Sedimentary rocks form when products of rock weathering and erosion, chemical precipitates, or biological remains are deposited and become solid. They cover about 75% of Earth's surface and contain important geological records, including fossils.
Basic Knowledge
Sedimentary rocks are divided into three types according to their formation process.
Clastic rocks form when clastic materials (gravel, sand, and mud) produced by weathering and erosion are deposited and compacted. Classification by grain size (largest first): conglomerate (grain size > 2mm), sandstone (2mm〜0.063mm), and mudstone (silt and clay: < 0.063mm). The process by which sediment becomes solid is called diagenesis (compaction and cementation).
Chemical rocks form when chemical components dissolved in seawater or lake water precipitate because of changes in solubility or evaporation. Typical examples include limestone (CaCO₃, chemical precipitation from seawater or biological origin), chert (a fine-grained SiO₂ rock), rock salt (NaCl evaporite), and gypsum (CaSO₄·2H₂O).
Biogenic rocks (organic rocks) form from accumulated biological remains. Examples include coal (altered plant remains), petroleum and natural gas (altered marine organic matter), and coral-reef limestone (corals and shells). Chert is also regarded as a biogenic rock when it forms from accumulated shells of radiolarians and diatoms (SiO₂).
Explain how chert forms and how it differs from limestone in its reaction to acid.
Answer: Chert forms when the remains of organisms such as radiolarians and diatoms, which have SiO₂ shells, accumulate on the deep-sea floor and become solid (a biogenic or chemical rock). Limestone (CaCO₃) dissolves and produces CO₂ when dilute hydrochloric acid is applied, whereas chert (SiO₂) does not dissolve in dilute hydrochloric acid (it dissolves in HF).
- Clastic rocks: conglomerate (> 2mm), sandstone (2〜0.063mm), mudstone (< 0.063mm)
- Chemical rocks: limestone (CaCO₃), chert (SiO₂), rock salt, gypsum
- Biogenic rocks: coal (plants), petroleum (marine organic matter), coral limestone
- Dilute hydrochloric acid on limestone → CO₂ is produced (identification test)
- Chert is very hard and does not dissolve in dilute hydrochloric acid
- Diagenesis: compaction + cementation
- Sedimentary rocks cover about 75% of Earth's surface
Points to Note
① Limestone and chert are different rocks despite their similar appearance (CaCO₃ vs. SiO₂). They can be distinguished by their reaction to acid. ② Mudstone is a general term that includes siltstone and claystone, and it has one of the smallest grain sizes. ③ Coal is a sedimentary rock (biogenic rock), not an igneous or metamorphic rock.
Practice
- Name the three types of clastic rocks in order of decreasing grain size.
- What is produced when dilute hydrochloric acid is applied to limestone?
- What kinds of organisms form the remains from which chert is made?