5.1 Weathering, soils & mass wasting

Weathering, Soils, and Mass Wasting

Weathering is the in-place breakdown of rock and minerals at or near Earth's surface, whereas erosion is the subsequent transport of that material. The ASBOG Fundamentals (FG) and Practice (PG) exams treat weathering, soil formation, and slope stability as a connected chain that converts fresh bedrock into regolith and ultimately drives many surficial hazards.

Physical (mechanical) weathering

Physical weathering fragments rock without altering its chemistry, greatly increasing the surface area available for later chemical attack.

• Frost wedging (freeze-thaw): Water expands about 9% when it freezes, prying open fractures. It is most effective where temperatures cycle repeatedly across 0 degrees C, producing angular talus and blockfields.
• Exfoliation (sheeting): Pressure release, or unloading, as overburden is stripped away lets a pluton expand, creating fractures parallel to the ground surface and forming rounded exfoliation domes.
• Thermal expansion (insolation): Repeated daily heating and cooling stresses mineral grains, an important process in deserts.
• Salt wedging (haloclasty): Salts crystallizing in pore spaces exert pressure, carving tafoni and honeycomb textures in arid and coastal settings.
• Biological wedging: Root growth and animal burrowing mechanically disaggregate rock.

Chemical weathering

Chemical weathering alters mineral composition, generally requires water, and is favored by warm, humid climates.

• Hydrolysis: The dominant reaction for silicates. Feldspars react with slightly acidic water to form clay minerals such as kaolinite, releasing dissolved silica and cations. This is the master process that makes soil.
• Oxidation: Iron-bearing minerals like olivine, pyroxene, and magnetite react with oxygen to form iron oxides such as hematite and limonite, producing the red and yellow staining of many soils.
• Dissolution: Carbonates dissolve in weak carbonic acid formed from carbon dioxide and water. This produces karst landscapes with sinkholes, caves, disappearing streams, and towers where limestone and dolomite are removed.
• Hydration: Water absorbed into a mineral structure, such as anhydrite altering to gypsum, causes expansion.

Bowen's reaction series predicts weathering susceptibility: minerals that crystallize at high temperature (olivine, calcium-rich plagioclase) weather fastest because they are farthest from surface equilibrium, whereas quartz is highly resistant and becomes concentrated in mature sediments.

Weathering rates and differential weathering

Weathering rate rises with warmth, moisture, and surface area, so a coarse jointed granite may weather along its fractures into rounded corestones by spheroidal weathering, in which corners and edges are attacked from multiple sides. Differential weathering occurs where rocks of contrasting resistance sit side by side: resistant sandstone caps stand out as ledges while weak shale erodes into slopes, and this contrast shapes cliffs, hoodoos, and cuestas. Climate is decisive: chemical weathering dominates in warm, humid regions, while physical weathering dominates in cold or arid regions where liquid water is scarce.

Soils and soil horizons

A mature soil profile develops downward into distinct horizons:

• O horizon: Organic litter at the top.
• A horizon (topsoil): Mineral matter mixed with humus; dark and biologically active.
• E horizon: Zone of eluviation where clays, iron, and aluminum are leached out, leaving a pale, sandy layer.
• B horizon (subsoil): Zone of illuviation where leached clays, iron oxides, and carbonates accumulate.
• C horizon: Weathered parent material, transitional to unaltered bedrock (R).

The five soil-forming factors are captured by CLORPT: CLimate, Organisms, Relief (topography), Parent material, and Time. Climate and organisms are generally the most active drivers, while time controls maturity. Pedalfers form in humid climates and are aluminum- and iron-rich; pedocals form in dry climates and are rich in calcium carbonate (caliche); laterites form in the tropics through intense leaching that leaves iron and aluminum oxides.

Mass wasting

Mass wasting is the downslope movement of rock and soil under gravity, without a transporting medium such as flowing water or ice. Slope stability is governed by the factor of safety (FS), the ratio of resisting forces (shear strength) to driving forces (shear stress). An FS greater than 1 is stable, and an FS less than 1 fails. Water is the great destabilizer: it adds weight, raises pore-water pressure, and reduces cohesion.

The angle of repose is the steepest angle (typically 30 to 37 degrees for dry granular material) at which loose material remains stable. Mass-wasting types are classified by material, type of motion, and rate:

• Creep: Extremely slow, imperceptible soil movement that tilts fences and poles and curves tree trunks.
• Slump: Rotational sliding along a curved failure surface, leaving a crescent-shaped scarp and back-tilted blocks.
• Rockfall: Free fall of rock from a cliff, accumulating as talus at the angle of repose.
• Debris flow and mudflow: Rapid, water-saturated slurry of mixed sediment; volcanic examples are lahars.
• Rock slide: Rapid translational movement along a planar surface, often a bedding plane dipping parallel to the slope.

A useful classification framework groups movements as falls, topples, slides (rotational slumps and translational slides), lateral spreads, and flows, spanning the full range from bedrock to saturated soil. Common triggers include heavy rainfall, snowmelt, undercutting by rivers or waves, earthquake shaking, and loading or oversteepening by construction. Sensitive (quick) clays can lose nearly all strength when disturbed, producing sudden lateral spreads. Mitigation reduces driving forces or raises resisting forces: regrading to flatten slopes, installing drains to lower pore pressure, adding retaining walls or rock bolts, and revegetating to bind soil. Recognizing hummocky topography, tension cracks, bowed or curved tree trunks, and displaced fences or vegetation is central to geologic hazard evaluation on both ASBOG exams.