Weathering and Soil Formation

1
Weathering and Soil Formation

• Weathering is the process that breaks down rock
  and other substances on Earths surface.
• Erosion is the movement of rock particles by
  wind, water, ice or gravity.

2
Chemical and Mechanical Weathering

• Mechanical
• Rock physically broken down into smaller pieces.
  This is achieved by freezing and thawing, release
  of pressure, growth of plants, actions of animals
  and abrasion.
• Abrasion is the grinding of rocks by other
  rocks carried by water, ice, wind or gravity.

3
Mechanical Weathering Cont..

• In cool climates, the most important mechanical
  weathering is done by freezing and thawing this
  is called ICE WEDGING!

4
Chemical Weathering
5
Related Factors for Chemical Weathering

• Water Weathers rock by dissolving it.
• Oxygen Iron combines with Oxygen in a process
  called Oxidation. The product is rust. Makes
  rocks soft and crumbly and gives it a red or
  brown color.
• Carbon dioxide. Dissolved in rain water then
  sinks through air pockets in the soil. Results in
  a weak acid called CARBONIC ACID. This acid
  easily weathers limestone and marble.

6
Chemical Weathering Continued

• Living organisms. Plants roots grow into rock
  making chemicals that weather the rock (weak
  acids).
• Acid rain When coal, oil and gas are burned
  sulfur, carbon and nitrogen compounds mix with
  water vapor forming acids. Acid rain causes very
  rapid chemical weathering.

7
Rate of Weathering

• What are the two most important factors that
  determine the rate of weathering?
• 1) Type of rock
• The minerals contained in rocks determine the
  rate of weathering. Rock made of minerals that do
  not dissolve in water quickly weather slowly.

8
Rate of Weathering Continued

• Climate
• Both mechanical and chemical weathering occur
  faster in wet climates.
• Chemical reactions occur faster in wet climates.
  Therefore, in hot and wet climates the rocks
  weather the fastest.

9
Soil Formation

• Soil forms as rock is broken down by weathering
  and mixes with the material on the surface.
• Soil is constantly being formed wherever bedrock
  is exposed.
• Soil is made up of rock particles, minerals,
  decayed organic material, air and water.

10
The type of soil

• Depends on two factors
• The bedrock that was weathered to form the soil
• The type of weathering.
• Decayed organic material is known as HUMUS which
  is rich in nitrogen, sulfur, phosphorus and
  potassium.

11
Soil Horizons

• A layer of soil that differs in color and texture
  from the layers above and below it.

12
How do soil horizons form?

• 1) The C horizon forms as bedrock weathers and
  rocks break up into soil particles.
• 2) Then, the A horizon develops from the C
  horizon as plant roots mechanically and
  chemically weather the rock.
• 3) The B horizon forms as the rain pushes clay
  and minerals from the A horizon.

13
So what is soil anyway?

• Rock particles
• Minerals
• Decayed organic material
• Air and water

14
So what about the size of the soil particles?

• From largest to smallest
• Gravel (2mm and larger)
• Sand (less than 2mm)
• Silt (less than 1/16 of an mm)
• Clay (less than 1/256 of a mm)

15
Life in the Soil

• As plants shed leaves, they form a loose layer
  called litter.
• Humus forms in a process called decomposition.
  Decomposers are organisms that break the remains
  of dead organisms into smaller pieces and digest
  them with chemicals.

16
Soil Types

• Farmers classify soil partly on the climate that
  the soil exists in.
• Tundra soil with little humus
• Prairie soil most fertile soil. Rich in humus.
• Forest soil low amounts of humus. Formed in
  warm wet climates.

17
Soil Conservation

• Why is soil so important?
• B/c everything that lives on land depends
  directly or indirectly on soil.
• It can take hundreds of years for even a few cm
  of soil to form.
• Sod is the thick mass of tough roots at the
  surface of the soil. Keeps soil in place and
  holds onto the moisture.

18
The Dust Bowl

• Happened in the 1930s.
• Plowing removed the grass from the Great Plains
  and exposed the soil. In times of drought, the
  topsoil quickly dried out, turned to dust, and
  blew away.
• The problem was most serious in the southern
  plains where these conditions lasted until 1938.
• Many people left the Plains and moved onto other
  jobs.

19
Soil Conservation

• As a result of the dust bowl, farmers developed
  new methods of protecting the soil.
• Contour plowing Plowing fields along the curves
  of the slope. This prevents excess runoff, and
  prevents the soil from washing away.
• Conservation plowing disturbs the soil and its
  plant cover as little as possible. This helps the
  soil stay in place because the plants cover the
  top soil.