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WEATHERING AND EROSION

  

The Rock Cycle- The rock cycle shows how one type of rocky material gets transformed into another in response to changing conditions.

 

Weathering, Erosion, and Transportation

 

Rocks exposed at Earth’s surface are constantly changed by the atmosphere, hydrosphere, and biosphere.

 

Weathering is the group of physical and chemical processes that change the character of rocks at or near Earth’s surface.  Weathering involves the breakdown of rock and the formation of new minerals.

 

Erosion is the physical removal and transportation of rock particles from the source by water, wind, or ice.

 

There are two types of weathering:

 

Mechanical weathering

-The physical disintegration of rock into smaller pieces.

-Facilitated by the action of ice, water, and wind.

 

Chemical weathering

-Chemical decomposition of rock from exposure to atmospheric gases (oxygen, water vapor and carbon dioxide) or liquid water (which is often acidic).

-New minerals form.

-The rate of chemical weathering increases as surface area of rock increases.

 

Weathering and the Earth Systems

 

Within our solar system, Earth seems to be the only planet with surface rocks actively being weathered by wind, water, and living organisms. 

 

Atmosphere- Gases like O2, CO2, and H2O are critical to the chemical weathering of rock.  These gases react with pre-existing minerals to produce new minerals that are in equilibrium with surface conditions.

 

            ex.        CO2 +  H2O à  H2CO (Carbonic Acid)

 

Chemical weathering is essential to life in that it removes carbon dioxide from the atmosphere, reducing green-house effect.               

 

Atmospheric wind also causes the mechanical breakdown of rock via a sandblasting effect.

 

Hydrosphere- Oxygen dissolves in water and then can oxidize iron in rocks:

 

ex.        FeO   +   O2  à   Fe2O3

 

Biosphere- Decaying organic matter in soils produces acidic moisture in soils.

 

Types of Mechanical Weathering

 

Running water/glacial ice- Mechanically loosens and abrades mineral and rock particles.

 

Thermal cycling- Large temperature changes fracture rocks via repeated expansion and contraction.

 

Frost action- The freezing and expanding of water causes frost wedging and frost heaving.

 

Pressure release- The removal of overlying rock from erosion allows expansion and fractures (sheet joints and exfoliation).

 

Plant growth- Growing roots widen fractures (root pry).

 

Burrowing animals- Lead to accelerated mechanical weathering.

 

Chemical Weathering of Minerals

 

Feldspars (the most common minerals in Earth's crust)- Slightly acidic rainwater (H2CO3) attacks feldspar, producing clay minerals (hydrated sheet silicates).  The K+, Na+, and Ca2+ ions are released into the water.

 

Minerals highest on Bowen’s reaction series are most susceptible to weathering.  Therefore, olivine and pyroxene weather quickly to produce clays and iron oxides.

 

Quartz is the least susceptible to chemical weathering.  It can be transported all the way to the oceans.

 

Soil - Soil is a layer of weathered, unconsolidated material on top of bedrockCommon soil constituents include:

Clay minerals (from feldspars)

Quartz

Water

Organic matter

Residual soil- Residual soil develops from weathering of underlying rock in situ

 

Transported soil- Soil brought in from elsewhere else.  Examples are floodplain deposits (from river-transported sediments) and wind-transported deposits called loess.

 

Soil Horizons

Soil composition is determined by the parent rock composition.  The soil composition evolves with time and chemical weathering.

 

    O horizon - uppermost layer; organic material

    A horizon - dark layer rich in humus, organic acids

    E horizon - zone of leaching; fine-grained components removed by percolating water

    B horizon - zone of accumulation; clays and iron oxides leached down from above

    C horizon - partially weathered bedrock

 

Soil thickness

- Increases with time

- Typically greater in wetter climates

- Greater in areas with shallow slopes vs. steep slopes

 

Soils and Climate- Soil thickness and composition are greatly affected by climate.  Warm, wet climate conditions maximize weathering.

 

Wet climates:

-More chemical weathering and thicker soils

-Soils in moderately wet climates tend to have significant clay-rich layers.

 

Arid climates:

-Less chemical weathering and thinner soils

-Subsurface evaporation leads to build-up of salts

-Caliche- Calcite-rich accumulation zones may form, cementing soil together into a hardpan called caliche.

-Desert varish- Wind-transported clays desert heat → dark varnish (manganese plays an important role; Mn levels are 50 times greater in varnish than the average crust)..

Extremely wet climates (e.g., tropical rainforest):

-Highly leached and unproductive soils (laterites and bauxite deposits)

-Most nutrients come from thick O/A horizons.