Sediments and Sedimentary Rocks: Processes of Formation | Geology Optional Notes for UPSC PDF Download

Sedimentary Processes

Rocks and Atmospheric Agencies

• Rocks are constantly exposed to atmospheric forces such as wind, water, and ice.
• This exposure causes rocks to break down into smaller particles.
• The particles are transported over varying distances before being deposited in suitable basins.

Weathering

Definition: Weathering is the physical disintegration and chemical decomposition of rocks.

It is caused by factors such as:

• Precipitation
• Temperature fluctuations
• Frost action
• Plant growth
• Chemical reactions

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Types of Weathering

1. Physical Weathering (Mechanical Weathering)

• Breaks rocks into smaller pieces without altering their chemical composition.
• More common in cold or dry regions.

Key processes:

• Abrasion: Rocks are worn down by water, wind, or glaciers.
• Frost Action: Water freezes in cracks, causing fractures through expansion.
• Unloading: Pressure release causes rocks to expand and crack.
• Thermal Cycling: Temperature changes cause stress, leading to fractures.
• Plant and Animal Activities: Roots and burrowing contribute to rock breakage.

Examples:

• Sediments eroded by glaciers and transported downstream.
• Rocks splitting due to tree roots.

2. Chemical Weathering

• Alters the chemical composition of rocks.
• Occurs through processes like:
  • Carbonation: Carbonic acid reacts with minerals, forming caves (e.g., limestone caves like Gupt Godavari caves in Madhya Pradesh).
  • Oxidation: Reaction with oxygen forms rust, giving rocks a reddish hue.
  • Hydrolysis: Water chemically breaks down minerals, forming clay.

Examples

• Carbonic acid dissolving calcite in limestone.
• Feldspar in granite converting to clay minerals due to rainwater.

3. Biological Weathering

• Caused by living organisms like plants, animals, and microbes.
• Activities include root wedging and burrowing.

Examples

• Mosses and lichens weakening rocks.
• Animals loosening soil through movement.

Erosion

• Definition: Transportation and deposition of weathered particles by natural agents.

Key Processes:

• Hydraulic Action: Water dislodges sediments from rock beds.
• Wind Erosion: Wind carries particles over distances.
• Glacial Erosion: Glaciers grind and carry rock materials.

Sediment Transportation

Types of Sediment Load

1. Dissolved Load

• Materials dissolved in water.
• Influenced by water temperature and solute supply.
• Example: Tropical rivers carrying larger dissolved loads.

2. Suspended Load

• Fine materials kept afloat by water currents.
• Example: Silt and clay in rivers.

3. Saltation Load

• Sand-sized particles bouncing along the streambed.
• Movement occurs slower than water flow.

4. Bed Load (Traction Load)

• Larger particles like gravel rolling or sliding along the streambed.

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Formation of Sedimentary Rocks

Processes

• Sedimentary rocks form through the accumulation and lithification of sediments.

Key Steps

• Cementation: Minerals bind sediment grains.
• Compaction: Pressure reduces pore space in sediments.
• Recrystallization: Minerals change form under pressure.

Deposition

• Occurs when transporting energy diminishes.
• Layers form, with older ones at the bottom.
• Structures include horizontal, cross-bedded, and graded layers.
• Can also result from chemical precipitation or evaporation.

Examples

• Chemical deposition forms salt flats.
• Evaporation leads to the formation of minerals like gypsum.

Factors Influencing Weathering and Sedimentary Processes

Rock Type

• Igneous rocks weather faster due to unstable minerals.
• Sedimentary rocks are susceptible to environmental conditions.
• Metamorphic rocks like quartzite weather slower due to high resistance.

Climate

• Hot, humid climates accelerate chemical weathering.
• Dry climates slow down weathering.
• Vegetation can protect rocks from weathering.

Transport Distance

• Sediment particle size decreases with longer transport distances.

Examples

• Coarser sediments near the source.
• Rounded clasts indicate longer transport distances.

Mineral Stability

• Minerals like quartz resist weathering due to their hardness and lack of cleavage.
• Goldich Stability Series ranks minerals based on weathering resistance.

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Weathering, Erosion, and Deposition 

• Weathering: Breaks rocks into smaller particles through physical, chemical, and biological processes.
• Erosion: Transports weathered materials through water, wind, and ice.
• Deposition: Deposits materials in layers in basins.

Key Features

• Sediments undergo transportation, sorting, and deposition based on energy conditions.
• Lithification transforms sediments into sedimentary rocks.

Examples

• Basalt weathering into clay minerals.
• Glacially transported sediments forming moraines.