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

8.4 SEDIMENTARY PROCESSES

Rocks and Atmospheric Agencies

• Rocks on Earth are constantly exposed to atmospheric agencies like wind, water, and ice.
• This exposure leads to the breakdown of rocks into small particles.
• These particles are then transported varying distances before being deposited in suitable basins.

Weathering

• Weathering involves the physical disintegration and chemical decomposition of rocks due to various processes.
• Factors causing weathering include precipitation, temperature changes, frost action, plant growth, and chemical actions.
• Physical and chemical weathering processes act together in breaking down rocks and minerals.

Types of Weathering

• Physical Weathering
• Chemical Weathering
• Biological Weathering

Erosion

• Erosion involves the processes of transportation and deposition of weathered particles.

Further Learning Resources

• For more information on weathering types and significance, watch this video: Weathering Types and Significance Video

Block 3

Physical Weathering

• Recall the concepts of physical weathering discussed in Unit 5 of BGYCT-131 course.
• Physical weathering, also known as mechanical weathering, involves the breakdown of rocks and minerals into smaller pieces due to exposure to atmospheric conditions like heat, water, ice, and pressure, without altering their chemical composition.
• It leads to the physical disintegration of rocks and is more prominent in cold or dry regions.
• Processes involved in physical weathering include:
• 
  
  • Abrasion: Sediments and clasts are worn down in size by the force of water and wind, cutting through rocks as they move.
  • Frost Action: Common in moisture-rich areas with fluctuating temperatures, where water freezing in rock cracks at night exerts pressure, causing fractures and eventually breaking rocks into angular pieces.
  • Unloading, thermal cycling, and plant and animal activities also contribute to physical weathering processes.

Examples and Explanations

• Abrasion: Sediments transported by water and wind can erode rocks, while glaciers can grind rocks and carry substantial materials.
• Frost Action: Water freezing in rock crevices expands, exerting pressure that leads to fractures, repeating cycles of freezing and thawing resulting in rock breakage.
• Unloading: Release of pressure on rocks can cause them to expand and crack.
• Thermal Cycling: Fluctuations in temperature can cause rocks to expand and contract, leading to stress and eventual breakage.
• Plant and Animal Activities: Activities such as root growth or burrowing can contribute to the breakdown of rocks over time.

Unit 8: Formation of Sedimentary Rocks

Here are the key concepts related to the formation of sedimentary rocks:

• Pressure Release or Unloading

  When intrusive igneous rocks like granite are formed at great depths, they bear tremendous pressure. As material is eroded from above, pressure on the rock is released, causing the outer surface to expand. This expansion can lead to stresses which cause the outer surface to break into sheets parallel to the rock surface. This process is known as 'exfoliation' or 'sheeting'.

  Example: Exfoliation in granite can be observed as the outer layers peel off due to pressure release.

• Thermal Cycling

  In arid regions, there are significant temperature differences between day and night. The resulting expansion and contraction of minerals in rocks due to temperature changes create stresses that can lead to fractures and the enlargement of joints in rocks, ultimately causing block breakage.

  Example: Sharp temperature changes in deserts can cause rocks to crack and break due to differential expansion.

• Plant and Animal Activities

  The growth of plant roots within rocks can exert pressure, widening existing cracks. Additionally, the burrowing activities of animals can contribute to the breakdown of rocks into smaller particles.

  Example: Tree roots growing through rocks can cause them to split apart over time.

• Chemical Weathering

  Chemical weathering involves the breakdown of rocks and minerals through direct exposure to atmospheric or biologically produced chemicals, leading to alterations or decomposition. This process occurs through chemical reactions between minerals and the interface of water or air, resulting in changes to the rock's chemical composition.

  Example: Carbonic acid reacting with limestone to form caves demonstrates chemical weathering.

• 
  

  • Agents of Chemical Weathering

    Main agents responsible for chemical weathering are water and weak acids like carbonic acid.

  • Process of Chemical Weathering

    Chemical weathering involves gradual processes like carbonation, oxidation, and hydrolysis.

  • Carbonation and Dissolution

    Carbonation, a common weathering process, occurs when carbonic acid reacts with minerals like calcite in limestone, leading to the formation of caves.

Block 3 in Limestone Country

• Formation of Caves in Limestone

  • Process:

    Etching of carbonic acid dissolves carbonate minerals in limestone when exposed to acidic water.

  • Example:

    Gupt Godavari caves near Chitrakoot, Madhya Pradesh, showcase this phenomenon.

• Oxidation and Its Effects

  • Definition:

    Oxidation is the reaction of a substance with oxygen, leading to the formation of rust on affected rocks.

  • Example:

    When iron reacts with chemically active oxygen, it forms iron oxide, giving rocks a reddish-brown color.

• Hydrolysis and Its Impact

  • Explanation:

    Hydrolysis involves the chemical breakdown of substances when combined with water, affecting silicate and carbonate minerals.

  • Example:

    Feldspar minerals in granite rock undergo hydrolysis, transforming into clay minerals due to acidic rainwater.

• Weathering of Rocks

  • Process:

    Chemical alteration of minerals like hornblende, augite, and biotite leads to the formation of clay and chloritic minerals.

  • Example:

    Table 8.1 illustrates the weathering of common rocks and the resulting products, including clay minerals and potassium ions.

Unit 8 Summary: Weathering and Sedimentary Rocks

Basalt and Its Mineral Composition

• Basalt is a common rock type composed of minerals such as feldspars, Fe-Mg minerals, and clay minerals.
• Clay minerals, along with quartz and oxide minerals, are primary products of chemical weathering.

Weathering by Living Organisms

• Living organisms contribute to weathering through activities like pedoturbation and root wedging.
• Animals, insects, and plants like mosses and lichens play a role in breaking down rocks through various mechanisms.
• Biological activities can weaken rocks and alter minerals, leading to the formation of clay.

Mineral Stability and Weathering

• Some minerals are more resistant to weathering than others.
• Minerals like quartz exhibit high resistance to weathering due to their hardness and lack of cleavage.
• The Goldich stability series ranks minerals based on their chemical stabilities during weathering processes.

Factors Influencing Weathering Processes

• Type of Rocks:
  • Igneous rocks weather faster due to the presence of unstable minerals, while sedimentary rocks break down quickly under environmental conditions.
  • Metamorphic rocks like quartzite and marble weather at a slower pace.
• Climate and Vegetation:
  • Hot and humid climates accelerate chemical weathering, whereas dry climates experience slower weathering rates.
  • Exposed rocks are more susceptible to weathering in extreme conditions, while vegetation-covered areas are more resistant.

Weathering, Erosion, and Transportation

Weathering: Types and Significance

• Weathering Overview: Weathering refers to the process through which rocks are gradually broken down by natural factors like water, wind, and glaciers.
• Types of Weathering: Weathering can occur in different forms such as mechanical and chemical weathering, each impacting rocks in specific ways.
• Significance of Weathering: Weathering plays a crucial role in shaping the Earth's surface over time by breaking down rocks into smaller particles.

Erosion

• Erosion Process: Erosion involves the disintegration and removal of rocks by natural agents like water, wind, and glaciers.
• Hydraulic Action: Streams exert hydraulic action to extract rock fragments and sediment particles from solid rock beds.
• Wind Erosion: Wind carries loose particles like sand, silt, and clay over varying distances based on its speed.

Transportation of Sediment

• Sediment Transport: This process involves moving particles from the site of erosion to the deposition site through agents like water, wind, and glacial ice.
• Types of Sediment Load:
  • Dissolved Load
  • Suspended Load
  • Saltation Load
  • Bed Load or Traction Load

Dissolved Load

• Definition: Dissolved load includes materials dissolved in water, influenced by factors like water temperature and solute supply.
• Example: Tropical rivers often carry a larger dissolved load compared to temperate regions.

Suspended Load

• Characteristics: Terrigenous materials move through rivers suspended in water columns due to turbulent water currents.
• Mechanism: Suspended sediments are kept afloat by upward currents balancing sediment fall-velocity.

Saltation Load

• Description: Sand-sized particles bouncing along stream beds fall under the saltation load category.
• Velocity: Particle movement during saltation occurs at a lower speed than the water flow.

Bed Load or Traction Load

• Composition: Larger clastic particles like gravel and coarse sand move along the stream bed through rolling and sliding actions.
• Movement: Shear stress supports the movement of gravel and coarse sand along the stream bed.

Unit 8: Sediment Transport and Deposition

Distance of Sediment Transport

• Sediments can be transported varying distances, influenced by the energy conditions of the transporting medium, sediment size, and ground slope.
• Particle size decreases with increased transport distance, with coarser sediments closer to the source and finer sediments traveling farther.
• Angular clasts indicate shorter transport distances, while rounded clasts suggest longer transport distances.
• Mature sediments are typically found farther from their source rocks compared to immature sediments.

Types of Sediment Transport

• Desse: Sediment transport through direct contact between particles and the surface.
• Traction: Movement of sediment by rolling, sliding, or dragging along the ground.
• Suspension: Fine sediment particles carried in suspension by fluid flow.
• OC: Sediment transport in dissolved form within a fluid medium.
• Saltation: Movement of sediment in a series of leaps or bounces along the ground.
• Solution: Sediment transport through dissolution in a fluid medium.

Formation of Sedimentary Rocks

• Sedimentary rocks form through the accumulation and lithification of sediments.
• Processes include cementation, compaction, and recrystallization.

Deposition

• Deposition occurs when the energy required for sediment transport diminishes, leading to the settling of particles.
• Sediments are deposited in layers in a basin, with older layers at the bottom and younger layers on top.
• Layers may exhibit different structures like horizontal, cross-bedded, or graded bedded, reflecting the energy conditions during deposition.
• Deposition can also result from chemical precipitation or evaporation in the depositional basin.

Sedimentary Processes

• Depositional processes include erosion, transportation, and lithification.
• These processes contribute to the formation of various sedimentary rock types.

Weathering & Erosion

• Weathering and erosion involve the gradual breakdown and transportation of rocks and soil.
• Land transport and deposition by water, wind, and ice are ongoing natural processes.

Transport & Deposition

• Ocean transport and deposition occur through currents and chemical precipitation.
• Materials undergo burial and lithification, transforming into sedimentary rock over time.

Lithification

• After deposition, sediments go through lithification to become sedimentary rocks.

Fig. 8.5: Block Diagram Showing Sedimentary Processes.

In the preceding sections, you have learned about the various processes involved in the formation of sediments.

Paraphrase the information provided, elaborate, and provide examples where necessary. Present all points in a bullet format with appropriate headings.