Weathering and Soil Formation Notes

Weathering

Weathering is the process by which rock breaks down physically and chemically over time, producing sediment and new minerals stable at the Earth's surface.
Weathering processes are gradual and typically not easily observed over short periods.

Physical Weathering

Physical weathering breaks down rocks into smaller fractions without changing the chemistry of the primary minerals.

Chemical Weathering

Chemical weathering involves chemical reactions that alter the mineralogy of rocks, transforming them into more stable compounds at the Earth's surface.

Time Scale of Weathering

Some weathering processes can be observed over a human lifetime (years to decades).
Example: Weathering of volcanic tuff in a road cut shows changes over 5, 15, and 25 years.
- 5 years: Sharp corners between joints and fractures.
- 15 years: Development of rills (small channels) from surface runoff.
- 25 years: Rounded edges and red staining (oxidation of iron).

Role of Joints and Fractures

Exposed bedrock is subject to both physical and chemical weathering.
Natural joints facilitate weathering by allowing water penetration, increasing the surface area exposed to weathering.
Joint fractures:
- Increase surface area.
- Permit water infiltration.
- Facilitate vegetation growth by trapping soil and water.
Joints in bedrock (e.g., El Capitan) form during uplift and unloading of overburden rock.
Joints also form during cooling and contraction of lava flows (columnar basalts, pillow basalts).

Physical Weathering Processes

Physical weathering breaks down rocks without changing their mineralogy.
Examples:
- Freeze-thaw cycles: Water trapped in cracks expands when freezing, exerting stress and breaking the rock.
- Important in alpine and subpolar environments where temperatures fluctuate around 0°C.
- Spalling: Rapid expansion and contraction of hydrous minerals during heating (e.g., forest fires, >900°C).
- Root penetration: Root growth in joint cracks exerts pressure.

Chemical Weathering

Chemical weathering changes the mineralogy of rocks.
Example: An Egyptian obelisk weathered in Central Park, New York, after surviving 3000 years in the Sahara Desert due to the change in environment.
Marble tombstone (calcium carbonate) dissolves over time via solution weathering.
- $CaCO3$ dissolves into $Ca^{+2}$ and $HCO3^{-1}$ ions.
Granitic tombstones are more resistant due to the stability of constituent minerals like quartz.

Bicarbonate Reactions

Bicarbonate ions hasten weathering.
Production of carbonic acid and bicarbonate ions:
- H2O + CO2
  ightharpoonup H2CO3
  ightharpoonup H^+ + HCO_3^{-1}
Carbonic acid reacts with feldspar, calcite, and other silicates producing bicarbonate ions and other weathering byproducts.

Climate System and Weathering

Atmospheric carbon dioxide influences weathering and climate.
Interactions:
- Rocks weather, influencing climate and the lithosphere.
- Variability in atmospheric $CO_2$ corresponds to changes in weathering rates.
- $CO2$ uptake by silicate weathering reduces atmospheric $CO2$ .
- Reduced weathering leads to increased atmospheric $CO_2$ , causing climate warming and increased weathering.

Hydrolysis Reactions

Hydrolysis involves hydrogen ions ( $H^{+1}$ ) from carbonic or other acids.
Reactions convert primary feldspars to clay minerals like kaolinite.
Example: Eocene oxisol in Ione, CA, with iron oxide laterite over kaolinite clay, formed on alluvium from Sierra Nevada volcanics in a tropical climate.

Oxidation Reactions

Conversion of $Fe^{+2}$ to $Fe^{+3}$ .
Goethite (FeOOH) dehydrates to form Hematite ( $Fe2O3 + H_2O$ ).

Chemical Weathering of Carbonates

Carbonates are easily soluble in water, especially with acid.
Calcium and magnesium are taken into solution.
CaCO3 + H2CO3 ightharpoonup Ca^{+2} + 2HCO3^{-}
Example: Acropolis in Athens, Greece.

Roman Concrete

Inclusion of quicklime clasts allows self-healing.

Solution Weathering and Karst Topography

Solution weathering of limestone creates hummocky topography and sinkholes.
Acidic groundwater dissolves carbonate rock, forming cavities.
Collapse of cave roofs leads to sinkhole formation.

Karst Landscape Formation

Conditions:
1. Limestone/Marble bedrock.
2. Uplifting tectonic landscape.
3. Warm, humid climate.
4. Absence of continental glaciation.
Karst towers form from long-term solution weathering by surface and groundwater.

Limestone Cave Formation

Carbonic acid in groundwater dissolves limestone.
Water table is lowered.
Limestone dissolves from rocks above caves.
Deposited as stalactites and stalagmites.

Speleothems

Formation of stalactites (roof) and stalagmites (floor) demonstrates reversible calcium carbonate dissolution.
$CaCO_3$ precipitates in concentric layers.
Factors influencing solubility: water temperature, acidity, pressure changes.
Example: Carlsbad Caverns, New Mexico.

Differential Weathering Rates

Minerals weather at different rates.
Plagioclase crystals may stand in relief compared to oxidized mafic minerals.
Stability of silicate minerals is inversely related to formation temperature (Olivine least stable, Quartz most stable).

Differential Weathering and Landforms

Frost-wedging and solution weathering create features like Bryce National Park amphitheaters.
Monuments and hoodoos form due to differential weathering.
Resistant cap rock (e.g., quartzite) protects weaker underlying rock (e.g., sandstone, shale).

Spheroidal Weathering

Solutions attack corners, edges, and sides of rock blocks at varying rates.
Corners round, eventually forming spheres.

Soils

Soils form in residual bedrock or unconsolidated sediment.
Composed of weathered minerals and decomposed organic matter.
Vital medium for plant growth providing nutrients.
Reduce surface runoff and flooding.
Buffer acidity in runoff.
Important carbon sinks.
Contain water, air, humus, and living organic matter.

Soil Degradation

Loss of soils is an environmental catastrophe.
Soil degradation is caused by surface runoff, wind erosion, tillage, over-grazing.

Agents of Erosion

Wind, water, ice, gravity.

Soil Formation Stages

Bedrock begins to disintegrate.
Organic materials facilitate disintegration.
Horizons form.
Developed soil supports thick vegetation.

Soil Horizons

O Horizon
A Horizon
B Horizon
C Horizon

Soil Forming Processes

Additions (organic matter, wind erosion).
Losses (leaching, erosion).
Transformations (weathering of primary particles).
Translocations (movement of inorganic and organic material).

Soil Horizonation

Controlled by soil forming processes.
E horizon forms in boreal forest soils (high acidic conditions).

Caliche Soil Formation

Calcium carbonate accumulates in the B (Bk) horizon in arid climates.

Soil Forming Factors (ClORPT)

Climate
Organisms
Relief
Parent material
Time

Climate and Soil Development

Tropical soils can extend to depths of exceeding 30 meters due to the high weathering and translocation depths.
Clay mineral formation depends on cation mobility.
Climate influences depth of B horizon (illuvial zone).

Vegetation and Soil Development

Vegetation influences soil nutrient replacement and pH.
Soils beneath conifer forests have lower pH values (higher acidity).

Soil pH

pH tends to be lower for humid region soils compared to arid regions.

Relief and Soil Development

Steeper slopes have poorly developed soils due to mass wasting.
Slope aspect influences soil development and vegetation distribution.

Parent Material and Soil Development

Lithologic content influences soil development rate.
Felsic and fine-grained lithologies weather slower than mafic and coarse-grained.
Bedding structure influences water penetration.

Time and Soil Development

Soil development increases with time.