Chapter 7

Weathering and Soils

Weathering –is the break up of rock due to exposure to the atmosphere. 

Conditions on the surface are much different than those found at depth (H2O

and O2). 

Materials formed from weathering do not remain in place; wind, rain, and moving water agents of erosion move the materials.

2 types of weathering:

1. Mechanical Weathering

aka – disintegration – when rock is split or broken into smaller pieces of the same material w/o changing it’s composition.

Types of Mechanical Weathering:

• Frost action or ice wedging

• Wetting and drying

• Actions of plants and animals

• Loss of overlying rock and/or soil (exfoliation)

Examples of Mechanical Weathering

http://www.classzone.com/books/earth_science/terc/content/visualizations/es1201/es1201page01.cfm?chapter_no=visualization

2. Chemical Weathering aka – Decomposition – when a rock’s minerals are changed into different substances – water and water vapor are most important agents.

Types of Chemical Weathering:

•Hydrolysis – chem. Rxn. Of water with other substances; feldspars react to form clays

•Oxidation – chem. Rxn. Of oxygen with other substances; ferrous magnesium minerals react to form iron oxide (rust).

•Carbon dioxide and some other gases readily dissolve in water to form acid and acid rain; play a key role in the formation of caves.

The most important factor affecting the rate of weathering – the amount of rock surface exposed. Smaller pieces – more surface area exposed.

Other factors i.e. hardness of the rock and climate;

• Warm and Wet climate = more chemical

• Cold and Dry climate = more mechanical

Examples of Chemical Weathering

http://www.gpc.edu/~pgore/geology/geo101/weather.htm

Erosion and Deposition

Erosion is the process of moving weathered materials via several natural agents. Land becomes increasing barren as soil is lost to wind and water erosion.

 

When the movement of transported materials slows down, materials are placed in another location. This process is known as deposition, the final stage of erosion.

Gravity pulls materials down hill. Gravity is agent of mass movements such as landslides, mudflows, and avalanches.

Water flowing down steep slopes has the greatest potential for moving weathered materials.

Erosion by water in small channels on the side of slopes is called rill erosion.

When channels become deeper and wider can evolve into in to gully erosion; can be as deep as 3 meters, major problem for farming.

Coastal Deposition and Erosion

Rivers carry enormous amounts of sediments and weathered materials.

Mississippi River carries 750 million metric tons of weathered and eroded material off the continent and into the Gulf of Mexico annually.

When a river enters a large body of water like an ocean or gulf, the speed of the water is reduced and sediments are deposited and built up. The build is called a delta.

                                                         

Comparison of Landsat multispectral scanner satellite scenes of the Mississippi River Delta, January 16, 1973, and March 3, 1989

Weathering and erosional processes continue in the ocean. Currents, waves and tides form many structures such as cliffs, arches, beaches, shorelines and is continually changing the shapes of beaches.

Sediments build up offshore and form sandbars that evolve into barrier islands as deposition continues.

Glacial Erosion

Erosional affects are wide spread and dramatic.  

Great density allows glaciers to carry large rocks and great amounts of debris over long distances. 

Landscape features as a result of glacial movement include waterfalls, lakes, and variously shaped deposits of sediments.

Wind Erosion

 Major erosional agent in areas of low precipitation and high temps.

Effects can be lessened by constructing “wind barriers”.

Erosion by Plants, Animals and Humans

 The effects of these activities are minimal.

Formation of Soils 

Soils are made of loose weathered rock and organic material in which plants with roots can grow. 

Parent material is the material from which soil is formed; commonly from bedrock. 

Residual soil has bedrock as parent material. 

Transported soils are soils formed from deposits left by winds, rivers and glaciers.A cross section of the earth exposed by digging down through layers is called a soil profile.

Most mature soils have 3

distinct zones or horizons:

• Horizon A (top soil) generally gray to black in color b/c it contains humus (organic material).

• Horizon B (subsoil) contains more clay, usually red or brown, caused by the oxidation of water leeching thru the topsoil layer.

• Horizon C is slightly weathered parent material and rock fragments; this layer sets on top of unweathered bedrock.

Soil Types and Climates 

• Tropical soils are relatively infertile because ….

• Grassland soils are rich in organic material and nutrients because …

• Forest soils are less than a meter thick and can be not very fertile because …

• Desert soils can be very infertile because …

• Arctic soils have poor drainage but the profile is very shallow, no distinct horizons; permafrost under thin soil layer.

Sometimes rocks weather by peeling off in sheets rather than eroding grain by grain. Exfoliation is scientific Latin for that process.

It can happen in paper-thin layers on individual boulders, or it can take place in thick slabs as it does here, in Yosemite Valley, California.