chemical and biological weathering lesson 5

CHEMICAL WEATHERIN

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Chemical Weathering

Decomposition of rock in situ resulting from chemical change

Water (rain, fog, moisture, ocean etc) is the key medium for chemical weathering.

Common in warm humid climate (high temperature & high rainfall)

Weathered rocks by this process have a dull pitted appearance

Five main processes of chemical weathering

1. Oxidation

2. Hydration

3. Hydrolysis

4. Carbonation

5. Solution

Oxidation Oxygen dissolved in

water reacts with mineral iron in the rock

The rock is converted to iron oxide

Always seen as reddish-brown crust

This conversion weakened the mineral bonding and consequently the whole structure of the rock

Hydration

Involves in the expansion of minerals resulting from the absorption of water

E.g. Clay minerals are capable of absorbing large quantities of water into their crystal structure

When this happen, they exerts stresses within the rock which may eventually cause it to break apart

Hydrolysis

Involves a chemical reaction

between hydrogen ions in water with certain minerals in the rock

E.g. Weakly acidic rainwater causes feldspar to be converted into white powdery clay (kaolin) which breaks down easily

Hydrolysis

Kaolinite mine

Carbonation Is a form of solution weathering

Common in rocks composed of calcium carbonate such as limestone and chalk

Rainwater mixed with carbon dioxide form carbonic acid

The acid water change the calcium carbonate to soluble calcium bicarbonate which is then easily washed away

Solution Simply the

dissolving minerals in water

E.g Halite (rock salt) dissolve in water

Quartz dissolves at an extremely slow rate

Spheroidical Weathering

A type of chemical weathering

Creates rounded boulders and domed monolith

Typically granitic in composition

Rock forms at great temperatures and pressures

Spheroidal weathering

BIOLOGICAL WEATHERING

Biological Weathering

Caused by the action living organisms

Can lead to physical break down of rock (Biophysical)

Also chemical attack (Biochemical weathering)

Biophysical Weathering

1. By plant roots

2. By animals such as rodents and earthworms

3. By human activities

Biophysical

By plant roots:

When roots penetrate into cracks, they will usually break rocks apart

By animals:

Help to widen and loosen the joints in rocks and expose underlying rocks to weathering processes as they burrow through the ground

Biophysical

By human activities:

Clearing of land for cultivation

Construction and mining activities

This activities break up rocks and expose them to the weathering processes

Biophysical

Biochemical

Organic acids from dead plants and root can help to decompose rock

Chelation is the effect of organic acid on rock

Waste and organic acids from dead animals help to weather rocks chemically

Human activities:

Industrialisation can cause pollution such emission of sulphur dioxide which can produce acid rain

Accelerate the chemical weathering process of hydrolysis, carbonation and solution

Biochemical