Geography: Weathering and Rock Types

WEATHERING

Definition of Weathering

• Weathering is the process of the breaking down of rocks into smaller pieces due to exposure to different elements.

Types of Weathering

1. Mechanical (Physical) Weathering

   • Definition: Physical breaking down of rock without changing their chemical composition.

2. Chemical Weathering

   • Definition: Breakdown of rocks through chemical composition/reactions, leading to alterations in their chemical structure.

3. Biological Weathering

   • Definition: Caused by living organisms (e.g., plants and animals) that can produce weak acids impacting the rock structure.

Types of Rock

1. Igneous Rocks

   • Formed from cooled and solidified magma.

   • Types:

     • Extrusive (Volcanic): Formed from lava that cools quickly on the earth's surface.

     • Intrusive (Plutonic): Formed from magma that cools slowly beneath the earth's surface.

   • Key Characteristics: Contains crystals; hard; no fossils.

2. Sedimentary Rocks

   • Formed from fragments of other rocks that are compressed and hardened in layers (strata).

   • Key Characteristics: Typically layered; soft; contains fossils.

3. Metamorphic Rocks

   • Formed from existing rocks that have undergone transformation due to heat or pressure.

   • Key Characteristics: Crystalline; hardness varies (soft to hard); thinly layered.

Types of Physical (Mechanical) Weathering

1. Freeze-thaw (Ice wedging/Frost shattering)

   • Process: Occurs in cold climates. Water seeps into cracks of rocks, freezes, expands, and exerts pressure, causing the rock to crack.

   • Illustration:

     • A: Rain or snow and ice melt allows water to seep into cracks.

     • B: At night, when temperatures drop below freezing, ice forms within the cracks, increasing volume and pressure.

     • C: Repeated expansion and contraction from freeze-thaw cycles eventually breaks the rock apart.

2. Exfoliation

   • Definition: Involves peeling off outer layers of rock due to thermal expansion and contraction.

   • Conditions: Common in hot deserts where rocks heat up during the day and cool rapidly at night, causing stresses that result in flaking.

3. Salt Crystal Growth

   • Process: Saltwater penetrates rock surfaces, and as the saline solution evaporates, it leaves behind salt crystals.

   • Effect: At elevated temperatures, the crystals can expand, exerting pressure which can cause disintegration of the rock.

4. Vegetation (Biological) Weathering

   • Process: Roots of plants penetrate rocks, exerting pressure and creating cracks that can prevent further weathering.

Types of Chemical Weathering

1. Solution

   • Definition: Removal of rock through solution by acidic rainwater, which can dissolve minerals.

2. Oxidation

   • Process: Reaction of oxygen with minerals (particularly iron) leading to rust formation (iron oxide).

   • Equation: 4Fe + 3O2 ightarrow 2Fe2O_3

3. Carbonation (Acid Rain)

   • Process: Rainwater containing carbon dioxide forms carbonic acid which can dissolve limestone and other carbonate rocks.

4. Hydrolysis

   • Process: Water reacts with silicate minerals to form clays, altering the rock structure. Factors, such as warm and damp environments, can accelerate this process due to increased reaction rates.

Factors Affecting Weathering

1. Climate:

   • Weathering rates can vary significantly based on climatic conditions.

2. Rock Type:

   • Different rock types have varying susceptibilities to weathering processes.

3. Rock Structure:

   • Rocks with larger grains tend to weather faster.

4. Vegetation:

   • Roots are influential in increasing biological weathering processes.

5. Relief:

   • The altitude and exposure can affect temperature and weathering rates.

6. Human Activity:

   • Pollutants can contribute to acid rain, influencing weathering processes.

7. Temperature (Van't Hoff's Law):

   • The rate of chemical weathering increases by approximately 2-3 times for every 10°C temperature rise, up to a maximum of 60°C.