A hummock (of uncertain derivation; cf. hump or hillock) is a boss or rounded knoll of ice rising above the general level of an ice-field, making sledge travelling in the
Arcticand Antarcticregion extremely difficult and unpleasant.
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.
An earlier use of this term also refers to lumpy terrain; or land that has an irregular shape; or a fertile, wooded area that is at a slightly higher elevation (less than 3 m or so) than nearby marshes or swamps. Hummocks are often made by decaying plants.
Hummock is a general geological term referring to a small knoll or mound above ground (Bates and Jackson 1984). They are typically less than 1.5 meters in height and tend to appear in groups or fields. It is difficult to make generalizations about hummocks because hummocks are diverse in their morphology and sedimentology (Grab 2005). The term hummock, or hummocky, is also applied to extremely irregular surfaces (Williams and Smith 1989). This article provides a brief overview of earth hummocks of cryogenic origin and hummocky terrain created by debris avalanches to demonstrate the diverse set of processes that form hummocky landscapes.
Earth Hummocks of Cryogenic Origin
Cryogenicearth hummocks go by many different names; in North American they are earth hummocks; thúfur in Greenland and Iceland; and pounus in Fennoscandia. These cold climate landforms appear in regions of permafrostand seasonally frozen ground (Grab 2005). 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 (Davis 2001).
Leading 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 (Grab 2005).
Hummock excavation normally reveals a disturbed soil profile, often with irregular streaks of organic matter or other colorations suggesting fluidity at some time past (Davis 2001). The disturbance, a form of
cryoturbationoften 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 (Williams and Smith 1989).
=Differential Frost Heave (Cryostatic Pressure Hypothesis)=
This is the most widely excepted explanation of cryogenic hummock genesis (Grab 2005). 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. (Williams and Smith 1989)
Cryogenic Earth Hummock Summary
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 (Grab 2005).
Resent 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 (Davis 2001)) they are an ideal landform to monitor for medium range environmental change (Grab 2005).
Hummocks created by Debris Avalanches
Debris Avalanches are caused by sudden collapses of large volumes of rock from the flanks of mountains, especially volcanoes (Reubi et all 2005). These events are fast-moving; gravity driven currents of saturated debris that do not necessarily include juvenile material (Francis and Oppenheimer 2003). 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 (Ui et all 2000).Debris Avalanche blocks are identifiable because they keep their internal stratigraphy. The blocks simply break off the mountain and slid down, completely intact, identifiable because they differ from the surrounding landscape (Francis and Oppenheimer 2003). The volume and height of hummocks is mostly dependent on their location; the closer to the source region, the larger they become (Ui et all 2000). 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 (Francis andOppenheimer 2003)
*Bates, Robert L. and Julia A. Jackson, ed. (1984). “hummock.” Dictionary of Geological Terms, 3rd Ed. New York: Anchor Books. p. 241.
*Davis, Neil. (2001). Permafrost: A Guide to Frozen Ground in Transition. Fairbanks, Alaska: University of Alaska Press. p. 133, 137-40, 146, 175-76.
*Francis, P, & Oppenheimer, C (2003). Volcanoes. Oxford: Oxford University Press.
*Grab, Stefan. (2005). “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.
*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, 1615-1627.
*Ui, T., Takarada, S., Yoshimoto, M., (2000). Debris Avalanches. In Sigurdsson, H, Houghton, B.F (eds), Enclopedia of Volcanoes. San Diego: Academic Press.
*Willams, Peter J. and Michael W. Smith. (1989). The Frozen Earth: Fundamentals of Geocryology. Cambride, UK: Cambridge UP, p. 149-163.
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