in the alluvial sediments. Alas floors typically have a chaotic topography, while their slopes develop thermocirques and landslides through backwearing.
Paul De Schutter