Introtoweathering

The Wonderful World of

Rock WeatheringMdm Masayu Mahmud

Department of HumanitiesInnova Junior College,

Singapore

What geologic processes causes these rocks to weather?

Dairy Farm Estate Granite Outcrop, Singapore

Describe this landform

Denudation

Includes all processes of wearing down of the rocks on the earth’s surface and their removal resulting in the lowering of the land surface.Major processes.1. Weathering.

2. Mass movement/mass wasting. 3. Erosion. 4. Transportation. 5. Deposition

Weathering

Weathering refers to the process of disintegration (physical breakdown) and decomposition (chemical decay) of rock in situ at or near the earth’s surface by climatic or biological elements.

Erosion

Erosion refers to the wearing away of weathered rock material by agents such as running water (rivers), tornadoes, mudflows, moving ice (glaciers), waves and wind.Movement of loose rock material involved.

Why Do Rocks Weather?

Formed at depthUnder pressureNo exposure to oxygen, carbon dioxide,water, temperature changesComposition of rocks – reaction?Weathering as a response to atmospheric and surface conditions?Slow process

Types of Weathering

Physical (Mechanical)

Chemical

Biological

Mechanical Weathering( MW)

Disintegration of rock into smaller particles by mechanical processes but without any change in the chemical composition of the rockWhere is it likely to occur?- devoid of vegetation

- deserts, high mountains, arctic regionsEnd product- sands

Chemical Weathering(CW)

Decomposition of rock resulting from chemical changeVarious components of rock – iron, mica, feldspar, calcium carbonate reacting with water, acids, heat etcDominant Location- warmer climates with vegetationEnd product- clay

Biological Weathering(BW)

Physical or chemical breakdown of rocks due to action of plants, trees, animals,algae, bacteria etc.

Root pry, animals burrowing, humic acids reacting with rock.

Products of Weathering

Type of weathering

Product of weathering and erosion

Type of sediment ultimately deposited

Mechanical weathering

Fragments containing the same minerals as the original rock (e.g. grains of quartz from a granite) Clastic sediments and

sedimentary rocks (e.g. sand and sandstone)

Chemical weathering

New solid mineral particles formed by chemical reactions during weathering (e.g. Clay minerals)

Material dissolved in water (e.g. Calcium ions)

Chemical and biological sediments (e.g. limestone and rock salt)

A Point to Note

Different types of weathering

- dominant in certain regions

- DO NOT occur in isolation

Factors Influencing Rate of Weathering

1. Climatic factor2. Rock Types3. Relief/

Topography/Slope4. Vegetation5. Impact of human

activities

1. Climatic Factor

Main influences- air temperature, humidity and rainfallCW dominant in hotter, wetter climatesHydrolysis, oxidation and solution common in equatorial regionsMW dominant in regions with extreme temperature fluctuations- Which ones?Deserts, Temperate regions and mountainous areas

2. Rock Types

Refers to

1. Chemical composition (constituents minerals) –determines resistance

2. physical structure

3. Presence of joints

4. Bedding planes

5. Permeability of rocks

Jointed Structures

Hard Rock Not?

Rocks composed of cemented particles less resistant compared to?

Rocks consisting of mainly carbonates?

Rocks of massive structure vs bedded structures

Presence of joints (naturally occurring cracks in rocks), cracks, fissures?

Jointing in Granite

3. Relief, Topography and Slope

Aspect- slopes exposed to wind, rain and heat vs. sheltered slope

- Slopes facing sun in high altitudes vs. cold and sunless slopesSlope

- high, steep slopes lead to down slope washing and prone to runoff and erosion of materialsElevation- Higher elevation > opportunity for water movement

- Flat terrain; slow removal of weathered material. Why? How does this affect weathering rates?

4. Effect of Vegetation

Dense vegetation- retains more water; aids CW

Decaying vegetation provides moisture and humic acids- aids CW

Roots of trees and plants pry rocks and joints- increase MW

5. Impact of human activities

Expose the rocks and accelerate weathering

1. Clearing forests

2. Road construction

3. Industrial development

4. Mining/quarrying

5. Trampling

Results of Weathering Processes

Joint blocks

Boulder fields

Exfoliated Rocks

Spheroidal boulders

Sand

Clay

Laterite

Boulder fields

Exfoliated Rocks and Spheroidal Boulders

Lateritic Soil

A Laterite soil cross section in Brazil. The deep red color is due to Fe3+ hydroxide minerals in the soil.