Scree
Scree is a collection of broken
The term scree comes from the Old Norse term for landslide, skriða,[2] while the term talus is a French word meaning a slope or embankment.[3][4]
In high-altitude
Description
The term scree is applied both to an unstable steep mountain slope composed of rock fragments and other debris, and to the mixture of rock fragments and debris itself.[8][9][10] It is loosely synonymous with talus, material that accumulates at the base of a projecting mass of rock,[9][11] or talus slope, a landform composed of talus.[12] The term scree is sometimes used more broadly for any sheet of loose rock fragments mantling a slope, while talus is used more narrowly for material that accumulates at the base of a cliff or other rocky slope from which it has obviously eroded.[9]
Scree is formed by rockfall,
Scree slopes are often assumed to be close to the angle of repose. This is the slope at which a pile of granular material becomes mechanically unstable. However, careful examination of scree slopes shows that only those that are either rapidly accumulating new material, or are experiencing rapid removal of material from their bases, are close to the angle of repose. Most scree slopes are less steep, and they often show a concave shape, so that the foot of the slope is less steep than the top of the slope.[16][17]
Formation
The formation of scree and talus deposits is the result of physical and chemical weathering acting on a rock face, and erosive processes transporting the material downslope.
There are five main stages of scree slope evolution: (1) accumulation, (2) consolidation, (3) weathering, (4) encroaching vegetation, and finally, (5) slope degradation.
Scree slopes form as a result of accumulated loose,
- Physical weathering
- Chemical weathering
- Bioticprocesses
- Thermal stresses
- Topographic stresses
Physical weathering processes
Scree formation is commonly attributed to the formation of ice within mountain rock slopes. The presence of joints, fractures, and other heterogeneities in the rock wall can allow precipitation, groundwater, and surface runoff to flow through the rock. If the temperature drops below the freezing point of the fluid contained within the rock, during particularly cold evenings, for example, this water can freeze. Since water expands by 9% when it freezes, it can generate large forces that either create new cracks or wedge blocks into an unstable position. Special boundary conditions (rapid freezing and water confinement) may be required for this to happen.[20] Freeze-thaw scree production is thought to be most common during the spring and fall, when the daily temperatures fluctuate around the freezing point of water, and snow melt produces ample free water.
The efficiency of freeze-thaw processes in scree production is a subject of ongoing debate. Many researchers believe that ice formation in large open fracture systems cannot generate high enough pressures to force the fracturing apart of parent rocks, and instead suggest that the water and ice simply flow out of the fractures as pressure builds.[21] Many argue that frost heaving, like that known to act in soil in permafrost areas, may play an important role in cliff degradation in cold places.[22][23]
Eventually, a rock slope may be completely covered by its own scree, so that production of new material ceases. The slope is then said to be "mantled" with debris. However, since these deposits are still unconsolidated, there is still a possibility of the deposit slopes themselves failing. If the talus deposit pile shifts and the particles exceed the angle of repose, the scree itself may slide and fail.
Chemical weathering processes
Phenomena such as acid rain may also contribute to the chemical degradation of rocks and produce more loose sediments.
Biotic weathering processes
Biotic processes often intersect with both physical and chemical weathering regimes, as the organisms that interact with rocks can mechanically or chemically alter them.
Lichen frequently grow on the surface of, or within, rocks. Particularly during the initial colonization process, the lichen often inserts its hyphae into small fractures or mineral cleavage planes that exist in the host rock.[24] As the lichen grows, the hyphae expand and force the fractures to widen. This increases the potential of fragmentation, possibly leading to rockfalls. During the growth of the lichen thallus, small fragments of the host rock can be incorporated into the biological structure and weaken the rock.
Interactions with surrounding landscape
Scree often collects at the base of glaciers, concealing them from their environment. For example, Lech dl Dragon, in the Sella group of the Dolomites, is derived from the melting waters of a glacier and is hidden under a thick layer of scree. Debris cover on a glacier affects the energy balance and, therefore, the melting process.[25][26] Whether the glacier ice begins melting more rapidly or more slowly is determined by the thickness of the layer of scree on its surface.
The amount of energy reaching the surface of the ice below the debris can be estimated via the one-dimensional, homogeneous material assumption of
,
where k is the
Debris with a low thermal conductivity value, or a high
The albedo, or the ability of a material to reflect incoming radiation energy, is also an important quality to consider. Generally, the debris will have a lower albedo than the glacier ice it covers, and will thus reflect less incoming solar radiation. Instead, the debris will absorb radiation energy and transfer it through the cover layer to the debris-ice interface.
If the ice is covered by a relatively thin layer of debris (less than around 2 centimeters thick), the albedo effect is most important.[27] As scree accumulates atop the glacier, the ice's albedo will begin to decrease. Instead, the glacier ice will absorb incoming solar radiation and transfer it to the upper surface of the ice. Then, the glacier ice begins to absorb the energy and uses it in the process of melting.
However, once the debris cover reaches 2 or more centimeters in thickness, the albedo effect begins to dissipate.[27] Instead, the debris blanket will act to insulate the glacier, preventing incoming radiation from penetrating the scree and reaching the ice surface.[27] In addition to rocky debris, thick snow cover can form an insulating blanket between the cold winter atmosphere and subnivean spaces in screes.[28] As a result, soil, bedrock, and also subterranean voids in screes do not freeze at high elevations.
Microclimates
A scree has many small interstitial voids, while an ice cave has a few large hollows. Due to cold air seepage and air circulation, the bottom of scree slopes have a thermal regime similar to ice caves.
Because subsurface ice is separated from the surface by thin, permeable sheets of sediment, screes experience cold air seepage from the bottom of the slope where sediment is thinnest.[6] This freezing circulating air maintains internal scree temperatures 6.8-9.0 °C colder than external scree temperatures.[29] These <0 °C thermal anomalies occur up to 1000m below sites with mean annual air temperatures of 0 °C.
Patchy permafrost, which forms under conditions <0 °C, probably exists at the bottom of some scree slopes despite mean annual air temperatures of 6.8–7.5 °C.[29]
Biodiversity
During the
Scree microclimates maintained by circulating freezing air create microhabitats that support taiga plants and animals that could not otherwise survive regional conditions.[6]
A Czech Republic Academy of Sciences research team led by physical chemist Vlastimil Růžička, analyzing 66 scree slopes, published a paper in Journal of Natural History in 2012, reporting that: "This microhabitat, as well as interstitial spaces between scree blocks elsewhere on this slope, supports an important assemblage of boreal and arctic bryophytes, pteridophytes, and arthropods that are disjunct from their normal ranges far to the north. This freezing scree slope represents a classic example of a palaeo refugium that significantly contributes to [the] protection and maintenance of regional landscape biodiversity."[6]
Ice Mountain, a massive scree in West Virginia, supports distinctly different distributions of plant and animal species than northern latitudes.[6]
Scree running
Scree running is the activity of running down a scree slope; which can be very quick, as the scree moves with the runner. Some scree slopes are no longer possible to run, because the stones have been moved towards the bottom.[33][34][35]
See also
- Blockfield - similar to talus and scree slopes, formed by frost weather instead of mass wastings
- Fellfield
- Lava stringer– Type of rock formation
- Mass wasting – Movement of rock or soil down slopes
- Stratified slope deposit
- Weathering – Deterioration of rocks and minerals through exposure to the elements
- Scree plot
References
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- ^ Harper, Douglas. "scree". Online Etymology Dictionary. Retrieved 2006-04-20.
- ^ Harper, Douglas. "talus". Online Etymology Dictionary. Retrieved 2008-12-01.
- ^ "Talus". bab.la language portal. Retrieved 2011-12-10.
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- ^ "scree". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
- ^ ISBN 0922152349.
- ^ ISBN 9780199653065.
- ^ Jackson 1997, "talus".
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- ^ Jackson 1997, "colluvium".
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- ^ Whalley, WB (1984). "Rockfalls". In Brunsden, D.; Prior, DB (eds.). Slope Instability. Chichester: John Wiley and Sons. pp. 217–256.
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- Encyclopaedia of New Zealand.
- ^ Short, David (2012-02-01). "Scree running madness". Wilderness. Retrieved 2020-12-21.
- Wildlife Trust. Retrieved 2020-12-21.