Weathering
Weathering - chemical (altered or dissolved minerals) and physical (fragmented rocks) processes that break down rocks IN PLACE at the surface.
Erosion - loosen and move soil and rock downhill or downwind.
Some rocks weather more rapidly than others
Different minerals - feldspar crystals corrode at earth's surface, quartz crystal do not. calcite completely dissolves
Structure of rock - shale splits easily along bedding planes.
Climate - High temperatures and heavy rainfall speed chemical weathering; Physical weathering my be active in climates with little chemical weathering (e.g., deserts)
Soils - fragments of rock, clays formed by chemical weathering and organic material. Soil retains water and hosts organisms which speed weathering
Length of Exposure - More time, more weathering
Chemical Weathering - some minerals dissolve and others combine with water, oxygen, and carbon dioxide to from new minerals
Feldspar to Kaolinite Clay
Feldspar + Carbonic Acid + Water -> Kaolinite + dissolved ions
Dissolved Carbon Dioxide makes water slightly acidic (e.g., Coca Cola)
Physical weathering and high temperature makes the process faster by increasing surface area and reaction rate
Chemical weathering makes physical weathering faster by making the rock easier to break
Other Silicates
Amphibole and mica weather to clays
Olivine (fast), pyroxene (fast) and quartz (slow) completely dissolve.
Iron Silicates to Iron Oxides (rust/red soils of Georgia)
Dissolution of Carbonates (Solid completely dissolves)
calcite + carbonic acid -> calcium ion + bicarbonate ion
Chemical Stability - tendency of a compound to spontaneously react to become a different chemical compound. Chemical stability is related to environment (e.g., temperature, water). At the Earth's Surface, Chemical Stability of Silicates is roughly inversely proportional to Bowen's reaction series. Olivine and Ca-Feldspar are unstable. Quartz is stable.
Minerals that form at High Temperature and High Pressure (olivine) are unstable at the earth's surface.
Fe, Mg, Ca rich and simple silicate structures (isolated, chain) are less stable. More ionic bonds which are not as strong as covalent bonds.
Calcite is other common rock forming mineral that weathers rapidly
Physical Weathering
Natural Zones of Weakness - Bedding Planes, fractures, joints
Activity by Organisms - tree roots
Frost Wedging - expansion of freezing water.
Mineral Crystallization - calcite, gypsum grow in cracks and pry them open (pyramids)
Exfoliation and Spheroidal Weathering - cracking and splitting off of layers from an outcrop or boulder (like layers of an onion).
Soils: Residue of Weathering - on moderate or gentle slopes a layer of loose, heterogeneous weathered material over bedrock. Weathered and unweathered parent rock, clays, metal oxides, and organic matter.
Soil Profile
A-Horizon - top layer, contains clays, quartz & organic matter
B-Horizon - precipitation of soluble minerals from A-Horizon, iron oxides, sparse organic matter
C-Horizon - slightly altered bedrock mixed with clays
Climate, Time, and Soil Groups
Wet: Laterites - thick soils, upper layer is completely leached of soluble and easily weathered minerals. Not productive for farming.
Dry: Pedocals - thin soils, much unweathered minerals, soils rich in calcium, low in organics
Temperate: Pedalfers - abundant, insoluble minerals, clays, and iron alteration products. Good for agriculture.
Weathering, Erosion and Humans
Agriculture, building, road construction accelerates erosion because they break up the soil and eliminates erosion-resistant natural plant cover
Soils may be long term sources of pollution from salt, pesticides and petroleum products.
2 billion tons of topsoil are lost to erosion each year in U.S. and Canada. Soils take thousands of years to form.