Weathering and Mass Movement Notes

Weathering: Meaning and Concept

Weathering is the disintegration and decomposition of rocks in situ. It's a static process involving breakdown and alteration of minerals near the Earth's surface to align with new physical-chemical conditions.
Rocks are never in permanent equilibrium; weathering changes rocks from massive to clastic states. It includes subaerial processes causing rock decay and disintegration without large-scale transport of loosened products.
Weathering involves physical (mechanical fracturing) & chemical decomposition of rocks by natural agents at the Earth's surface.

Controlling Factors of Weathering

Weathering varies by location due to agents of weathering, rock lithology/structure, height, slope, climate, topography, and flora/microfauna.
Disintegration is effective in hot, dry regions and where frost action dominates. Chemical decomposition prevails in hot, humid, and temperate humid areas.
Rock Composition/Structure: Mineral composition, joint patterns, layering, faulting, and folding all affect weathering. Soluble minerals in carbonate rocks are susceptible to chemical weathering. Jointed rocks undergo mechanical disintegration. Vertical strata rocks are easily broken, while horizontal beds are more compact and less affected.
Ground Slope: Steep slopes promote mechanical disintegration and mass movement of weathered products. Gentle slopes experience less disintegration.
Climatic Variations: Climate dictates weathering types. Chemical weathering dominates in humid tropics due to water and high temperatures, enhancing leaching and rock solution. Mechanical weathering is dominant in tropical and semi-arid regions due to temperature-induced expansion and contraction. Limestone weakens in humid climates but resists weathering in hot deserts.
Floral Effects: Vegetation either protects or weakens rocks. Roots bind rocks, protecting them, but also cause breakage. Dense vegetation shields from sun, while microorganisms promote decomposition.

Types of Weathering Processes

Weathering is divided into physical, chemical, and biochemical processes.

Physical Weathering Agents

Moisture, water, frost, insolation (temperature), and wind.

Chemical Weathering Agents

Oxygen, carbon dioxide, and hydrogen.

Biological Weathering Agents

Vegetation and animals (mainly microorganisms).
Weathering is divided into 3 major types based on weathering agents:

Physical or Mechanical Weathering

Block disintegration (temperature/frost)
Granular disintegration (temperature)
Exfoliation/onion weathering (temperature/wind)

Chemical Weathering

Oxidation, carbonation, solution, hydration, chelation, and hydrolysis.

Biotic & Biochemical Weathering

Plant, animal, anthropogenic, and biochemical weathering.
Physical weathering involves rock fragmentation due to temperature, frost, wind, and pressure release.

Specific types of Physical Weathering:

Block disintegration: Temperature changes cause expansion/contraction, creating joints. Unloading contributes.
Granular disintegration: Coarse grains of different colors in hot deserts absorb insolation differently, causing differential expansion/contraction.
Shattering due to rain & heat: Sudden showers on heated rocks create cracks, leading to granular disintegration.
Block disintegration due to frost: Freeze-thaw cycles expand water in cracks, disintegrating rocks. Frost weathering in periglacial areas forms frost-riven polygons.
Exfoliation: Peeling off concentric rock shells due to heat and wind (common in crystalline rocks).
Disintegration due to unloading: Removal of overlying rocks releases pressure, causing cracks and joints.
Boulder Cleaving: Breaking of granite/basalt boulders due to thermal expansion.
Dirt Cracking: Fracture of boulders containing dirt due to thermal expansion.
Slaking Weathering: Disintegration due to cycles of wetting and drying.
Salt Weathering: Disaggregation caused by salt crystal growth.

Chemical Weathering

Decomposition of rocks via chemical reactions, with water as the medium. Key reactions: oxidation, carbonation, solution, hydration, chelation, hydrolysis, and base exchange.

Specific types of Chemical Weathering:

Solution: Dissolving soluble particles with water. Limestones are prone to solution, influenced by temperature, $CO_2$ content, and pH.
Oxidation: Reaction with atmospheric oxygen to form oxides (e.g., iron oxide), weakening rocks. It often involves iron-bearing rocks reacting with oxygen dissolved in water to produce rust.
Carbonation: Reaction of carbonate or bicarbonate ions with minerals, dissolving carbonate rocks, and forming calcium bicarbonate.
Hydration: Addition of water to minerals, increasing volume and causing stress. Feldspar turns into kaolinite clays (kaolinization).
Hydrolysis: Chemical reaction between mineral and water, forming new mineral compounds. It commonly affects silicate minerals. For example: $2KAlSi3O8 + 2H2CO3 + 9H2O \rightarrow Al2Si2O5(OH)4 + 4H4SiO4 + 2K^+ + 2HCO3^-$
Chelation: Metallic cations are incorporated into hydrocarbon molecules, disrupting mineral lattices. It's a form of chemical weathering by plants extracting minerals.
Products of Chemical Weathering: Solutes, Clays, and Mineral Residuals.

Biotic Weathering

Breakdown of rocks controlled by plants and animals, with humans accelerating the process via modern technologies.

Specific types of Biotic Weathering:

Faunal Weathering: Burrowing animals mix soil and expose fresh materials to weathering.
Floral Weathering: Roots widen cracks, and dense vegetation creates microclimates that promote chemical weathering.
Anthropogenic Weathering: Mining, blasting, and quarrying lead to rapid disintegration of rocks.

Biochemical Weathering

Decomposition of rocks due to organic materials (flora and fauna). Organic acids (humic, bacterial, microfaunal) facilitate decomposition.

Geomorphic Importance of Weathering

Production of Rock Wastes: Weathering disintegrates rocks, producing rock wastes (regoliths). Weathered materials aid soil formation and expose minerals.
Aids Erosional Processes: Weathering loosens rocks, facilitating erosion by running water, wind, glaciers, and waves.
Lowering of Surface: Continuous removal of weathered materials lowers the surface height.
Evolution and Modification of Landforms: Differential weathering shapes landforms such as stone lattices, tors, buttes, and talus cones.

Mass Movement (Mass Wasting)

Downslope transport of soil & rock material under gravity. It detaches rock materials and transports them downslope en bloc. Water, ice, and air presence accelerates it.
Mass wasting focuses on gravity as the sole force, but water plays a crucial role by over-steepening slopes via surface erosion and generating seepage forces through groundwater flow.

Classification of Mass Movements:

Categorized by causative factors (rate, direction, type, and lubricating substance).

Direction of Movement

Vertical, Lateral, Diagonal.

Types of Movement

Vertical

Rockfall, Collapse Earthfall.

Lateral

Block Slide, Spread, Cambering, Sackung.

Diagonal

Soil Creep, Rock Creep, Talus Creep, Rockslide, Debris Slide, Slump, Debris Flow, Mudflow, Solifluction, Avalanche.

Factors of Mass Movement

Ratio between shearing forces (stress) and material's shearing resistance. Mass movement occurs when shearing forces increase or shearing resistance decreases.
$Fs = \frac{Shearing\,resistance\,of\,materials}{Magnitude\,of\,shearing\,forces}$

Factors Increasing Shearing Forces

Removal of lateral support (natural or anthropogenic), surcharge (loading of slope), transitory earth stress, removal of underlying support, lateral pressure.

Factors Reducing Shear Strength

Weathering, changes in intergranular forces (water content), structural changes, organic factors.

Landslides

Collective term for mass movements, generally divided into falls, slides, topples, flows, and lateral spreads.

Types of Landslides

Fails

Instantaneous fall of weathered rock. Subdivided into rock fall, debris fall, and earth fall.

Slides

Downslope displacement of rock or soil along a defined shear plane. Further subdivided into slump, rock slides, debris slide, and earth slide. Slumping involves intermittent sliding along a curved plane. Rock slides: Large rock blocks slide downhill. Debris slide: Mixture of soils and rock fragments.

Flows

Diagonal downslope movement with enough water. Earthflow, mudflow etc.

Creep

Imperceptible downslope movement of colluvium. Soil creep (solifluction) and rock creep.

Topographic Expressions of Mass Wasting and Mass Movement

Distinctive morphological features (scars, ripple marks, terraces) on hillslopes, river valley sides, and coastal lands. The resulting morphological features remain almost uniform.