Question

Evaluate the following statements about the formation of sedimentary rocks: Which of the statements are correct?

1. Rocks of the Earth's crust are broken into fragments via weathering.
2. These fragments must remain stationary to coalesce into sedimentary rocks.
3. External forces play a role in the transport and deposition of these fragments.

• A. 2, 3
• B. 1, 3
• C. 3 only
• D. All

Answer 1

Answer:

B. 1, 3

Formation of Sedimentary Rocks

Key Processes Involved:

• Weathering: The process by which rocks are broken down into smaller pieces or fragments. This can be physical (e.g., frost wedging, abrasion) or chemical (e.g., dissolution, oxidation). These fragments are called sediments.

• Erosion and Transportation: Sediments are moved from their source area by external forces. The primary agents of transport include:

  • Water: Rivers, streams, ocean currents, and waves are highly effective in carrying sediments.

  • Wind: Particularly effective in transporting fine-grained sediments like sand and silt in arid or semi-arid regions.

  • Ice: Glaciers can transport vast amounts of rock debris.

  • Gravity: Plays a role in mass wasting events like landslides, which move fragments downslope.

• Deposition: When the transporting agent loses energy, the sediments are dropped or laid down. This typically occurs in environments like ocean basins, lakes, river floodplains, and deltas.

• Compaction and Cementation (Lithification): Over time, the accumulated sediments are buried under more layers. The weight of the overlying material compacts the sediments, reducing pore space. Dissolved minerals in groundwater percolate through the sediments and precipitate in the pore spaces, acting as a natural glue to bind the fragments together. This process is called cementation, and together with compaction, it transforms loose sediments into solid sedimentary rock.

Addressing the Statements:

• Statement 1 is correct: Weathering is the initial step where rocks are broken down into fragments (sediments).

• Statement 2 is incorrect: Sediments do not need to remain stationary; in fact, they must be transported and deposited to form sedimentary rocks.

• Statement 3 is correct: External forces (water, wind, ice, gravity) are crucial for moving sediments from their origin to a depositional site.

Types of Sedimentary Rocks:

• Clastic: Formed from pre-existing rock fragments (e.g., sandstone, shale, conglomerate).

• Chemical: Formed from precipitation of minerals from solution (e.g., rock salt, some limestones).

• Organic: Formed from the accumulation of organic debris (e.g., coal, some limestones like chalk).