GLG 101

Chapter 5

Weathering, Erosion, and Soil

weathering: physical breakdown (mechanical) and chemical alteration (chemical) of rocks near or at the surface

erosion: removal of weathered material

transport: by wind, water, glaciers

mechanical weathering:

frost action: repeated freezing and thawing; product is called talus; frost wedging; frost heaping

pressure release: from intrusive bodies of rock (e.g. batholiths)

exfoliation produces exfoliation domes

thermal expansion: response to heating and cooling, not very important

salt crystal growth: active in arid areas

activities of organisms: bacteria, roots of plants, animals (worms)

chemical weathering:

most important agents: water, acids, oxygen

solution: a solid substance dissolves (e.g. NaCl)

oxidation: reaction with oxygen to form oxides, hydroxides (e.g. rust)

hydrolysis: reaction between hydrogen ions and hydroxyl ions in water with mineral (e.g. feldspar hydrolysis => clay minerals)

Factors controlling rate of chemical weathering

particle size: the smaller, the faster

climate: the warmer, the faster

parent material (see Bowen's reaction series): high-temp. minerals weather faster

Soil

regolith: unconsolidated rock and mineral fragments

soil: regolith which consists of weathered material, water, air, and organic matter

humus: organic matter, decomposed by bacteria

residual soil: weathered residual accumulated over the bedrock

transported soil: eroded and transported (e.g. wind blown) and deposited elsewhere (e.g. loess)

Soil profile:

soil consists of distinct layers (horizons) that differ in texture, structure, composition, and color

O horizon: organic matter (humus); top

A horizon: top soil, intense biological activity

B horizon: subsoil, less organic material, zone of accumulation of minerals leached out of A horizon

C horizon: partially altered and unaltered parent rock; bottom

Factors controlling soil formation:

climate: most important factor

parent material: not that important

organic activity: from single-celled organisms to worms and gophers

relief and slope: slope angel, direction of slope

time: geologically fast