HKDSE Geography/E1/Mass Movement

What better way to start this section with another definition?

Mass movement/Mass wasting - the spontaneous movement of slope materials en masse downhill due to gravity

Stress and Strength

The occurrence of mass movement is determined by the interplay between two main factors:

Stress - the force that tries to pull slope materials downhill, i.e. gravity

Strength - the forces that maintain slope materials in situ and prevent them from falling, i.e. the internal cohesion of slope materials and internal friction

Mass movement occurs when stress exceeds strength.

Factors

Factors that affect the occurrence of landslides affect stress, strength or both.

• Water acts as a lubricant to weathered materials and increases pore water pressure. It also adds weight to weathered materials. Thus both the coherence of slope materials and friction will drop during rainfall. When the amount of rainwater reaches a certain level the coherence of the slope materials will break, leading to mass movement.
• Slope angle is another factor. Slope materials move slower on slopes with gentler gradient as the stress is smaller, and faster on slopes with steeper gradient as the stress is greater.
• Some rocks are more resistant to mass movement than others. Granite is more resistant than siltstone.
• The degree of weathering is important. An area of active weathering has more loose weathered materials, which decreases the internal cohesion of slope materials and thus strength, and favours mass movement.
• Vegetation roots prevent mass movement as they strengthen the bonding between soil particles. However, they also secrete organic acid, which weathers rocks below. This will weaken the rocks, decreasing the internal cohesion of slope materials and thus strength.
• Strong earthquakes can trigger earthquakes. The 2008 Sichuan earthquake and 2011 Japanese earthquake are examples.

Landslides

Landslide - the rapid movement of rocks and soils of all sizes along a slide plane. (It may slide on a flat slide plane or slump on a curved slide plane)

A scar is left when a landslide occurs. Landslides will be discussed in detail later.

Rockfall

Rockfall - the sudden and rapid free fall of rock fragments from steep or overlying slopes

The rocks vary in size, from small grains to huge rocks. Tors are a typical example. If they stand on steep slopes, their bases may be removed by rain, seawater, or some other agent. Strong winds and the construction of roads are also common culprits. They will lose support and the stress will increase greatly, exceeding the strength. Thus they will fall suddenly.

Rock fragments called screes are found at the foothill. A scree slope is formed.

Soil creep

Soil creep is caused by alternate wetting and drying, thermal expansion and contraction or freeze and thaw.

1. When soil expands (i.e. wets/freezes/expands thermally), it expands at right angles to the slope.
2. When soil contracts (i.e. dries/thaws/contracts thermally), it falls back vertically to the slope of the bedrock.
3. The material is moved downslope after each successive movement.

Several good indicators of soil creep include terracettes, bent trees, bulging fences, tilted walls and soil accumulation.