The photo shows Bryce Canyon, Utah which formed from weathering and erosion--weathering is an important process in the formation of soils

I. Nature of weathering and erosion

A. Weathering

B. Erosion

II. Kinds of weathering

A. Physical or mechanical

1. Frost (ice) wedging

·         is the alternating freezing and thawing of moisture in the openings or cracks of rocks resulting in a breakdown of the rock--this is the single most abundant form of physical weathering

·         a slope of gravel material called talus can form at the base of a cliff or mountain--this accumulation of material is an example of mass wasting--frost wedging loosens pieces of bedrock which then fall and accumulate

·         click here to see a talus slope

·         in humid northern latitudes during the late Autumn or early Spring frost wedging causes "potholes" in streets and highways

·         click here to see frost wedging

2. Unloading--exfoliation

·         erosion of upper rock portions causes remaining underlying rocks to expand (like an inflated balloon after pressure on it is released) resulting in cracking and peeling off in slabs similar to onion skin layers

·         click here to see exfoliation

3. Organic activity

·         activities of plants and burrowing animals can cause a rock material to disintegrate

·         click here to see tree roots causing rocks to disintegrate

B. Chemical weathering

III. Factors which affect the rate and/or extent of weathering

A. Total surface area of mineral or rock

B. Climate

C. Composition of rock or mineral substance

1. Rate and extent of chemical weathering

·         as mentioned earlier silicate composition substances weather extensively by hydrolysis while carbonate substances do so primarily by leaching-solution and iron and manganese minerals weather primarily by oxidation

2. Silicate mineral weathering series

·         silicate minerals higher in Bowen's mineral series will decompose at a greater rate and extent than those lower in the series based on composition alone--a rock containing olivine, pyroxene and plagioclase as does gabbro should weather at a greater rate and more extensively than granite which contains quartz, orthoclase and hornblende, all other weathering factors being the same

·         which should weather more based on composition alone, a rhyolite or a basalt?

·         click here to see the silicate weathering series

D. Time

IV. Soil

A. Definition and basic nature of soil

B. Soil profile and soil horizons

C. Soil creep

D. Solifluction



page 108
click to return

page 201
click to return

page 110
click to return

page 111
click to return

page 113
click to return

Cleopatra's Needle in Egypt
click to return

Cleopatra's Needle in New York
click to return

page 119
click to return

page 123
click to return

page 209
click to return