Physical Geology
Weathering and Erosion
Weathering:
The disintegration or decomposition of rx on the Earth's surface.
- - eventually breaks down rx to form sediments in rivers, beach sand, and soil.
- - 2 types: Mechanical and chemical
Mechanical Weathering:
- breaks rocks by physical forces into smaller and smaller pieces, each retaining the characteristics of the original piece.
- with each new pieces, the area available for more weathering increases, so the process increases logarithmicly
- four important processes: Frost wedging, Unloading, Thermal expansion, and Organic activity.
- also: wind, glacial ice, running water.
Frost Wedging: Alternate freeze and thawing.
- water expands 9% when it freezes.
- - creates and outward force.
- - in nature, water gets down into cracks and fissures in rx, expands, and enlarges the openings. Cycles over and over again the rx finally break into smaller fragments.
- - in mountainous regions, this ends up producing a talus slope which forms at the base of steep outcrops.
Unloading:
- when large masses of igneous rx are exposed (like Table Rock, SC), it tends to exfoliate like an onion - skin by skin. This is termed sheeting.
- - thought to occur because of the reduction of overlying pressure as the outer layers of rx are stripped away.
- - fractures develop parallel to the surface topography, giving the outcrop a domed look. Domes are termed exfoliation domes.
Thermal Expansion:
- Heating and cooling of rx in very hot desert regions cause stress on the outer surface of rx. Eventually, the outer shell will crack and fall off.
Organic Activity:
- - Plant roots grow into rx fractures in search of water and mineral nutrients. As roots grow, fractures widen.
- - Burrowing animals move fresh material to the surface, allowing it to weather quicker than it would undergound.
- - Decaying organisms produce acids, which contribute to chemical weathering.
Chemical Weathering:
- alters internal structure of mins by removing/adding elements.
- original material erodes into something stable in the surface environment.
- Water is the most important agent in chemical weathering.
- - can oxidize (like rust on a fender) and make a material weak & friable.
- - combines w/CO2 to form carbonic acid: H2CO3
- orthoclase weathers into clay & silica when reacts w/H2CO3.
- Quartz is very resistant, doesn't weather easily.
-Ends up being able to be transported in rivers great distantces without being broken down. That's why beaches tend to be quartz-rich (sand is quartz).
- quartz --> quartz
- feldspars --> soft clay minerals
- amphibole --> clay, hematite
- olivine --> hematite
- resistance to weathering is just the opposite of Bowen's Reaction Series.
- - olivine is least resistant, while quartz is most resistant.
- - termed differential weathering (+ avaiability of joints, climate, mins, etc.)
- - causes spectacular formations in some places.
- climatic factors (temp, avail of water, altitude, etc. affect the rate of weathering.
- chemical weathering can cause angular fragments in a rock mass to become rounded and take on a spherical shape.
- attacks the corners & edges first (greater surface area available)
- called spheroidal weathering.
Back to the selection page.
|
