SLOPE PROCESSES AND LANDFORMS

Controls On Mass Wasting:
Mass wasting occurs on slopes (even very gentle slopes) because weathering attacks the surface rock allowing the resulting weak soil and regolith to be pulled downslope by gravity. Since weathering is a continuous process, many slopes become weaker and weaker until eventually they can no longer withstand the pull of gravity and move downslope (i.e. shear stress becomes larger than strength).

Types of Mass Wasting:
Based on:
1. MATERIAL TYPE (SIZE): Rock, Soil (debris=coarse sediment; earth & mud =fine sediment). Note: the deposits of mass wasting are collectively known as COLLUVIUM.

2. TYPE OF MOTION:
Falls: Free-fall of material under gravity - forms cliffs and talus slopes.
Slides: movement of slope material over a well-defined planar surface. These commonly occur where there are joints, bedding planes or fractures parallel to the surface of a slope.
Slumps: In thick, saturated, unconsolidated sediments, a slide can be rotational - producing a slump.
Flows: High fluid content; behaves like a viscous liquid flow. Coarse material mixed with mud = DEBRIS FLOW. Fine material alone referred to as an EARTHFLOW (unconfined) or MUDFLOW if confined to a channel. When mudflows emerge from channels they build ALLUVIAL FANS. Mudflows are common in flash floods in arid regions. Where mudflows emerge from channels onto flat valley floors, alluvial fans are built.

Creep/Heave: Slow movement of near-surface material (e.g. cms/year). Common on most slopes above a few degrees. Caused by gravity, freeze/thaw, shrinking/swelling, rain splash, burrowing. Shows up as curved tree trunks, bent walls, "wrinkled" slope surface (terracettes).

In North Texas, high-clay-content soils on steep constructed slopes provide good examples of heave. Movement is probably initiated by large cracks that form in the dry summers (review lab 3).

Slope Form
Much of the earth's surface is made up of valley side slopes (hill slope processes act in combination with fluvial processes to create river valleys surrounding river channels). From the point of view of geomorphology, the RATE and TYPE of hill slope erosion processes can have a major impact on landscape evolution by influencing the SHAPE of slopes. The question is, "What controls slope form (shape)?" - the answer suggested by geomorphological study is that the shape of a slope reflects the dominant weathering, transportational or depositional processes at work on the slope. In other words, there is a link between geomorphic processes (e.g. rockfall, creep, solution, deposition…) and slope shape (e.g. steep, gentle, straight, curved...).

Convex Segments Occur on upper soil-covered slopes - result of solution, rain splash, sheet wash and soil creep, which increase down slope causing an increasing angle down slope (e.g. interfluves).

Straight Segments Occur where mass movement of coarse sediments and rock dominate e.g. rock falls, rock slides (uniform depth of removal maintains a straight slope).

Concave Segments Occur on lower slopes subject to deposition (eg. alluvial fans).

Climatic Controls On Slope Evolution
Both vegetation cover and hill slope processes are strongly influenced by CLIMATE - therefore, it is not surprising that contrasting slope form and slope evolution are characteristic of different climates.

Moist climates promote soil and vegetation and are dominated by creep and solution processes that create "smoothed" landscapes. Arid climate regions often lack soil and vegetation and are dominated by mechanical weathering and mass wasting of coarse sediment (rock falls and slides, sheetwash, rills, gullies), that create angular landscapes.

Humid-Temperate Climates
The pattern of slope evolution in these climates is one of faster erosion on steeper upper slopes and slower erosion + possibly deposition on lower slope -> overall slope "flattening out" or SLOPE DECLINE:

Arid Climates The lack of soil and dry conditions means that mass movements dominate. In the absence of "smoothing" by creep; slope erosion tends to be fairly uniform, producing straight (erosional) and concave (depositional) slope segments.

The pattern of slope evolution in these climates reflects the balance between mass wasting and removal of sediment by erosion - if material accumulates at the slope base faster than it can be removed -> SLOPE REPLACEMENT:

If material is removed by erosion as fast as it is moved down slope by mass wasting -> PARALLEL RETREAT and PEDIMENT formation:

Example Questions:

1. Describe, with the aid of diagrams, the following types of mass wasting: rockfall, rockslide, slump, mud flow, earth flow, debris flow, creep.

2. Explain, with the aid of diagrams, the following models of slope evolution: a. slope decline b. slope replacement c. parallel retreat.