,
Svalbard, Norway 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. In high-altitude
arctic and
subarctic regions, scree slopes and talus deposits are typically adjacent to hills and river valleys. These steep slopes usually originate from late-
Pleistocene periglacial processes. There are five main stages of scree slope evolution: • accumulation • consolidation • weathering • encroaching vegetation • slope degradation. Scree slopes form as a result of accumulated loose,
coarse-grained material. Within the scree slope itself, however, there is generally good sorting of sediment by size: larger particles accumulate more rapidly at the bottom of the slope.
Cementation occurs as
fine-grained material fills in gaps between debris. The speed of consolidation depends on the composition of the slope;
clayey components will bind debris together faster than
sandy ones. Should
weathering outpace the supply of sediment, plants may take root. Plant roots diminish
cohesive forces between the coarse and fine components, degrading the slope. The predominant processes that
degrade a rock slope depend largely on the regional
climate (see below), but also on the thermal and topographic stresses governing the parent rock material. Example process domains include: •
Physical weathering •
Chemical weathering •
Biotic processes •
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.
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. 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. 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. 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.
Freeze-thaw action of the entire lichen body due to microclimatic changes in moisture content can alternately cause thermal contraction and expansion, which also stresses the host rock. Lichen also produce a number of
organic acids as metabolic byproducts. These often react with the host rock, dissolving minerals, and breaking down the substrate into unconsolidated sediments. ==Interaction with glaciers==