Sedimentary rocks form when transported sediments, the remains of organisms, volcanic ejecta, and other materials accumulate and become solid. Their origins differ greatly depending on the type.
Basic Knowledge
Sedimentary rocks are broadly classified into three types.
① Clastic rocks: Made of particles formed by the weathering of rocks. Conglomerate, sandstone, and mudstone.
② Biogenic rocks(including chemical rocks): Formed from the remains of organisms or precipitation from aqueous solutions. Limestone(CaCO₃, remains of corals and foraminifers, effervesces with hydrochloric acid) and chert(SiO₂, remains of radiolarians and diatoms, produces sparks when struck with a hammer).
③ Volcaniclastic rocks: Rocks formed from accumulated volcanic ejecta. Tuff(rock formed from consolidated volcanic ash, often becoming a useful key bed for correlating strata).
Consolidation includes compaction, in which sediments are compressed by the weight of overlying strata, and cementation, in which calcium carbonate, iron oxide, and other substances seep between particles and bind them together.
Conglomerate(A sedimentary rock formed from gravel. Particles are 2 mm or more)
Sandstone(A sedimentary rock formed from sand. Particles are 1/16〜2 mm)
Mudstone(A sedimentary rock formed from mud. Particles are less than 1/16 mm. Includes siltstone and claystone)
Limestone(Mainly composed of CaCO₃. Effervesces as CO₂ is produced with hydrochloric acid)
Chert(Mainly composed of SiO₂. Extremely hard and produces sparks when struck with a hammer)
Tuff(Rock formed from volcanic ash. Used as a key bed to correlate strata)
In Depth (Background and Significance)
Limestone(CaCO₃) is an important domestic resource in Japan and a raw material for cement. Although Japan depends on imports for petroleum and iron ore, it can meet 100% of its limestone needs domestically. Karst landforms such as Akiyoshidai in Yamaguchi Prefecture, Hiraodai in Fukuoka Prefecture, and Ateshidai in Okayama Prefecture formed when limestone dissolved in rainwater.
Chert is hard and difficult to process, but it was used as a material for stone tools in ancient times and is still used as an abrasive today. Because it accumulates slowly in the deep sea over long periods, it is essential for precise age determination using radiolarian fossils.
Tuff is likely to be distributed over a wide area even when its layer is thin, so it is extremely useful as a key bed for correlating strata far apart. Analysis of volcanic ash, or tephra, is crucial for determining the ages of Quaternary strata.
- Clastic rocks = conglomerate, sandstone, and mudstone
- Biogenic rocks = limestone(corals, etc.) and chert(radiolarians, etc.)
- Volcaniclastic rocks = tuff(which becomes a key bed)
- Limestone: effervesces with hydrochloric acid(CaCO₃+2HCl→CaCl₂+H₂O+CO₂)
- Chert: does not effervesce with hydrochloric acid and is extremely hard
- Consolidation = compaction + cementation
- Tuff = useful for correlating distant strata
Points to Note (Common Confusions)
① Distinguish limestone(CaCO₃, effervesces with hydrochloric acid) from chert(SiO₂, does not effervesce, and is hard). ② Tuff originates from volcanic ash, not from clastic rock particles. ③ The boundaries among gravel, sand, and mudstone are based on particle size(2 mm and 1/16 mm). ④ Limestone becomes marble through metamorphism.
Practice
- Which sedimentary rock produces bubbles when hydrochloric acid is applied?
- What rock forms when volcanic ash becomes consolidated?
- What extremely hard rock forms from accumulated radiolarian shells?