A retrogressive thaw slump (RTS) is a type of landslide that occurs in the terrestrial permafrost region of the circumpolar Northern Hemisphere when an ice-rich section thaws. RTSs develop quickly and can extend across several hectares (then often called 'megaslump') modifying Arctic coastlines and permafrost terrain. They are the most active and dynamic feature of thermokarst—the collapse of the land surface as ground ice melts. They are thermokarst slope failures due to abrupt thawing of ice-rich permafrost or glaciated terrains. These horseshoe-shaped landslides contribute to the thawing of hectares of permafrost annually and are considered to be one of the most active and dynamic features of thermokarst. They are found in permafrost or glaciated regions of the Northern Hemisphere—the Tibetan Plateau, Siberia, from the Himalayas to northern Greenland, and in northern Canada's Northwest Territories (NWT), the Yukon Territories, Nunavut, and Nunavik and in the American state of Alaska. The largest RTS in the world is in Siberia—the Batagaika Crater, also called a "megaslump"—is one kilometre long and 100 metres (330 ft) deep and it grows a 100 feet (30 m) annually. The land began to sink, and the Batagaika Crater began to form in the 1960s, following clear-cutting of a section of forested area.
Development and formation
As ice-rich permafrost and glacial terrain thaws, the melting ground ice causes the land surface to collapse through a series of processes resulting in the formation of an irregular land surface, called thermokarst, composed of hummocks and hollows. Retrogressive thaw slumps are the "most active geomorphic features of thermokarst permafrost terrain". Permafrost dates back hundreds of thousands of years; thermokarst and its features—such as retrogressive thaw slumps—which are slope failures, have been initiated by terrain disturbance associated with clear-cutting forests, the construction of seismic lines and roads. The retrogressive thaw slump forms on massive ice or ice-rich permafrost, which is often covered in a layer of tundra vegetation under which a layer of peat may lie. The RTS surface is convex and is located on the shoulder of the hillslope. The most thawing occurs on south- and west-facing slopes. Ballantyne describes how, as scarp ice thaws it causes rapidly evolving retrogressive slope failure or slumping. This landslide "exposes a fresh face" of ice-rich permafrost. As thawing takes place, the ice-rich, steep, erosional headscarp retreats as it collapses. An active layer of basal sediment accumulates flowing down a low-gradient slump floor. This flows downslope as it "collapses to the base of the exposure". The floor or base of the retrogressive thaw slump is covered in then sediment—mudflows and braided hills. As the headscarp progressively retreats, the slump floor extends. As the slopes thaw, the ice-rich permafrost is exposed and turns into a mud slurry. "Thermokarst processes may cause lakes to enlarge, peatlands to collapse and landslides or thaw slumps to develop." "Retrogressive thaw slumps are among the most active geomorphological features in permafrost terrain." A 2009 study classified slumps as active, stable, and ancient. An active slump is one that has a clearly defined headwall and bare areas; and a stable slump is one that has clearly defined boundaries and is completely covered in vegetation. The headwall relief of an ancient slump is a subdued scar on the terrain that is covered in tundra vegetation.
Geomorphic terminology RTS morphology comprises a vertical headwall, an inclined headscarp, a floor filled with flow deposits, and a lobe that conveys thawed sediments downslope. RTS morphology comprises the headwall, headscarp; a floor, and a lobe. The vertical headwall is steep and ice-rich; the downsloped headscarp is a "low-angled scar zone" composed of thawed slurry; the lobe is a tongue of debris in active slumps which is composed of saturated materials that flowed downslope. Retrogressive thaw slumps are slope failures due to abrupt thawing of ice-rich permafrost. They have also been called ground-ice slumps, thermocirques, tundra mudflows, retrogressive flow slides, and bi-modal flows. These terms are no longer recommended by the National Snow and Ice Data Center (NSIDC). Yedoma are deposits of highly organic-rich and ice-rich permafrost with ice content representing from 50 to 90% of its volume. Much of the yedoma deposits have been frozen since 10,000 years ago, in the Pleistocene age. As of 2011, the Yedoma domain covered 1,000,000 square kilometres (390,000 mi2; 100,000,000 ha; 250,000,000 acres) of the northern permafrost zone, mostly in Siberia, including northern Yakutia, and also in Alaska, and in the north of Canada, including the Yukon Territories. A retrogressive thaw slump is a slow landslide caused by thawing yedoma. Because yedoma deposits are ice-rich, they are "especially prone to rapid-thaw processes" and "highly vulnerable to disturbances such as thermokarst and thermo-erosion processes". According to the definition of the Multi-Language Glossary of Permafrost and Related Ground-Ice Terms compiled by the International Permafrost Association (IPA)'s Terminology Working Group, "retrogressive thaw slumps consist of a steep headwall that retreats in a retrogressive fashion due to thawing, and a debris flow formed by the mixture of thawed sediment and meltwater that slides down the face of the headwall and flows away."
Geographic distribution Retrogressive thaw slumps are forms of the permafrost or glaciated regions and may be found in the Northern Hemisphere and the Tibetan Plateau, from the Himalayas to northern Greenland, in northern Canada and Alaska. RTSs "are commonly found on the banks of northern rivers and lakes and along the arctic coast, especially where undercutting is active."
Alaska
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