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Weathering

Objectives:

    1.    Describe how mechanical weathering breaks rocks down.

    2.    Describe how chemical weathering breaks rocks down.

    3.    Name three factors that affect weathering rates.

Notes:

Weathering is the name given to the process of breaking-down rocks at Earth's surface.  This is accomplished due to the exposure to oxygen through the process of oxidation and to the exposure to water in its various forms (liquid, solid, & gas).  Weathering can be further broken down into 2 different categories; chemical & mechanical.

Mechanical weathering:

PAK Diagram of Mechanical Weathering (Unloading, Frost Wedging, Tree-Root Wedging, Temperature Changes)

 

(www.members.aol.com/rhaberlin/ images/mwmech.gif)

 

 

 

Mechanical weathering is the process of erosion where larger rocks are broken into smaller rocks.  Water is the primary agent for mechanical weathering.

Frost wedging
water that accumulates in the cracks of rocks wedges the rocks apart when freezing
water takes up around 10% more space as it freezes
occurs in areas where frequent freezing and thawing occurs
frequent wetting and drying can break apart porous rocks (shale, sandstone, clay) 

(regentsprep.org/.../units/weathering/ frostwedging.gif)

Rock split by frost wedging.(www.suite101.com/files/articles/ 88000%5C88697/brokenrk.jpg)

 

Abrasion
wears away the surface of rocks via friction
wind can blow sand and small particles across the surface
gravity can cause larger rocks and boulders to move across the surface

(www.u.arizona.edu/ic/nats1011/ lectures/ch12/FIG12_002A.JPG)

Plants
roots act as wedges breaking rocks apart as they grow

(comp.uark.edu/~sboss/ rockwx02.jpg)

Animals
animals that burrow into the substrate leave holes and spaces that allow water accumulation and aid in weakening the rock structures

 

Upward expansion (exfoliation)
as rocks formed from pressure are uplifted to the surface, the pressure above the rocks is lessened causing rocks to fracture in parallel bands to the surface

 

Chemical Weathering:

All chemical weathering takes place in the presence of water (sometimes known as the universal solvent).  It is the chemical breakdown of the bonds that hold the rock together (also known as hydrolysis).  Other types of chemical weathering involves both oxygen and acids.

Water
The breaking down of substances in the presence of water is known as hydrolysis
The breakdown occurs as the ions in the minerals are dissolved
The rate of hydrolysis is increased by the presence of acids in the water
hydrogen liberating (releasing) compounds (ex: carbonic acid (acid rain))
acid rain is produced by the mixing of atmospheric water with greenhouse gases.  The source of these gases are mostly industrial.  Examples include:
carbonic acid- H2O + CO2
nitric acid - H2O + NO
sulfuric acid - H2O +SO2
Oxidation
Oxidation is the reaction of chemical of oxygen with other minerals in the presence of water
rust is an example of the reaction of oxygen and iron    (O2 +2Fe = 2FeO)
once the material is oxidized it is chemically changed permanently

 

Rates of Weathering:

Weathering is generally a very slow process.  It is not static, though, as its rate is affected by different factors.  The two biggest contributors to weathering are surface area and the climate.

Surface area is the amount of the rock's surface that is exposed at Earth's surface.
the greater the surface area the greater the weathering
surface area increases as an object gets smaller

 

Rock composition
rocks with high soluble mineral (halite, calcite, augite) contents tend to weather faster than rocks containing little soluble minerals
many rocks are only as durable as the cement that holds them together.
sedimentary rocks formed via precipitation may be most susceptible to this type of weathering

 

Climate
warm & wet
high mechanical and chemical weathering
cold & dry
mostly mechanical
 

 

 

 

 

 

Last modified: November 14, 2003