Weathering & soils

Chemical and Mechanical Weathering

Rocks at the Earth's surface are subjected to either physical disintegration or chemical decomposition. 

Mechanical weathering processes include abrasion, fluctuations in temperature, and loading/unloading. Abrasion is primarily associated with glacial movement whereby the rocks are abraded under the grinding weight of the encroaching or receding ice. Rapid fluctuations in temperature serve to crack exposed igneous and metamorphic rocks. Fine vernacular cracks within these rocks that contain water upon freezing expand and thereby serve to fracture the rock. 

Chemical weathering processes include dissolution reactions and reaction with acids. An example of a dissolution (and acid reaction) reaction is the formation of caves in limestone. Rain water entering a limestone aquifer (rain water is slightly acidic) dissolves CaCO3along fracture flow paths, gradually expanding into caverns and caves.

Rock Weathering and Soil Formation

Weathering processes include:

Physical processes that break rocks apart
1. Freeze-Thaw
2. Unloading
3. Thermal expansion and contraction


Chemical processes involving reactions between minerals in rock and water:
1. Hydrolysis - reaction with water
2. Dissolution - mineral completely dissolves in water
3. Oxidation - iron in minerals react with oxygen to form iron oxides.

Soil is a loose mixture of weathered rock, organic matter, water and air that differs from parent material physically and chemically. Parent material is weathered rock. Residual soil forms on weathered rock on top of bedrock, Transported soil forms on material that was transported and deposited.

Soils have layered or banded appearance. These bands are called horizons. A soil profile is a description of soil horizons starting at the ground surface and working downwards. Soils may contain these horizons: O horizon, A horizon, B horizon, and C horizon.

O horizon: abundant organic matter, dark coloured; 

A horizon: region from which materials are removed (especially calcite (CaCO3), iron oxide (FeO3) and clays; lighter colour than O horizon but still contains organic matter so usually darker than underlying horizons; 

B horizon: material removed from A horizon may accumulate here; colour depends on presence/absence iron oxides (FeO3) etc; 

C horizon: partially altered parent rock material, but looks similar to parent rock.

Important soil properties include: 

Texture - Size of individual soil particles: Sand (sand sized grains) feels gritty; Silt (finer than sand but can see coarser silt particles) feels slightly gritty; Clay (grains so small that you can't see individual grains) feels smooth. Loam is a mixture of sand, silt and clay. 

Permeability - Sandy soils are very permeable, Clay soils have low permeability. 

Strength - Strength is a measure of the stress a soil can bear. Stress may be compression or shear. Strength comes from cohesion (clays stick together) and internal friction (amount of needed for individual grains to move). Moisture content affects strength - a little moisture aids cohesion whereas saturation weakens soil. 

Shrink/Swell Potential - Some soils contain certain clays that absorb water and expand (montmorillonitic [smectite] and illitic clays, not so much kaolinitic clays). Expansion and contraction of soil can damage building foundations and other structures. 

Sensitivity - Some soils loose strength when shaken (e.g. earthquakes). Fine grained soils are more likely to be 'sensitive'.