Unit 4: Rocks and mass movements

I. Geomorphic framework

Geomorphology examines how Earth’s surface materials, internal forces, climate, gravity, water, and time produce and modify landforms. Rocks provide the geological structure and material; weathering breaks material down in place, while mass wasting moves it downslope under gravity. These processes interact with erosion, transport, and deposition to create landscapes.

A. Defining principles

• Rock cycle: Igneous, sedimentary, and metamorphic rocks can transform into one another through melting, cooling, weathering, burial, pressure, and heat.
• Weathering versus erosion: Weathering is the breakdown or alteration of rock in situ; erosion includes the removal and transport of weathered material by water, wind, ice, or gravity.
• Gravity as the driving force: Mass wasting occurs when the downslope component of gravity exceeds the resisting strength of soil, regolith, or bedrock.
• Resistance and structure: Mineral composition, grain size, joints, bedding, faults, and permeability influence how quickly rock weathers and fails.
• Climate control: Temperature and moisture govern chemical reactions, frost action, soil formation, and vegetation cover.
• Relief and slope: Steep, high slopes possess greater gravitational potential and commonly experience more rapid runoff, weathering, and slope failure.
• Threshold behaviour: Slopes may remain stable until rainfall, earthquakes, stream undercutting, freeze–thaw, or human excavation reduces resistance below a critical threshold.

II. Rocks — types and classification

Rocks are naturally occurring aggregates of one or more minerals, mineraloids, or organic materials. Their origin, mineral composition, texture, and structure determine their engineering strength and their susceptibility to weathering and mass movement.

A. rocks – types and classification

The three major rock groups are classified primarily by origin: crystallisation from molten material, deposition and lithification of sediments, or alteration of pre-existing rock.

• Igneous rocks: These form when magma or lava cools and crystallises.
  • Intrusive rocks cool slowly underground, producing large crystals; granite contains visible quartz, feldspar, and mica.
  • Extrusive rocks cool rapidly at the surface, producing fine-grained or glassy textures; basalt is a dark, fine-grained volcanic rock.
  • Texture and stability: Coarse crystals and cooling joints influence permeability and slope strength; columnar joints in basalt can form steep cliffs but also provide failure planes.
• Sedimentary rocks: These form through deposition, compaction, cementation, chemical precipitation, or accumulation of organic remains.
  • Clastic rocks contain fragments, such as sandstone and shale.
  • Chemical rocks precipitate from solution, as in limestone or rock salt.
  • Organic rocks develop from biological material, as in some limestones and coal.
  • Layering: Bedding creates planes of weakness; shale commonly weathers into clay-rich slopes, whereas well-cemented sandstone may form escarpments.
• Metamorphic rocks: These are altered by heat, pressure, and chemically active fluids without complete melting.
  • Foliated rocks show aligned minerals or banding; slate, schist, and gneiss differ in degree of metamorphism.
  • Non-foliated rocks lack pronounced layering; marble develops from limestone, while quartzite develops from sandstone.
  • Structural control: Foliation dipping toward a slope can encourage planar sliding, especially when lubricated by groundwater.
• Classification by composition: Silicate minerals dominate the crust; quartz is relatively resistant, while feldspar commonly alters to clay minerals. Carbonate rocks such as limestone dissolve readily in weakly acidic water.
• Classification by texture: Grain size, crystal arrangement, porosity, and cementation indicate cooling or depositional history and help predict permeability and mechanical resistance.
• Rock cycle connections: Granite exposed at the surface may weather into sediment, become sandstone after lithification, and later become quartzite during metamorphism.

B. Geomorphic significance

Rock type controls relief, drainage, soil development, and the form of weathering and mass movement.

• Differential resistance: Resistant rocks form ridges, uplands, and waterfalls; weaker rocks form valleys, basins, and lowlands.
• Permeability: Jointed sandstone may allow infiltration, whereas impermeable shale promotes surface runoff and saturation of slopes.
• Mineral stability: Quartz usually resists chemical weathering better than feldspar, olivine, or pyroxene.
• Jointing and faults: Fractures permit water entry, enlarge through weathering, and divide rock masses into blocks liable to fall or slide.
• Example: Limestone landscapes commonly contain enlarged joints, underground drainage, caves, sinkholes, and steep karst scarps because carbonic acid dissolves calcium carbonate.

III. Weathering — processes and landforms

Weathering is the physical disintegration and chemical decomposition of rock at or near Earth’s surface. It reduces intact bedrock to regolith and soil, increases surface area for further reactions, and prepares material for erosion and mass wasting.

A. weathering - processes and landforms

Weathering processes are commonly grouped into mechanical, chemical, and biological types, although they usually operate together.

• Mechanical weathering: Rock breaks into smaller pieces without a major change in mineral composition.
  • Freeze–thaw action: Water enters a joint, freezes, expands by approximately 9% in volume, and exerts pressure on the fracture. Repeated cycles widen the joint and produce scree.
  • Thermal expansion: Repeated heating and cooling cause differential expansion of minerals, encouraging granular disintegration and exfoliation in exposed rock surfaces.
  • Salt crystallisation: Saline water enters pores; evaporation leaves crystals that grow and exert pressure. This is significant in arid and coastal environments.
  • Pressure release: Removal of overlying material allows expansion and sheet joints to develop, producing exfoliation domes.
• Chemical weathering: Minerals are altered or dissolved through reactions with water, oxygen, carbon dioxide, and acids.
  • Hydrolysis: Feldspar reacts with acidic water to form clay minerals and dissolved ions, weakening granite.
  • Carbonation: Carbon dioxide dissolves in rainwater to form weak carbonic acid, which reacts with limestone:

    CO₂ + H₂O ⇌ H₂CO₃
    CaCO₃ + H₂CO₃ → Ca(HCO₃)₂

Here, CO₂ is carbon dioxide, H₂O is water, H₂CO₃ is carbonic acid, CaCO₃ is calcium carbonate, and Ca(HCO₃)₂ is soluble calcium bicarbonate.

• Oxidation: Oxygen combines with iron-bearing minerals to form iron oxides; the reddish-brown staining of weathered basalt is a common result.
• Solution: Soluble minerals, including halite and gypsum, dissolve directly in water.
  • Biological weathering: Organisms physically disrupt or chemically alter rock.
• Plant roots grow into fractures and exert pressure.
• Lichens produce organic acids that attack mineral surfaces.
• Burrowing animals expose fresh material and mix soil horizons.
  • Controls on rate: Warm, wet climates favour chemical weathering; cold climates favour frost action; arid climates commonly enhance salt weathering and thermal stress.
  • Worked example—granite: Quartz remains comparatively resistant, while feldspar hydrolyses into clay and mica may oxidise. The granite becomes weaker, develops grus, and may form rounded corestones where joints are chemically enlarged.

B. Weathering landforms

Weathering produces distinctive landforms by exploiting mineral differences, joints, bedding, and variations in climate.

• Tors and corestones: Deep chemical weathering along granite joints creates rounded blocks; erosion of the softened material leaves isolated tors or balanced corestones.
• Exfoliation domes: Sheet joints formed by pressure release and thermal expansion produce curved slabs and dome-shaped granite hills.
• Karst landforms: Carbonation of limestone produces lapies, swallow holes, dolines, caves, stalactites, and stalagmites.
  • Doline: A closed depression formed by solution or collapse.
  • Stalactite and stalagmite: Calcite deposits hanging from a cave roof and rising from its floor, respectively.
• Scree slopes: Freeze–thaw shatters cliff faces; angular fragments accumulate at the base, forming a cone or apron of debris.
• Badlands: Rapid runoff on weak, clay-rich sediment creates dense gullies, sharp ridges, and sparsely vegetated slopes.
• Laterite: Intense tropical weathering removes silica and concentrates iron and aluminium oxides, often producing a red, iron-rich soil horizon.

C. Applications and limitations

Weathering is essential to soil production and landscape evolution, but its effects vary with environmental and structural conditions.

• Engineering importance: Weathered granite may lose strength and become unstable even when the underlying fresh rock is strong.
• Soil development: Weathering releases mineral nutrients and creates clay, silt, sand, and dissolved ions needed for soil formation.
• Rate limitations: Chemical weathering may be rapid in warm, humid regions but slow where rock is dry, protected, or chemically resistant.
• Interaction with vegetation: Roots can accelerate physical breakdown while vegetation cover reduces runoff and protects soil from removal.

IV. Mass wasting — processes and landforms

Mass wasting, or mass movement, is the downslope movement of rock, soil, or regolith under gravity, with little or no dependence on a transporting medium. Movement may be extremely slow, as in soil creep, or sudden and destructive, as in a rock avalanche.

A. mass wasting – processes and landforms

Mass-wasting processes are classified by the type of material, the manner of movement, and the speed of movement.

• Controls on slope stability: Failure is favoured when driving stress exceeds resisting strength.
  • Driving factors: Slope angle, material weight, groundwater pressure, and external vibration.
  • Resisting factors: Cohesion, friction, root reinforcement, rock interlocking, and intact bedding.
  • Triggering events: Intense rainfall, snowmelt, earthquakes, volcanic activity, stream undercutting, road cutting, and removal of vegetation.
• Creep: Soil or regolith moves imperceptibly downslope, often a few millimetres to centimetres per year.
  • Evidence: Bent tree trunks, tilted fence posts, displaced walls, and terracettes.
  • Mechanism: Expansion and contraction from wetting–drying or freezing–thawing gradually shift particles downslope.
• Solifluction: Waterlogged soil slowly flows over impermeable or permanently frozen ground in cold regions.
  • Landform: Lobes and sheets of saturated debris develop on gentle slopes, especially where a thawed active layer overlies permafrost.
• Slides: A coherent mass moves along a recognisable shear surface.
  • Rotational slide: Movement occurs on a curved, concave-upward surface, producing a head scarp and back-tilted blocks.
  • Translational slide: A slab moves along a relatively planar surface such as bedding, foliation, or a fault.
• Flows: Material deforms internally and moves like a viscous fluid.
  • Mudflow: Water-rich fine sediment travels rapidly through channels.
  • Debris flow: A dense mixture of water, soil, rock fragments, and vegetation moves down a gully; its deposits commonly form lobes or fans.
  • Earthflow: Fine-grained, relatively slow material moves with a tongue-shaped form.
• Falls and topples: Rock detaches from a steep face and moves through air or pivots forward.
  • Rockfall: Produces talus or scree at the cliff base.
  • Topple: A block rotates outward around a pivot point, commonly along steeply dipping joints.
• Avalanches: Snow, ice, rock, or mixed debris moves rapidly downslope; a rock avalanche may travel far beyond the source because fragmented material becomes highly mobile.

B. Mass-wasting landforms

Mass movement modifies slope profiles and creates deposits that record the process and its energy.

• Head scarp: A steep break at the upper margin of a slide, formed where material detaches.
• Rotational slump bench: A stepped, irregular slope develops when several rotational slides leave benches and scarps.
• Talus cone: Repeated rockfalls accumulate as angular debris in a cone below a cliff or mountain gully.
• Debris fan: Flowing sediment spreads where a confined channel opens onto a flatter valley floor, producing a fan-shaped deposit.
• Landslide dam: A large slide may block a river, forming a temporary lake; later failure of the dam can generate catastrophic flooding.
• Earth hummocks and lobes: Slow flow and frost-related movement produce uneven, tongue-shaped or mound-like microrelief.
• Valley-side morphology: Undercutting by a river steepens the toe of a slope, while repeated slides widen the valley and create a stepped profile.

C. Applications and limitations

Understanding mass wasting requires linking process, material, trigger, and landform rather than relying on slope angle alone.

• Rainfall and pore pressure: Infiltrating water raises pore-water pressure, reduces effective friction, and may transform a stable slope into a flow.
• Vegetation: Roots reinforce shallow soil, intercept rainfall, and increase evapotranspiration; however, tree weight and root wedging can locally destabilise fractured rock.
• Human modification: Quarrying, road construction, deforestation, irrigation, and building loads alter slope geometry or water conditions.
• Hazard management: Drainage channels, retaining structures, rock bolts, terracing, re vegetation, and early-warning systems reduce risk, but cannot eliminate failure where geological weakness and extreme triggers coincide.