Mass Movements

Weathering Weathering is the breaking down of rock into smaller fragments ‘in situ’. Weathering is classified into three main types: Physical Chemical Biotic

Physical Weathering Physical weathering occurs where physical forces (mainly temperature changes) cause rock to disintegrate into smaller pieces. The main type of physical weathering dealt with at Higher is frost shattering. Water enters cracks and pore spaces in rock Temperature changes cause water to freeze and expand exerting pressure on the rock The rock eventually shatters into jagged fragments, often building up into screes

Chemical Weathering Chemical weathering occurs when there is a chemical reaction between water or a weak acid and rock, causing the minerals in the rock to alter. Rainwater is a dilute solution of the weak acid, carbonic acid. Water passing over decaying vegetation also produces acids. The new materials produced by chemical weathering are very small particles called clays. Chemical weathering is mainly dealt with in the Limestone section of Lithosphere.

Biotic Weathering Biotic weathering is a combination of physical and chemical weathering which is caused by plant and animal action. An example of this type of weathering is where burrowing animals can expose rocks to weathering.

Mass Movement Mass movement is the movement of weathered rock and soil downhill, often assisted by water. At Higher, four mass movements are studied: Rockfalls Mudflow Soil Creep Landslip

Rockfalls Rockfalls are a quick, dry mass movement. They occur on very steep slopes where soil or vegetation is lacking. The rocks are disintegrated by physical weathering – frost shattering or exfoliation. Scree slopes form at the bottom of the steep slope.

Rockfalls cont… Lines of weakness Talus / Scree slope Angle of rest on Talus / Scree slope is usually º Heaps of sharp debris accumulate at slope foot Rock fragments bounce rather than roll or slide Physical weathering – frost shattering / exfoliation – disintegrates rock Bare rock Free face

Mudflow Mudflows are a quick, wet mass movement. They are the rapid movement of saturated soil and water down hollows in a slope. They occur in areas of sparse vegetation, subject to torrential downpours. The impermeable subsoil allows the topsoil to flow freely after heavy rain. Sandy soils, when saturated, flow over impermeable subsoils.

Mudflow cont… Topsoil Impermeable sub-soil Hollow Angle must be >10º for mudflow to occur Mudflow ‘fan’ / ‘delta’ Solifluction occurs as soils move downhill due to alternate freezing and thawing

Soil Creep Soil creep is an extremely slow, dry mass movement. It occurs on slopes greater than about 5-6º.

Direction of soil creep Soil Creep Trees, pylons, fences, signs bent by soil creep Tension gashes in road surface When rain falls and water sinks in, the soil expands at right angles to the slope As it dries out, it contracts in the vertical direction Net downward result of soil creep (Can also be alternate freezing and thawing) Terracettes Soil builds up behind wall causing it to break Increasing soil depth at foot of slope

Landslip Are occasional, rapid movements of a mass of earth or rock sliding along a concave plane. They can occur after a period of heavy rainfall and the soil becomes saturated making it heavy and liable to slide. Undercutting of a steep slope by a river or sea erosion weakens the rock above and slumping is likely.