Weathering may be by mechanical or chemical processes.
Mechanical weathering may be by the action of water (streams, rivers, surf), ice (frost, glaciers), or wind.
Chemical weathering may be by oxidation, hydration, or biological activity.
Here granite is weathering chemically by oxidation and hydration of the surface layer which causes them to peel off like the layers of an onion, leaving odd, rounded shapes like these in Zimbabwe. This is called spheroidal weathering.
Susceptibility of silicate minerals to chemical weathering follows Bowen's reaction series with olivine being most susceptible and quartz most impervious.
Oxidation is the result of the chemically corrosive free oxygen in our atmosphere which may change the oxidation state (valence) of some elements, notably sulfur (S) and iron (Fe), but also manganese (Mn), copper (Cu) and uranium (U).
Hydration occurs as the result of acidic solutions and can transform feldspars to clays, and dissolve carbonates.
Acid rain is the result of oxidation of sulfur in fossil fuel to form SO3 which hydrates to form H2SO4. Acid rain is neutralized by carbonate bedrock, but remains acidic in surface and ground water on granitic bedrock.
Acid mine drainange is the result of oxidation of pyrite (FeS2) to H2SO4 plus FeOOH (limonite). This leaves a yellow rust stain on stream boulders and can kill aquatic life. This is very common and is a major environmental problem here in Colorado. You may have seen it Clear Creek just below the town of Blackhawk. Not only does the acid kill aquatic life, but all of the heavy, toxic, chalcophile metals like lead, zinc, cadmium, and mercury are mobilized in the process.
Here an old gold mine is draining very acidic solutions and depositing iron hydroxide minerals in Summit County, CO.
Erosion is the reduction of exposed landforms.
Transportation is the movement of eroded material downslope.
A soil is a accumulation of weathered rock material together with organic matter at the Earth's surface.
Soils are classified into pedalfers and pedocals.
Pedocals are soils of arid regions and are characterized by calcite (CaCO3) cementation due to upward movement of water (evaporation). Extreme pedocals may contain salt and borate deposits.
Pedalfers are soils of humid regions and are characterized by highly leached clays due to downward movement of water. Extreme pedalfers are called laterites (soils of rain forests) which, if cemeneted, become bauxites. The most insoluble elements are Al3+ and Fe3+.
Here is a laterite soil cross section in Brazil. The deep red color is due to Fe3+ hydroxide minerals in the soil.
Here are some interesting links for further exploration of:
GEOL 1010 Class Note 7
GEOL 1010 Class Note 5
Joe Smyth's Home Page