Fix the mistake: Strata and Fossils

Weathering, Erosion, Transportation, and Deposition

Rocks are broken down, transported, and deposited by wind and water. This series of processes forms the basis of strata.

Basic Knowledge

Weathering is the process by which rocks break down at Earth's surface through changes in temperature and the effects of water and air. It includes physical weathering(freezing and thawing, and temperature changes) and chemical weathering(reactions with water and acids).
Erosion is the action of running water, wind, waves, and glaciers wearing away the surface. Transportation is the action of moving the eroded sediments. Deposition is the action of those sediments accumulating.
In the upper reaches of a river, erosion is active and V-shaped valleys form; in the middle reaches, transportation forms alluvial fans; and in the lower reaches, deposition forms deltas. Classified by particle size, the order from largest to smallest is gravel(2 mm or more), sand(1/16〜2 mm), and mud(less than 1/16 mm). When these become cemented, they form conglomerate, sandstone, and mudstone.

Key Terms
Weathering(The mechanical and chemical breakdown of rocks at Earth's surface)
Erosion(The action of running water and other agents wearing away the surface)
Transportation(The action of running water and other agents carrying sediments)
Deposition(The action of transported sediments accumulating)
V-shaped valleys, alluvial fans, and deltas(Landforms of the upper, middle, and lower reaches)
Gravel, sand, and mud(Classification by particle size. Large → small)

In Depth (Background and Significance)

Weathering, erosion, transportation, and deposition are fundamental processes that support the cycling of materials at Earth's surface. Throughout Earth's history, the cycle of wearing down continents and filling the seafloor has repeated and accumulated strata.
Whether particles can be transported is determined by flow speed and particle size; this relationship is shown in the Hjulström diagram. Coarse particles(gravel) require fast currents to be transported, so they are deposited near river mouths, while fine mud is carried to and deposited in quiet deep seas.
Climate change also alters the rate of weathering. Chemical weathering, in which CO₂ dissolves in water to form carbonic acid that dissolves rocks, is part of Earth's temperature-regulation mechanism that controls atmospheric CO₂ concentrations over long periods.

Key Points
  • Weathering → erosion → transportation → deposition, in that order
  • Physical weathering and chemical weathering
  • Upper reaches: V-shaped valleys / middle reaches: alluvial fans / lower reaches: deltas
  • Particle sizes: gravel 2 mm or more / sand 1/16〜2 mm / mud less than 1/16 mm
  • Coarse particles are deposited near river mouths, while fine particles travel farther
  • The particle size that can be transported is determined by flow speed
  • Chemical weathering absorbs CO₂

Points to Note (Common Confusions)

① Distinguish weathering(breaking down in place) from erosion(being worn away). ② Transportation and erosion are separate stages. ③ V-shaped valleys are found in the upper reaches, while deltas are found in the lower reaches. ④ The boundaries among gravel, sand, and mud are 2 mm and 1/16 mm(0.0625 mm).

Practice

  1. What is the phenomenon in which rocks mechanically break down at Earth's surface called?
  2. Name the landform that forms at a river mouth in the lower reaches.
  3. What are particles 2 mm or more in size called?

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