Weathering:
- Is the process of breaking rocks into smaller pieces to form soil.
Types of weathering
- Physical weathering
- Chemical weathering
- Biological weathering
Physical weathering:
- The breaking down of rocks by natural forces without changing the chemical composition of rocks.
Agents of physical weathering:
a) Flowing water/Running water/Falling water:
- Water carries stones that hits against each other and small pieces break-off to form soil and the rock becomes smooth.
b) Strong wind:
- Wind blows sand particles against rocks, causing pieces of rocks to break-off and form soil.
c) Temperature change:
-When temperature increases rocks expand and when temperature cools the rocks contract.
-This causes rocks to become weak and break to form soil.
d) Freezing water:
- Water in the cracks of rocks freezes causing the rock to expand and break into smaller pieces to form soil.
Chemical weathering:
- Breaking down of the rocks into soil by changing the chemical composition of the rock.
Agents of chemical weathering:
a) Water:
- Water combines with carbon dioxide to form a weak carbonic acid, that causes rocks to dissolve.
b) Oxygen:
- Combines with minerals such as iron and sulfur in the rocks to form oxides (rust) that weakens rock joints causing the rock to peel off and form soil.
c) Carbon dioxide:
-Combines with rainwater to form weak carbonic acid which dissolve rocks, and cause the rocks to break and form soil.
Agents of chemical weathering:
a) Water:
- Water combines with carbon dioxide to form a weak carbonic acid, that causes rocks to dissolve.
b) Oxygen:
- Combines with minerals such as iron and sulfur in the rocks to form oxides (rust) that weakens rock joints causing the rock to peel off and form soil.
Biological weathering:
- Breaking down of rocks by the actions of living organism.
Agents of biological weathering:
a) Plant roots:
- Roots grow and penetrate in the cracks of rocks causing the rock to expand and break to form soil.
b) Moving animals:
- When land animals like elephants and cattle move and graze, they step on rocks causing rocks to break and form soil.
c) Burrowing animals:
- Burrowers like termites and ants, create tunnels underground making the rocks to break and allow air and water to enter.
d) Microorganism:
- Bacteria and fungi produce organic acids during, decomposition of organic matter, these acids dissolve rocks causing rocks break and form soil.
e) Human activities:
- Activities such as ploughing or construction breaks rocks into smaller pieces to form soil
Describe the formation of soil through the decay of dead matter:
- Microorganisms decompose dead plant and animal remains o form humus.
- During decomposition of these organic materials, organic acids are produced, which dissolves rocks to form soil.
Define soil profile
- Is a vertical cross section of the soil that shows the horizons/layers of the soil.
OR - A straight sided hole dug in the ground to show different layers of the soil.
Describe the various soil horizons/layers in terms of color, organic matter and texture.
A horizon (top soil)
B horizon (subsoil)
C horizon (weathered parent rock)
D horizon (parent rock)
Top soil (horizon A)
- dark in color
- contains lots of humus, high organic content.
- mixture of fine and coarse texture
- good drainage and aeration
- contains many plant roots and soil living organisms.
Subsoil (horizon B)
- lighter in color
contains less humus, contains less organic content. - mixture of fine and coarse texture.
- poor drainage and aeration.
- contains leached minerals.
- have a few plant roots and living organisms.
Weathered parent rock (horizon C)
- light in color.
- contains very little organic content.
- have a coarse texture as rocks are partially broken down.
- very few roots.
Weathered parent rock (horizon C)
- light in color.
- contains very little organic content.
- have a coarse texture as rocks are partially broken down.
- very few roots.
Parent rock (horizon D)
- very light in color.
- no organic content.
- consist of rocks that are yet to be weathered.
- no roots.
