E&E 151 S Introductory Geology: Chapter 5 (Weathering and Soil) Definitive Study

Weathering

The group of processes at or near Earth's surface that change the physical and chemical character of rock stationary or moving.

Erosion

The physical picking up or removal of rock particles by an agent such as running water, ocean waves, glaciers, or wind.

Transportation

The movement of eroded particles by agents such as rivers, waves, glaciers, or wind.

Mechanical Weathering

The physical disintegration of rock into smaller pieces with little or no chemical change.

Chemical Weathering

The decomposition of rock from exposure to water and atmospheric gases, forming new chemical compounds.

Spheroidal Weathering

Chemical weathering that attacks the corners and edges of rocks faster than flat faces, resulting in rounded boulder shapes.

Differential Weathering

The process where different rocks or different parts of the same rock weather at different rates.

Pressure Release

The reduction of pressure on a rock body when overlying rock is eroded away, causing the rock to expand and crack.

Sheet Joints

Fractures developed parallel to the rock surface due to expansion from unloading.

Exfoliation

The process of spalling off of concentric rock layers or slabs from an exposed rock mass.

Exfoliation Domes

Large, rounded landforms developed in massive crystalline rock by exfoliation.

Frost Action

Mechanical weathering caused by the freezing and thawing of water in rock cracks.

Frost Wedging

The prying apart of rock when water freezing in cracks expands by 9% in volume.

Frost Heaving

The vertical lifting of soil and rocks caused by ice lens formation beneath the surface.

Talus

An accumulation of loose, angular rock fragments at the base of a steep cliff or slope.

Root Wedging

Mechanical weathering where growing plant roots pry into rock crevices and force them apart.

Thermal Expansion

Mechanical weathering caused by extreme temperature variations in desert environments or fires, causing rock expansion and cracking.

Salt Crystal Growth

Mechanical weathering caused by the pressure exerted by growing salt crystals as water evaporates in rock pores.

Surface Area Effect

The physical breakdown of rock into smaller fragments that exponentially increases total surface area exposed to chemical attack.

Oxidation

The reaction of oxygen with minerals or iron in rocks to form oxides, such as iron rusting.

Hematite

A brick-red iron oxide mineral (Fe2O3) formed by the chemical weathering of iron-bearing minerals.

Limonite

A yellowish-brown, amorphous hydrated iron oxide mineral formed during chemical weathering in the presence of water.

Acid Attack

Chemical weathering process where hydrogen ions (H+) replace other positive ions within mineral crystal lattices.

Carbonic Acid

A weak acid (H2CO3) formed when carbon dioxide dissolves in water, serving as the primary agent of chemical weathering.

Acid Mine Drainage

Environmental pollution caused by sulfuric acid produced from the weathering of exposed iron sulfide minerals like pyrite.

Solution Weathering

Chemical weathering where minerals like calcite dissolve completely in water or acidic solutions.

Hydrolysis

The chemical reaction between feldspar, carbonic acid, and water that forms clay minerals like kaolinite.

Kaolinite

A hydrous aluminum silicate clay mineral produced by the chemical weathering of feldspar.

Diamond Concentration

The accumulation of resistant diamonds in surface deposits after surrounding kimberlite rock chemically weathers away.

Regolith

The layer of unconsolidated, weathered rock material and mineral fragments covering bedrock.

Soil

A layer of weathered mineral and organic matter, water, and air at Earth's surface capable of supporting plant growth.

Soil Texture

The relative proportions of sand, silt, and clay sized particles in a soil.

Loam

Soil containing an ideal, balanced mix of sand, silt, and clay particles for agricultural plant growth.

Cation Exchange

The process where plant roots release hydrogen ions in exchange for nutrient cations held on negatively charged clay surfaces.

Soil Horizons

Distinct horizontal layers of soil distinguished by appearance, texture, and chemical composition.

O Horizon

The surface layer of soil consisting primarily of organic matter, decaying leaves, and plant litter.

A Horizon

Dark topsoil rich in organic matter and high in biological activity.

E Horizon

Light-colored zone of leaching (eluviation) where fine clay and soluble minerals are washed downward.

B Horizon

Subsoil zone of accumulation (illuviation) containing leached clays, iron oxides, or calcite.

C Horizon

Soil layer composed of partially weathered parent material directly above unweathered bedrock.

Topsoil

The combined O and A soil horizons, forming the most fertile upper layer.

Solum

The true soil consisting of the O, A, E, and B horizons.

Residual Soil

Soil formed in place directly from the underlying bedrock.

Transported Soil

Soil formed on parent materials brought from elsewhere by wind, water, or ice.

Pedalfer

Soil formed in humid temperate climates, characterized by subsoil enriched in iron and aluminum oxides.

Pedocal

Soil formed in arid or semi-arid climates, characterized by subsurface accumulation of calcium carbonate (caliche).

Laterite

Deeply weathered, red soil formed in hot, wet tropical climates, composed mainly of insoluble iron and aluminum oxides.

Bauxite

An aluminum-rich laterite deposit formed by intense tropical weathering, serving as the principal ore of aluminum.

Paleosol

An ancient, buried soil layer preserved in the geological rock record.

Splash Erosion

The dislodging of soil particles caused by the impact of falling raindrops.

Sheet Erosion

The uniform removal of soil in thin layers by overland water flow.

Rill Erosion

The formation of tiny, shallow channels carved into soil by running water.

Gully Erosion

Deep, wide channels carved into soil when rills merge, requiring heavy equipment to fill.

Dust Bowl

1930s environmental disaster in the U.S. Great Plains caused by drought, wind, and over-plowing, producing massive dust storms.

Alfisols

Soil order of moderately weathered soils with a clay-rich B horizon, common in humid temperate regions.

Andisols

Soil order formed in recent volcanic ash deposits.

Aridisols

Dryland soils of arid regions characterized by low organic matter and caliche accumulation.

Entisols

Young soils with little or no horizon development.

Gelisols

Cold-climate soils underlain by permafrost within 2 meters of the surface.

Histosols

Organic-rich soils formed in wet environments such as bogs and marshes.

Inceptisols

Young soils showing weakly developed horizons.

Mollisols

Highly fertile soils with dark, organic-rich A horizons typical of temperate grasslands.

Oxisols

Severely weathered tropical soils rich in insoluble iron and aluminum oxides.

Spodosols

Acidic soils formed beneath coniferous forests on sandy parent materials.

Ultisols

Strongly weathered, acidic soils with clay-rich B horizons in warm, humid regions.

Vertisols

Clay-rich soils that expand when wet and contract deeply when dry.

Formation of Carbonic Acid

CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻ (Carbon Dioxide + Water → Carbonic Acid → Hydrogen Ion + Bicarbonate Ion)

Dissolution / Solution of Calcite (Limestone)

CaCO₃ + H⁺ + HCO₃⁻ → Ca⁺² + 2HCO₃⁻ (Calcite + Carbonic Acid → Soluble Calcium Ion + Bicarbonate Ions)

Chemical Weathering of Potassium Feldspar to Kaolinite Clay

2KAlSi₃O₈ + 2H₂CO₃ + H₂O → Al₂Si₂O₅(OH)₄ + 2K⁺ + 2HCO₃⁻ + 4SiO₂ (Potassium Feldspar + Carbonic Acid + Water → Kaolinite Clay + Soluble Potassium + Bicarbonate + Soluble Silica)

Oxidation of Iron (Hematite Formation)

4Fe⁺³ + 3O₂ → 2Fe₂O₃ (Iron + Oxygen → Hematite)