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In this article, we would discuss the slow mass movements.
What are slow mass movements?
1. It involves slower movements and are seen in moderately steeper slopes. The movement of materials is slow and less damaging compared to the rapid mass movements.
2. There are two types of slow mass movements: soil creep and soil flow.
3. Soil creep:
i) It is slow, gradual yet continuous movement of the soil downwards.
ii) The rate of soil creep depends on factors like – type of sediment and material, steepness of the slope, water content and the type of vegetation on it.
iii) They are more common in damp soils and the movement is not very noticeable unless it is observed for extended periods of time.
iv) Gradual tilting of trees, fences and posts rooted in the soil are examples of soil creep mass movement.
v) Creep can also occur due to expansion of material-like clay. Though water helps sand particles bind with each other by cohesion, excess of it fills the pores between sand particles and causes them to slip away creating a creep.
4. Soil flow:
i) Also known as solifluction, it is the complete saturation of soil with water, which almost makes the soil particles suspend in it.
ii) Such soil acts like a fluid and moves easily over the underlying rock.
iii) It results in the slow downslope movement of water-saturated sediment, which carries with it rock debris and other underlying substances.
iv) Occurrence of soil flow depends on the composition of the slope. Higher the impermeability of materials, higher the chances of solifluction. However, soils with larger grain size are less prone since they get less saturated and permit greater filtration.
v) In temperate regions, soil flow of the surface of frozen ground occurs due to freeze-thaw effect (liquefying of a substance due to warming up).
vi) In areas of peat soil (soil that contains partially decomposed plants or organic matter), the moisture absorbed by it is very high. However, at its saturation point, the soil starts to flow downslope. In Ireland, this phenomenon is also known as ‘bog-bursts’.