Weathering and Erosion_Engineering Geology & Geomorphology

1. WEATHERING & EROSION
ENGR. MD. SHAMIM AHMED
LECTURER
DEPT. OF CIVIL ENGINEERING
SOUTHERN UNIVERSITY BANGLADESH

2. WEATHERING & EROSION
Weathering:
The physical disintegration or chemical
alteration of rocks at or near the Earth’s
surface.
Erosion:
The physical removal and transportation of
weathered material by water, wind, ice, or
gravity.

3. Types of Weathering
I. Mechanical (physical) weathering is the physical disintegration and reduction in the size
of the rocks without changing their chemical composition.
Examples: exfoliation, frost wedging, salt wedging, temperature changes, and
abrasion.
II. Chemical weathering decomposes, dissolves, alters, or weakens the rock through
chemical processes to form residual materials.
Examples: carbonation, hydration, hydrolysis, oxidation, and solution.
III. Biological weathering is the disintegration or decay of rocks and minerals caused by
chemical or physical agents of organisms.
Examples: organic activity from lichen and algae, rock disintegration by plant or root
growth, burrowing and tunneling organisms, and acid secretion.

4. Mechanical (physical) weathering
 Exfoliation
 Frost wedging
 Temperature changes
 Abrasion
Exfoliation: the peeling away of large sheets of
loosened materials at the surface of a rock.

5. Mechanical (physical) weathering …
Frost wedging: Caused by the freeze-thaw action of
water that is trapped between cracks in the rock. This
process gradually weakens, cracks, and breaks the
rock through repetitive freeze-thaw weathering cycles.
Temperature Changes: Warmer temperatures may
cause some minerals to expand, and cooler
temperatures cause them to contract. This gradual
expansion and contraction of mineral grains weakens
the rock causing it to break apart into smaller fragments
or to fracture.

6. Mechanical (physical) weathering …
Abrasion: occurs when rocks collide against each other while they are transported by water,
glacial ice, wind, or gravitational force.
Chemical weathering
 Carbonation
 Hydrolysis
 Hydration
 Oxidation
 Solution
The rate of Chemical weathering depends on various factors like -
 Temperature
 Amount of surface area and
 Availability of water or natural acid.

7. Chemical weathering …
Carbonation: Carbonation is a process by which carbon dioxide and rainwater or moisture in
the surrounding environment chemically react to produce carbonic acid, a weak acid, that
reacts with carbonate minerals in the rock.
CO2 + H2O → H2CO3
CaCO3 + H2CO3 → Ca(HCO3)2
Hydration: attachment of water molecules to crystalline structure
of a rock, causing expansion and weakness.
Hydrolysis: combination of hydrogen and oxygen in water with
rock to form new substances.
Oxidation: Oxidation occurs when oxygen and water react
with iron-rich minerals and weaken the structure of the mineral.
Solution: Solution occurs when minerals in rock dissolve directly
into water.

8. Biological weathering
a) Organic activity from lichen and algae
b) Rock disintegration by plant growth
c) Burrowing and tunneling organisms
d) Secretion of acids
Fig: (a)
Fig: (b)
Fig: (c)
Fig: (d)

9. Factors influencing weathering
They are:
 Topography
 Climatic conditions – temperature and humidity
 Physical characteristics of rocks
 Chemical and structural characteristics of rocks- their mineralogy and structural
features, and
 Vegetation- their abundance and type, including the micro and macro
vegetation.
Weathering involves no moving agent of transport.

10. Rates of weathering
There are several factors determining the rate of weathering. The notable ones are:
 Climate: which is usually measured in terms of temperature and moisture, can drastically
affect the rate of weathering.
 Organisms
 Time: The longer that a rock has been exposed to the weather, the faster it weathers.
 Mineral Composition: Those minerals that are most reactive with acids, water, and air are
weathered at a more rapid rate. Minerals that are soft can be easily abraded and blasted.
 Porosity, faults and shears in materials
 Exposure: Rocks that are exposed to the atmosphere tends to weather much quicker.
 Particle Size: The smaller the particle size, the faster it will weather.
 Slope: On steep slopes weathering products may be quickly washed away by rains. On
gentle slopes water may stay in contact with rock for longer periods of time, and thus
result in higher weathering rates.

11. THANK YOU