Understanding Weathering Processes and Their Effects

Weathering

The breakdown and alteration of Earth's rocks and minerals.

Endogenic Processes

Internal energy / tectonic processes.

Exogenic Processes

Processes that wear away and/or rearrange landforms.

Landmass Denudation

The process of wearing away the land surface.

Geomorphology

The study of landforms and the processes that shape them.

In situ process

Material altered in place, 'not moved'.

Regolith

Loose, broken-up bedrock material that helps make soil and move material.

Laterite

Highly weathered tropical soil found along the Rio Negro, Brazil.

Weathering Effects

1. Lowers resistance, 2. Drives soil development, 3. Forms unique landforms.

Material Resistance to Weathering

A function of internal resistance of material and magnitude/type of external forces.

Factors Influencing Weathering

Includes composition of rocks, areas of weakness, climate, temperature, and precipitation.

Composition of Rocks

Mineralogy including Quartz (SiO2), Calcium carbonate (CaCO3), Clay Minerals (Feldspars), etc.

Planes of Weakness

Stress at zones of weakness within material exceeds strength of the material.

Porosity

Pore space within a rock.

Permeability

Ability of rock to transmit water.

Vegetation

Deflects water, stabilizes rock, and contributes to weathering.

Microclimates

Climates of small areas influenced by local variations in temperature and precipitation.

Mechanical Weathering

Also known as physical weathering, it involves the disintegration of rocks into smaller pieces.

Chemical Weathering

Involves the decomposition of rocks through chemical processes.

Freeze Thaw Weathering

Water freezes in cracks, increasing volume ~9%, leading to tensile stress that exceeds rock strength.

Talus

Rock apron at the base of steep slopes from mass wasting.

Angle of Repose

The maximum surface angle maintained by loose material.

Wetting and Drying

Minerals expand/contract in response to moisture changes, affecting rocks.

Salt Crystallization

The formation of salt crystals, which can contribute to weathering.

Physical Weathering

The process of breaking down rocks through physical forces without changing their chemical composition.

Salt Weathering

The process where salt enters in solution, water evaporates, crystals form, and crystals hydrate and expand, leading to disintegration of material.

Expansion of Clay Minerals

The increase in volume of clay minerals when wet, with specific expansions: Ca-montmorillonite (45-185%), Na-montmorillonite (1400-1600%), illite (15-120%), and kaolinite (5-60%).

Desiccation Cracks

Cracks that can form in clay-rich soils as a result of drying.

Expansive Clays

Clays that can cause damage to structures; estimated to affect 1/4 of all homes in the United States.

Exfoliation

The process of rock layers peeling away due to pressure release.

Thermal Expansion

The expansion of rocks and minerals due to temperature changes, such as those caused by fire or forest fires.

Biological Activity

The influence of living organisms, such as plants and animals, on the weathering of rocks.

Chemical Weathering Equation

The reaction where primary minerals react with water to produce dissolved ions and secondary minerals.

Role of Acidity

Acidity from the reaction of CO2 with water, which can enhance weathering processes.

Solution Weathering

The process where minerals dissolve in water into their constituent ions.

Carbonate Weathering

The reaction of carbonate minerals with acids, leading to the dissolution of minerals like limestone.

Carbonic Acid Formation

The formation of carbonic acid (H2CO3) from the reaction of water and carbon dioxide.

Pitting and Etching

Surface features that result from chemical weathering, particularly on limestone structures.

Surface Area Calculation

The measurement of surface area affected by weathering, with examples: 16 cm on a side = 1,536 cm²; joints 4 cm apart = 3,072 cm²; joints 1 cm apart = 24,576 cm².

Chemical Weathering Agents

Water and organic acids that facilitate the chemical weathering process.

Expansive Soils

Soils that expand and contract significantly, causing structural damage.

Half Moon Dome

An example of an exfoliation dome located in Yosemite National Park, CA.

CaCO3 + H+

HCO3

Oxidation

Reaction between metallic elements and oxygen, very effective with increased temperature and precipitation.

Oxidation of Iron

Fe2SiO4 + 4H2CO3 → 2Fe2+ + 4HCO3

Oxide

Metal + oxide ions (O2-).

Oxidized minerals

Often have yellowish brown to red color.

Basalt

Igneous rock that can show weathering effects.

Hydrolysis

Chemical reactions that produce different compounds, often involving water.

Hydrolysis of K-feldspar

2KAlSi3O8 + 2H+ + 9H2O → H4Al2Si2O9 + 4H4SiO4 + 2K+.

Weathering of feldspar in granite

A common form of hydrolysis weathering.

Residual clays

Form into soils and/or shale stone (sedimentary rock).

Quartz (SiO2)

Sand transported to beaches.

Differential Weathering

Not all materials weather at the same rate; varies over many scales.

Corestones

Relatively unaltered rock surrounded by weathered materials.

Grus

Coarse rock fragments from granular disintegration of rock.

Saprolites

Deeply weathered rock, largely converted to clay and Al or Fe oxides.

Weathering pits

Positive feedback loop in weathering processes.

Joints in sandstone

Formed by uplift and collapse, leading to weathering.

Chemical weathering activity

More active in warm, wet climates.

Forms of mechanical weathering

Include exfoliation, thermal expansion, and frost action.

Freezing of water in joints

Associated with mechanical weathering.