WEATHERING, MASS WASTING, EROSION AND SOIL

What is weathering?

It is the process by which rocks on the earth's surface are broken down into pieces such as boulders, pebbles, sand, grains, silt and clay.

Why is weathering important?

a) It breaks down rocks to form soils.

b) It breaks up minerals in rocks to release important nutrients into the soil.

The breaking down of the rock materials occurs in place with little or no movement.

What are the 2 basic types of weathering?

a) Mechanical or physical weathering and

b) Chemical weathering

Mechanical weathering: is the physical breakdown of rocks into smaller pieces and fragments. It does not alter the chemical composition of the original rock.

What are the causes of Mechanical Weathering?

a) Frequent temperature changes (exfoliation)

b) Freezing and melting of liquid water (frost wedge)

c) Roots of plants break up rocks

d) Burrowing the ground by such animals as rabbits

Mechanical weathering is common is Deserts and Tundra climates.

Chemical Weathering: involves the decay of rocks exposed to moisture and air. It occurs in a variety of chemical reactions.

Chemical weathering results in changes in minerals that make up the rocks

Types of Chemical Weathering

a) Oxidation: a process in which iron, a common element in many rocks, combine with moisture and oxygen to form iron oxide or rust.

b) Solution: water enters rock substances to dissolve them e.g. sodium chloride (salt) & calcium sulfate (gypsum).

Chemical weathering occurs faster in hot wet climates such as Rainforests

MASS MOVEMENTS

Once rocks are broken down, the particles are carried downhill under the force of gravity in a process called mass movement.

In a mass movement, rocks roll, slide, or free-fall downhill under the force of gravity.

What are the common forms of mass movement?

a) Soil Creep: gradual movement of soil materials down slope. It may only be noticed as curved tree trunks, bent telegram posts and broken retaining walls on roadsides.

b) Rock or Landslide. a dramatic fall of rocks and soil materials over very steep slopes.

c) Slump or Earth Flow: A fluid mud flowing down a steep slope.

SOIL FORMATION AND EROSION

FUNCTIONS OF SOIL

1. A habitat for several plants and animals

2. A storage bank for water and nutrients

3. Foundation of the world's food chain

4. Provides an anchor for plants

SOIL FORMING FACTORS

1. Parent Material: the underlying bedrock over which

a soil develops. Soils forms from a variety of parent

materials such as:

a) Till soil in New England = Glacial transport.

b) Alluvial soils (i.e, Nile Delta) = Water transport

c) Loess soils in Iowa & Illinois = Wind transport

2. Climate: Water & Temperature help break up rocks.

3. Biological Activity (Organisms): The activities of

bacteria, people earthworm and roots of plants.

4. Topography (relief of the land): Various sections of

a slope produce different soils (color & grain size)

5. Time: Soil takes a long time to develop and mature

SOIL COMPOSITION

1. Organic Matter (Humus - decomposed materials):

2. Mineral Nutrients contained in rocks

3. Gas Content (Oxygen, Nitrogen, C02 etc.,)

4. Moisture (Water)

SOIL PROFILE

A horizontal layer of a soil is called a horizon. A cross-sectional view of the various soil horizons is known as a Soil Profile.

SOIL PROFILE

a) Humus layer (remains of plants and animals)

b) A horizon: (The top soil) is a zone of leaching and

the habitat of many organisms.

c) B horizon: (Subsoil) a zone where soluble salts and

organic matter leached from the A horizon is

deposited (accumulated).

d) C horizon: weathered portion of the rock that forms

the parent material over which a soil develops

e) D horizon: Unweathered part of the parent material

SURFACE EROSION

Erosion is the picking up and removal of earth's material (e.g., soil) by wind, running water and glacier.

TYPES OF EROSION:

a) Splash Erosion: results when water droplets break soil particles apart and carry them a few distance away

b) Rill Erosion: a rill is the smallest channel eroded by the flow of water or any liquid.

c) Gully Erosion: the last stage of erosion during which running water, wind and glacier create deep valley channels on the surface

IMPACTS OF EROSION

1. Eroded soil materials pollute rivers,

2. The materials increase stream load to cause floods

3. Erosion leaves the land bare and unproductive.

AGENTS OF EROSION:

1. Running Water,

2. Wind, and

3. Glacier (Ice)

HUMAN CAUSES OF EROSION

1. Over cultivation by farmers,

2. Overgrazing by herds of cattle, sheep etc,

3. Deforestation by local groups

4. Irrigation by farmers

5. Urbanization

EROSION CONTROL PRACTICES

1. Contour ploughing along sloping grounds

2. Strip Cropping across slopes

3. Terracing along slopes

4. Damming of Gullies

6. Removing cropland from production to allow soil

renew itself.

7. Creating windbreaks to reduce wind speed and erosion.

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Review Questions:

Answer the following questions and submit your answers in class

Consult your textbook for help.

1. Explain the terms; a) Tectonic forces and b) Gradational forces and give 2 examples of landforms that each of the forces produce on the earth’s surface

2. List the three main classes of rocks and explain how each of the three categories of rocks differs from each other.

3. List three features on the earth’s surface that result from erosion caused by the following agents:

a) Running water including rivers and streams

b) Winds

c) Glacier

4. Make a sketch of a typical soil profile and label the main horizons. Explain the distinctive processes that take places in horizons A & B.