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

• weathering is simply the chemical and/or physical breakdown of a rock material--weathering involves specific processes acting on rock materials at or near the surface of the Earth

B. Erosion

• is the removal or transportation of material by agents as running water, ice, wind, etc.
• different degrees of weathering in rocks and erosion work together to form some interesting landforms such as Bryce Canyon, National Park, Utah
• click here to see Bryce Canyon

II. Kinds of weathering

A. Physical or mechanical

• is the breakdown of a substance by disintegration

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

• is the breakdown of a substance by decomposition resulting in the formation of new mineral matter--water and acids especially carbonic acid (H2CO3) are important agents in this type of weathering

• Kinds of chemical weathering
  • hydrolysis--is the chemical reaction of water with the substance in which the (OH) in water (HOH) becomes part of the new formed substance-- often silicate minerals will weather this way to form clays

  • solution (leaching)--is the dissolving of substances by acid in waters usually leaving residual non-solubilized materials--carbonate minerals are especially affected by leaching

  • oxidation--is the reaction of a substance with oxygen causing a rust to form--manganese (Mn) and especially iron (Fe) minerals are affected

• although there are individual chemical and physical weathering types often combinations of these can affect the total weathering of a substance

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

A. Total surface area of mineral or rock

• chemical and physical weathering increases proportionately with the amount of open space (cracks, holes, etc.) at the surface of the rock and extending through the rock

B. Climate

• temperature and the amount of moisture can influence the kind and magnitude of weathering--a humid hot climate should experience alot of hydrolysis, leaching, oxidation and physical weathering from biological activity but there would be very little or no frost wedging

• a good example of climate affecting weathering is a granite obelisk called Cleopatra's Needle--for almost 3500 years it was in Egypt with barely a weathered trace but present in New York City for only about 80 years it has been weathered (pitted) alot
• click here to see Cleopatra's Needle in Egypt
• click here to see Cleopatra's Needle in New York

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

• obviously time is a factor in all geologic processes including weathering

IV. Soil

A. Definition and basic nature of soil

• is an unconsolidated material formed from the chemical and/or physical weathering of substances as bedrock or outcrops and varying degrees of organic material present--soil can be found in situ (in place) or transported

• obviously soil is an important product of weathering and based on the factors of weathering there is a classification of soil types (pedocals and pedalfers)
• click here to see definition of soil types

B. Soil profile and soil horizons

• a typical soil profile includes 3 horizons--the 2 upper horizons A and B are known as the solum--the A horizon is concentrated in organic matter and acids and represents an area of high leaching or solution effects--the B horizon is concentrated in clay size materials and rich in iron (Fe)--the C or bottom horizon is comprised primarily of fragmented bedrock

• eluviation is the removal of clay and soluble materials from the A horizon and illuviation is the precipitation of eluviated materials from the A horizon in the B horizon

• click here to see the soil profile

• click here for more on weathering and soil formation

C. Soil creep

• is a form of mass wasting of earth materials--creep is often initiated if the ground becomes saturated with water allowing gravity to pull the water logged soil downslope--freezing and thawing of soil can also cause heaving thus loosening the soil
• click here to see soil creep

D. Solifluction

• is a form of mass wasting and soil creep that occurs in regions underlain by permafrost in arctic climates--this form of creep occurs in the layer called the active layer above the permafrost zone---the active layer becomes saturated with water during the summer months since the underlying permafrost layer is impermeable--this results in a creep down-slope of the active layer

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Cleopatra's Needle in Egypt click to return

Cleopatra's Needle in New York click to return

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