Account for the occurrence of large-scale mass wasting in the highland areas of East Africa

Candidates are expected to have the following approach

• Definition of mass wasting
• Identification of areas affected
• Description of types of types of mass wasting; (illustrations may be drawn)
• Causes to be explained in relation to types of mass wasting in areas affected.

Definition: It is the movement i.e. creeping, flowing, sliding or falling of weathered rock  materials down slope under the influence of gravity.

Areas affected include; Mt Elgon, Rwenzori, Kenya, Kilimanjaro, Muhavura, Kigezi highland slopes etc.

The types/ processes can be slow or rapid and include;

• Soil creeping e. very slow movement of fine, soil and other unconsolidated weathered materials on very gentle slopes at a very slow rate.
• Soliflution e. slow movement of saturated soil, gravels and other weathered rocks over frozen ground on moderate slope/ gentle slope. Common on glaciated mountains of Rwenzori, Kenya, Kilimanjaro.
• Talus creep /creeping e. movement of angular rock particles/ screes of all sizes, over moderate to steep slope, at a moderate or fast steep.
• Mudflow e. intermittent movement of semi liquid mud mixed with gravels and boulders on moderate to steep slopes at fast speed e.g. slopes of Elgon /Bududa.
• Rock slumping I slump e. large masses of rock and debris on over – steepened slopes/scarps/cliffs /road cuttings moving down at a fast speed e.g. Kilimanjaro, Elgon slopes etc.
• Rock slidese. rapid movement of large masses of detached rock particles /debris rolling down fast along slippery over steepened slope e.g. Rwenzori
• Rock fail i.e. very fast / rapid movement of free fall of individual rocks and boulders over very steep slopes/vertical slopes.
• Avalanche – sudden down fall of rock materials imbedded in ice / snow.

Explain the causes for mass wasting clearly in relation to various types as follows:

Climatic conditions

• Prolonged/torrential rainfall such as EI-Niño or snow melting /thawing providing a lot of weight, increasing weight, saturating and lubricating/ over loading rock materials , making ground /slope slippery, accelerating mud fall, rock slump, rockslides etc. e.g. Bududa, Bulucheke (Elgon slopes) etc.
• Temperature changes i.e. alternate heating and cooling effect on soil, weakening it hence accelerating soil creeping.
• Drying and wetting conditions in semi-arid areas influencing mud flows etc.
• Freezing and thawing action on glaciated mountains of Kilimanjaro, Kenya hence frozen, slippery ground along which weathered materials move down slope in form of Soliflution, Talus creep, rock slump etc.

Nature of slope /relief

• Very gentle slope encouraging soil creeping.
• Gentle / moderate slopes / steep for solifluction, talus creeping, mud flows.
• Steep/ over – steepened slopes such as scarps, cliffs accelerating rock slumping, rock sliding , avalanches
• Very steep slopes /vertical slopes speeding tip rock fall. The above can be evidenced on slopes of Elgon, Rwenzori, Kilimanjaro and Kenya.

Nature of rock material

• Permeable weathered saturated rocks overlying impermeable rock layers being lubricated by water or impermeable rock overlying permeable rock encouraging mud flow, talus creep, rock slide, solifluction etc.
• Well jointed rock large masses/thinly bedded or steeply dipping boulders encourage rapid mass wasting inform of rock slump, slide or rock fall, avalanches e.g. Mt Rwenzori etc.
• Alternating belts / layers of hard and soft rocks encouraging rock fall, rock slide, rock slump, mud flow on slopes of Elgon, Kilimanjaro etc.
• Massive layers of semi liquid materials/ mud leading to mud flow on slopes of Elgon etc.
• Angular rocks encourage talus creep etc.

The nature of the soil

• Fine, dry soil materials for soil creeping.
• Heavy wet massive clay soils encouraging mud flow, rock slumping etc.
• Undercutting of base of river valley or lake causing rock slump etc..
• Overloading /accumulation of rock debris in large masses on the over steepened slope hence rock slump, rock slide, mud flow etc.
• Tectonic forces i.e. Earthquakes causing vibrations, triggering off landslides, rock slump, rock slides, rock fall on mountain Rwenzori, Bundibugyo etc.
• Vulcanicity i.e. basic lava flowing down slopes of volcanoes, mixed with weathered rocks like ashes, hence mud flowing, rock slumping etc.
• Living organisms i.e. burrowing effects of rodents etc., trampling effects of animals like elephants encouraging mass wasting.

Human factor

• Mining and quarrying (using explosives) causing vibrations steeping or exposing slopes triggering of mudflows, rock slump, rock slide and rock fall.
• Deforestation along slopes exposing rock material /soil to soil creeping, rock slumping etc.
• Over cultivation on steep slopes leading to soil creeping, mud flowing, etc. e.g. on Mt. Elgon, Kigezi
• Building/ construction of houses, roads along steep slopes encouraging rock slumps, mud flows etc.
• Overgrazing on steep slopes.
• Moving-of heavy vehicles like trailers, buses causing vibrations along groad cuttings etc. encouraging rock slump, rock fall etc. e.g. Kabale – Kisoro Road, Fort Poratal – Bundibugyo, Simu – Kapchorwa Road etc.