Hummock

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A hummock is a small knoll or mound above ground. [1] They are typically less than 15 meters in height and tend to appear in groups or fields. It is difficult to make generalizations about hummocks because of the diversity in their morphology and sedimentology. [2] An extremely irregular surface may be called hummocky. [3]

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An ice hummock is a boss or rounded knoll of ice rising above the general level of an ice-field. Hummocky ice is caused by slow and unequal pressure in the main body of the packed ice, and by unequal structure and temperature at a later period.

Bog hummocks

Hummocks in the shape of low ridges of drier peat moss typically form part of the structure of certain types of raised bog, such as plateau, kermi, palsa or string bog. The hummocks alternate with shallow wet depressions or flarks.

Swamp hummocks

Swamp hummocks are mounds typically initiated as fallen trunks or branches covered with moss and rising above the swamp floor. The low-lying areas between hummocks are called hollows. [4] A related term, used in the Southeastern United States, is "hammock".

Earth hummocks of cryogenic origin

Cryogenic earth hummocks on Mount Kenya Frost upheaval.jpg
Cryogenic earth hummocks on Mount Kenya

Cryogenic earth hummocks go by various names; in North America they are known as earth hummocks; the Icelandic term þúfa/thúfa (pl. þúfur/thúfur) is also used to describe them in Greenland and Iceland, and the Finnish term pounu (pl. pounut) in Fennoscandia. These cold climate landforms appear in regions of permafrost and seasonally frozen ground. [2] They usually develop in fine-grained soils with light to moderate vegetation in areas of low relief where there is adequate moisture to fuel cryogenic processes. [5]

Cryogenic earth hummocks appear in a variety of cold-ground environments, making the story of their genesis complex. Geologists recognize that hummocks may be polygenetic and form by a combination of forces that are yet to be well understood. [2]

Recent research on cryogenic hummocks has focused on their role as environmental indicators. Because hummocks can both form and disintegrate rapidly (well within a human lifetime) [5] they are an ideal landform to monitor for medium range environmental change. [2] There are several explanations of earth hummock formation.

Cryoexpulsion of clasts

Hummocks may form as a result of clasts migrating to the surface through frost push and pull mechanisms. As the clasts rise they push up on the ground above forming bulging mounds. [2]

Cellular circulation

Hummock excavation normally reveals a disturbed soil profile, often with irregular streaks of organic matter or other colorations suggesting fluidity at some time past. [5] The disturbance, a form of cryoturbation often extends to a depth roughly equal to the hummock’s height. This has been explained by some as the result of convection processes whereby warmer soil and water at depth expands, becomes less dense and rises, while gravity forces denser soil downwards. Circulation has also been explained as driven solely by density of soil material, not temperature induced density changes. [3]

Differential frost heave (cryostatic pressure hypothesis)

This is the most widely accepted explanation of cryogenic hummock genesis. [2] Irregularities in preexisting ground conditions (differences in grain size, ground temperature, moisture conditions of vegetation) cause surface downwards freezing during the winter to spread unevenly. Encroaching frost exerted increasing pressure on the adjacent unfrozen soil. Trapped between the freezing surface soils and the buried permafrost layer the soil material is forced upwards into hummocks. While this is currently the most commonly accepted hypothesis, there is still only limited evidence of this happening. [3]

Hummocks created by debris avalanches

Debris avalanches are caused by sudden collapses of large volumes of rock from the flanks of mountains, especially volcanoes. [6] These events are fast-moving, gravity-driven currents of saturated debris that do not necessarily include juvenile material. [7] Debris avalanche deposits are characterized by the debris-avalanche block (hummocks) and the debris-avalanche matrix. Debris avalanches are diagnosed for landscapes where the volcano has an amphitheater at the source with hummocky terrain downhill. In some cases, such as Mount Shasta in California, the amphitheater has been filled in by later volcanic activity and all that remains are the hummocks. [8]

Debris avalanche blocks are identifiable because they keep their internal stratigraphy. The blocks simply break off the mountain and slide down, completely intact, identifiable because they differ from the surrounding landscape. [7] The volume and height of hummocks is mostly dependent on their location; the closer to the source region, the larger they become. [8] The bottom layer of a debris avalanche deposit is the fine-grained matrix which forms due to the shear at the base of the large, turbulent moving mass. [7]

Related Research Articles

Glacier Persistent body of ice that is moving under its own weight

A glacier is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. Glaciers slowly deform and flow due to stresses induced by their weight, creating crevasses, seracs, and other distinguishing features. They also abrade rock and debris from their substrate to create landforms such as cirques and moraines. Glaciers form only on land and are distinct from the much thinner sea ice and lake ice that form on the surface of bodies of water.

Moraine Glacially formed accumulation of unconsolidated debris

A moraine is any glacially formed accumulation of unconsolidated glacial debris that occurs in both currently and formerly glaciated regions on Earth, through geomorphological processes. Moraines are formed from debris previously carried along by a glacier, and normally consist of somewhat rounded particles ranging in size from large boulders to minute glacial flour. Lateral moraines are formed at the side of the ice flow and terminal moraines at the foot, marking the maximum advance of the glacier. Other types of moraine include ground moraines and medial moraines.

Sedimentary rock Rock formed by the deposition and subsequent cementation of material

Sedimentary rocks are types of rock that are formed by the accumulation or deposition of small particles and subsequent cementation of mineral or organic particles on the floor of oceans or other bodies of water at the Earth's surface. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. Before being deposited, the geological detritus was formed by weathering and erosion from the source area, and then transported to the place of deposition by water, wind, ice, mass movement or glaciers, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.

Permafrost soil frozen for a duration of at least two years

In geology, permafrost is ground, including rock or (cryotic) soil, with a temperature that remains at or below the freezing point of water 0 °C (32 °F) for two or more years. Most permafrost is located in high latitudes, but at lower latitudes alpine permafrost occurs at higher elevations. Ground ice is not always present, as may be in the case of non-porous bedrock, but it frequently occurs, and it may be in amounts exceeding the potential hydraulic saturation of the ground material. Permafrost accounts for 0.022% of total water on Earth, and the permafrost region covers 24% of exposed land in the Northern Hemisphere. It also occurs subsea on the continental shelves of the continents surrounding the Arctic Ocean, portions of which were exposed during the last glacial period.

Volcanic cone Landform of ejecta from a volcanic vent piled up in a conical shape

Volcanic cones are among the simplest volcanic landforms. They are built by ejecta from a volcanic vent, piling up around the vent in the shape of a cone with a central crater. Volcanic cones are of different types, depending upon the nature and size of the fragments ejected during the eruption. Types of volcanic cones include stratocones, spatter cones, tuff cones, and cinder cones.

Galunggung mountain in West Java, Indonesia

Mount Galunggung is an active stratovolcano in West Java, Indonesia, around 80 km (50 mi) southeast of the West Java provincial capital, Bandung. Mount Galunggung is part of the Sunda Arc extending through Sumatra, Java and Bali, which has resulted from the subduction of the Australian plate beneath the Eurasian plate. Galunggung had its first historical eruption in 1822 that produced pyroclastic flows and lahars that killed 4,011 people.

Aeolian processes Processes due to wind activity

Aeolian processes, also spelled eolian or æolian, pertain to wind activity in the study of geology and weather and specifically to the wind's ability to shape the surface of the Earth. Winds may erode, transport, and deposit materials and are effective agents in regions with sparse vegetation, a lack of soil moisture and a large supply of unconsolidated sediments. Although water is a much more powerful eroding force than wind, aeolian processes are important in arid environments such as deserts.

Geologic hazards

A geologic hazard is one of several types of adverse geologic conditions capable of causing damage or loss of property and life. These hazards consist of sudden phenomena and slow phenomena:

Augustine Volcano mountain

Augustine Volcano is a stratovolcano consisting of a central complex of summit lava domes and flows surrounded by an apron of pyroclastic, lahar, avalanche, and ash deposits. The volcano is frequently active, with major eruptions recorded in 1883, 1935, 1963–64, 1976, 1986, and 2006. Minor eruptive events were reported in 1812, 1885, 1908, 1944, and 1971. The large eruptions are characterized by an explosive onset followed by the quieter effusion of lava. It forms Augustine Island in southwestern Cook Inlet in the Kenai Peninsula Borough of southcentral coastal Alaska, 174 miles (280 km) southwest of Anchorage. Augustine Island has a land area of 32.4 square miles (83.9 km2), while West Island, just off Augustine's western shores, has 2 sq mi (5.2 km2). The irregular coastline of Augustine Island is due to the repeated catastrophic collapse of the summit dome, forming debris avalanches down the flanks and into Cook Inlet.

Thermokarst A land surface with very irregular surfaces of marshy hollows and small hummocks formed as ice-rich permafrost thaws

Thermokarst is a land surface characterised by very irregular surfaces of marshy hollows and small hummocks formed as ice-rich permafrost thaws, that occurs in Arctic areas, and on a smaller scale in mountainous areas such as the Himalayas and the Swiss Alps. These pitted surfaces resemble those formed by solution in some karst areas of limestone, which is how they came to have karst attached to their name without the presence of any limestone. Small domes that form on the surface due to frost heaving with the onset of winter are only temporary features. They then collapse with the arrival of next summer's thaw and leave a small surface depression. Some ice lenses grow and form larger surface hummocks, which last many years and sometimes become covered with grasses and sedges, until they begin to thaw. These domed surfaces eventually collapse either annually or after longer periods and form depressions which contribute to uneven surfaces. These are included within the general label of thermokarst.

Pingo A mound of earth-covered ice

A pingo, also called a hydrolaccolith or a bulgunniakh, is a mound of earth-covered ice found in the Arctic and subarctic that can reach up to 70 metres (230 ft) in height and up to 600 m (2,000 ft) in diameter. The term originated as the Inuvialuktun word for a small hill. The plural form is "pingos". The term is also used for depressions, often water filled, formed by the melting of ice at the end of the last glaciation.

Rock glacier Landform of angular rock debris frozen in interstitial ice, former "true" glaciers overlain by a layer of talus, or something in between

Rock glaciers are distinctive geomorphological landforms, consisting either of angular rock debris frozen in interstitial ice, former "true" glaciers overlain by a layer of talus, or something in-between. Rock glaciers may extend outward and downslope from talus cones, glaciers or terminal moraines of glaciers.

A periglacial lake is a lake bordering a glacier, usually found along the fringes of large ice sheets.

Palsa A low, often oval, frost heave occurring in polar and subpolar climates

A palsa is a low, often oval, frost heave occurring in polar and subpolar climates, which contain permanently frozen ice lenses. Like pingos, palsas consist of an ice core with overlying soil, but they are generally smaller than pingos, often occur in groups and may develop from ground water without additional hydrostatic pressure. Palsas are characteristically found in areas with discontinuous permafrost, and in such areas they may be the only reliable surface evidence of permafrost. Surface water, found in bogs, enhances palsa formation in areas called palsa bogs.

Frost boil

A frost boil, also known as mud boils, a stony earth circles, frost scars, or mud circles, are small circular mounds of fresh soil material formed by frost action and cryoturbation. They are found typically found in periglacial or alpine environments where permafrost is present, and may damage roads and other man-made structures. They are typically 1 to 3 metres in diameter.

The permafrost carbon cycle is a sub-cycle of the larger global carbon cycle. Permafrost is defined as subsurface material that remains below 0o C for at least two consecutive years. Because permafrost soils remain frozen for long periods of time, they store large amounts of carbon and other nutrients within their frozen framework during that time. Permafrost represents a large carbon reservoir that is seldom considered when determining global terrestrial carbon reservoirs. Recent and ongoing scientific research however, is changing this view.

A hummock is a mound or knoll, usually of earth or ice; it, with its derivatives hummocks and hummocky, may also refer to:

Tittivilla

Tittivilla is an eroded volcano in the Andes of Bolivia, on the isthmus separating the Salar de Coipasa in the north from the Salar de Uyuni. The maximum slope in its summit area is 30°. The stratovolcano and neighbouring Tunupa formed 3.5-2.5 mya on NE-SW and NNE-SSW structures, it belongs to the Tungapujo-Guachacoyo volcanic chain. The volcanoes are formed from andesite and dacite belonging to potassium-rich calc-alkaline series.

Stratified slope deposits or grèzes litées are accumulations of debris that are traditionally associated with periglaciation but that can also form in other settings. The deposits have a weak sorting and a coarse bedding. Stratified slope deposits are usually found at the lower slopes of valleys where thicknesses vary but may exceed 10 meters. Periglacial stratified slope deposits are thought to be the result of rock fragmented by frost being accumulated downslope.

References

  1. Bates, Robert L. and Julia A. Jackson, ed. (1984). “hummock.” Dictionary of Geological Terms, 3rd Ed. New York: Anchor Books. p. 241.
  2. 1 2 3 4 5 6 Grab, Stefan. (2003). “Aspects of the geomorphology, genesis and environmental significance of earth hummocks (thufur, pounus): miniature cryogenic mounds.” Progress in Physical Geography 29, 2. p. 139-155.
  3. 1 2 3 Willams, Peter J. and Michael W. Smith. (1989). The Frozen Earth: Fundamentals of Geocryology. Cambridge, UK: Cambridge UP, p. 149-163.
  4. http://www.amjbot.org/content/91/8/1251.full
  5. 1 2 3 Davis, Neil. (2001). Permafrost: A Guide to Frozen Ground in Transition. Fairbanks, Alaska: University of Alaska Press. p. 133, 137-40, 146, 175-76.
  6. Reubi, O, Ross, P. S., & White, J.D.L. (2005). Debris Avalanche deposits associated with large igneous province volcanism: An example from the Mawson Formation, central Allan Hills, Antarctica. Geological Society of America Bulletin. p. 117, 1612-1627.
  7. 1 2 3 Francis, P, & Oppenheimer, C (2003). Volcanoes. Oxford: Oxford University Press.
  8. 1 2 Ui, T., Takarada, S., Yoshimoto, M., (2000). Debris Avalanches. In Sigurdsson, H., Houghton, B.F (eds), Encyclopedia of Volcanoes. San Diego: Academic Press.