Weathering

Charity I. Mulig

Weathering is the …

physical breakdown (disintegration) and chemical alteration (decomposition) of rocks at or near the Earth’s surface.

Types of Weathering

Mechanical Weathering

Accomplished by physical forces that break rock into smaller pieces without changing its mineral composition

Examples of Mechanical Weathering

Frost Wedging

• Caused by repeated freesing and thawing of water

Talus Slopes- Result of frost wedging.- Accumulated rocks

usually piles at the base of steep rocky cliffs

Unloading

• Reduced pressure on igneous rock causes it to expand and allows slabs of outer rock to break off in layers in a process called exfoliation.

Sheeting is the process of exfoliation.

Biological Activity

• organisms, including plants, burrowing animals, and humans, can cause mechanical weathering

Examples of Chemical Weathering

Chemical Weathering

• The original rock decomposes into substances (mineral/compounds) that are stable in the surface environment.

• Products of chemical weathering will remain essentially unchanged as long as they remain in an environment similar to the one in which they formed.

Chemical Weathering

• “water is the most important agent of chemical weathering”

• Water tend to:– Dissolve oxygen which then oxidizes minerals– Combine with carbon dioxide and form carbonic

acid which acts on rocks

Chemical Weathering…of Granite …of Silicate

• produces clay minerals, soluble salt (potassium bicarbonate), and silica in solution

•quartz remains substantially unaltered

• produces insoluble iron oxides and clay minerals

Chemical Weathering

• Spheroidal Weathering– Process which leads to

rocks taking a more rounded shape

– May lead to spalling

Factors Affecting Rates of Weathering

Mechanical Weathering

Increases the surface area available for chemical attack

Rock Characteristics

• Encompasses all the chemical traits of rocks including mineral composition, solubility and physical features such as joints or cracks.

– Silicates are relatively more resistant to chemical weathering than calcites

– Silicates weather in the same sequence as their order of crystallization.

Climate

• Temperature and moisture are the most crucial factors.

• Chemical weathering is most effective in areas with high temperatures and abundant moisture.

Rate of Weathering

• Differential Weathering–Masses of rock do not weather

uniformly–Caused by variations in composition–May create spectacular rock

formations and landforms

Other Links

• http://geology.about.com/od/geoprocesses/ig/chemweathering/basaltrind.htm