Question

Regarding the classification of sedimentary rocks: Which of the statements is/are correct?

1. Mechanical rocks are formed from fragments of existing rocks.
2. Chemical rocks form through mineral precipitation from water.
3. Coal is a primary example of a mechanically formed rock.

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

Answer 1

Answer:

C. 1, 2

Classification of Sedimentary Rocks

1. Mechanical Sedimentary Rocks (Clastic Rocks)

• Formation: These rocks are formed from the accumulation and cementation of fragments (clasts) of pre-existing rocks and minerals. These fragments are transported by agents like water, wind, or ice.

• Examples:

  • Conglomerate and Breccia (large fragments)

  • Sandstone (sand-sized fragments)

  • Shale and Mudstone (silt and clay-sized fragments)

• Key Process: Weathering, erosion, transportation, deposition, compaction, and cementation.

2. Chemical Sedimentary Rocks

• Formation: These rocks form when dissolved minerals in water precipitate out of solution. This often occurs due to evaporation of the water body or changes in chemical conditions.

• Examples:

  • Rock Salt (Halite) - formed by evaporation of saltwater

  • Gypsum - formed by evaporation of sulfate-rich water

  • Limestone (some types, like travertine) - formed from calcium carbonate precipitation

  • Chert (some types)

• Key Process: Dissolution, transportation of ions in solution, precipitation, and lithification.

3. Organic Sedimentary Rocks (Biochemical Rocks)

• Formation: These rocks are formed from the accumulation of organic matter, such as plant or animal remains.

• Examples:

  • Coal: Formed from the accumulation and compression of plant material over millions of years, typically in swampy environments. It is not a primary example of a mechanically formed rock, but rather an organic one.

  • Limestone (some types, like coquina and chalk) - formed from shells and skeletal fragments of marine organisms.

• Key Process: Accumulation of organic debris, burial, and transformation through heat and pressure.