Chemical & Mechanical Weathering How is soil formed?

Weathering - the disintegration and decomposition of rock at or near the surface of the earth.  It affects the rocks in place and no transport is involved.  This distinguishes weathering from erosion.

Mechanical/physical weathering - physical disintegration of a rock into smaller fragments, each with the same properties as the original.  Occurs mainly by temperature and pressure changes.

Chemical weathering - process by which the internal structure of a mineral is altered by the addition or removal of elements.  Change in phase (mineral type) and composition are due to the action of chemical agents.  Chemical weathering is dependent on available surface for reaction temperature and presence of chemically active fluids. 

Types of Mechanical Weathering: Frost Wedging - water expands when it freezes.  This photograph shows the individual layers within the sedimentary rock breaking apart through repeated cycles of freeze-thaw. 

Thermal Expansion and Contraction - heating causes rock to expand, cooling results in contraction; different minerals expand and contract at different rates. 

Mechanical Exfoliation - rock breaks apart in layers that are parallel to the earth's surface; as rock is uncovered, it expands (due to the lower confining pressure) resulting in exfoliation. 

Abrasion - physical grinding of rock fragments Abrasion - physical grinding of rock fragments.  Here, the photo shows some pits that have been eroded into the rock by sandblasting.  Another photograph which shows the powerful effect of wind generated abrasion is the Double Arch from Arches National Park.

Types of Chemical Weathering: Dissolution Dissolution is very common in areas that have a great deal of limestone.  Acidic waters (from pollution or natural) dissolve limestone allowing for additional water to gain entrance.  H2O + CO2 + CaCO3  --> Ca+2 + 2HCO3- water + carbon dioxide + calcite dissolve into calcium ion and bicarbonate ion

Oxidation (rust) 4Fe+2 +3O2 --> 2Fe2O3 ferrous iron + oxygen combine to form ferric iron oxide (hematite) Will happen to all iron-bearing silicates to varying degrees.  Common reaction minerals are hematite, limonite, and goethite.

Hydrolysis 2KAlSi3O8 + 3H20 --> Al2Si2O5(OH)4 + 4SiO2 + 2K(OH) Silicate minerals (unstable at the earth's surface) weather to form clay minerals.

Factors that influence chemical weathering: 1. Climate 2 Factors that influence chemical weathering: 1.Climate 2.Living Organisms     a.decomposition 3.Time 4.Mineral composition     a. Chemical Weathering Products     b.Clays     c.Metals ores     d.Rounding of boulders

Soils and Soil Formation Dependence of weathering type on the mean temperature and annual rainfall. Weathering rates depend on the composition of the rock, temperature range and rainfall amount.  Weathering produces soils.  Soils may or may not remain in place, and any soil may be a combination of residual and transported material.

The two categories of soil are: Residual soil: Remains in place; has not been transported (gruss). Transported soil: Transported by wind or water and deposited.  

A complete soil profile will have the following components: O horizon:  Organic debris and leaf litter on the surface. A horizon: Topsoil - leaching, water movement down, Organic and Mineral material transported downward. B horizon: Subsoil - accumulation of dissolved material and fine clays, hardpan. C horizon: Partially altered parent rock material. Bedrock:  Unweathered parent rock material.

Factors in Soil Formation Climate: The greater the rainfall amount, the more rapid the rate of erosion and leaching.  Topography: The steeper the surface slope, the more likely any eroded material is to be transported out of the system. Parent Material: Sandstones are more resistant to weathering than limestone in humid climates, but limestone are more resistant than sandstones in arid climates. Plant and Animal activity: Plant and animal activity produces acids that are powerful erosion agents.  Time: Reaction rates are slow, the longer a rock unit has been exposed, the more likely it is to be weathered.