Weathering

• The break up of rock due to exposure to the atmosphere.
• Weathering will occur on all rock surfaces, but at varying rates.
• Caused by such events as wind, water, rain, freezing, and chemical breakdown (acid rain)
• Materials are weathered and then carried away through a process called Erosion - the removal and transport of material by natural agents

Mechanical Weathering

• Disintegration
• Rocks are split or broken into smaller pieces of the same material without changing its composition
• Ex. - the breaking of a rock cliff into boulders and rocks

Chemical Weathering

• Decomposition
• Rock minerals are changed into different substances
• Water is an important agent of chemical weathering
• Ex. - formation of clay from feldspar minerals by water is a good example.

• Both processes occur almost always together.
• Since water vapor is in the atmosphere almost everywhere on earth, both occur.

Types of Mechanical Weathering

• Water takes up 10% more space when it freezes
• This expansion and pressure will cause rocks to easily break under the stress
• This is called Ice Wedging
• Ice wedging also causes potholes on city streets in a process called Ice Heaving
• Clay-based rocks that shrink and swell with the moisture conditions cause a breakdown of the rock in a process called Wetting and Drying
• Small plant life can cause a weathering effect on rock as well - the root systems put increased pressure on the rock and hold moisture in the rock, creating a chemical weathering effect
• Some rocks have the ability to peal away in layers. This is called Exfoliation

Types of Chemical Weathering

• Chemical reaction of water with other substances is called Hydrolysis - feldspar is a common reactant with water
• Chemical reaction of oxygen (air) with other substances is called Oxidation - iron-bearing minerals are a common reactant
• Carbon dioxide dissolves easily in water, creating a weak carbonic acid - feldspar and mica are common minerals that react with this acid creating weathering
• Calcite is especially reactant with this acid!

Strong Minerals (non-reactant)

• Quartz does not react with anything
• Minerals made of quartz are usually very hard to weather chemically
• Common strong rocks are sandstone, conglomerate, and quartzite
• Shales are probably the weakest of the common rocks - easily broken with water
• Marble and limestone are strong mechanically, but weaken with chemical weathering

Rate of Weathering

• Rocks differ in rate of weathering
• Rate is affected by amount of rock surface exposed to the elements - smaller pieces weather faster than one large piece of rock
• Climate can also affect the rate of weathering - chemical weathering favors warmer climates; mechanical weathering favors cold climates
• Remember, weathering of any kind is VERY slow - Limestone dissolves at only 1/20^th of a centimeter per 100 years!

Soils and Mass Movement

• Soil is defined a loose, weathered rock and organic material in which plants with roots can grow
• The material in which soil is formed is called the Parent Material - often the bedrock formation beneath the soil
• Soil that has the bedrock beneath it as its parent material is known as Residual Soils
• Soils that are found in areas where they were not originally formed (probably carried by river water or glaciers) are called Transported Soils

The Soil Profile

• This is a cross section of earth's dirt!
• Composed of multiple layers: (horizons)

1. A-horizon - (the Topsoil) mostly silts, clays, and fine sands with many organic particles
2. B-horizon - (the Subsoil) mostly sands and clays (the non-organics)
3. C-horizon - weathered parent material (rocks, pebbles, sands, gravels, etc.)

Soil Types (by Climate)

• Tropical soil - soil that is produced in a tropical area; mostly humid region; contains very little nutrient material
• Grassland soil - soil that is produced in usually flat regions; contain nutrients for grass life; not suited for growing trees/forests
• Forest soil - soil that is perfect for growing hardy tree species; usually found in much cooler climates; not fertile for much grass growth
• Desert soil - soil that forms in very dry climates; not suited for much growth-lack of water; soil is surprisingly nutrient rich!
• Arctic soil - soil that forms at high altitudes and high latitudes (cold climate); usually very thin layers of soil horizons

Mass Movement

• Soils are easily moved by various actions
• That includes water, elevation, gravity, wind, etc.
• Any movement of loose earth material down a slope is defined as Mass Movement
• Creep is a slow downslope movement of the soil - like watching fence posts slowly tip downhill in a farmer's field
• Talus is the material that forms at the base of steep slopes or cliffs - the material that was weathered from high above and dropped as the erosional processes took affect (gravity)
• A Landslide is a sudden movement of a mass of bedrock or loose rock down a hill
• An Avalanche is simply the movement of a mass of snow down a hill
• A Mudflow is a sudden movement of wet, loose soil material down a slope - water serves as a great lubricant to cause the mud to slide down due to gravity

Soil Conservation

• Modern society has developed a series of techniques that we can use to conserve soil
• Soil cannot easily be replaced once it is lost
• Nutrients in the soil are also easily lost
• Here are some techniques:

1. Windbreaks - trees planted in alternating rows at the edge of a field cause the winds to not easily move the soil particles
2. Contour Farming - instead of planting rows up and down a hill, crops are planted in rows parallel to the slope of the hillside
3. Terraces - flattening the hillslopes into level areas to plant crops (can be very expensive)
4. Strip Cropping - alternating bare-ground crops with ground-cover crops to prevent soils from eroding away (ex. corn & wheat)
5. No-Till - farmer plows, plants, and fertilizes all at the same time; then leaves the field vacant for the entire growing season to prevent unnecessary or too much activity in the field