Weathering, Erosion, and Sediment Transport

Weathering breaks rock down in place, erosion removes the loosened material, and transport carries it across the landscape to where it is eventually deposited as sediment.

Definition

Weathering is the in-place breakdown of rock by physical and chemical processes, erosion is the removal of the resulting particles, and sediment transport is their movement by water, wind, ice, or gravity to a site of deposition.

Scope

This topic covers the production and movement of sediment: physical and chemical weathering, the agents of erosion, and the modes of transport by water, wind, ice, and gravity, including the relationship between flow energy and grain size. It is the upstream half of the sedimentary system.

Core questions

• What physical and chemical processes break down rock?
• What controls whether a particle of a given size is eroded, transported, or deposited?
• How do different transport agents sort and shape sediment?

Key theories

Competence and the Hjulström relationship

Hjulström's experiments related flow velocity to whether grains of a given size are eroded, carried, or deposited, showing that fine cohesive particles require higher velocities to erode than to keep in suspension.

Weathering regimes and climate

Physical weathering dominates in cold and arid settings while chemical weathering dominates in warm and humid ones, so climate exerts a first-order control on the rate and products of rock breakdown.

Mechanisms

Physical weathering fragments rock through processes such as frost wedging and exfoliation, while chemical weathering alters minerals by dissolution, hydrolysis, and oxidation, producing clays and dissolved ions. Eroded particles move as bed load, suspended load, or dissolved load depending on grain size and flow energy; as transport energy declines, the coarsest grains settle first, sorting sediment by size.

Clinical relevance

Weathering and erosion control soil formation and agricultural productivity, sediment loads that affect reservoirs and water quality, and landslide and erosion hazards, while the dissolved products of silicate weathering influence the long-term carbon cycle and climate.

History

Quantitative study of sediment transport began with nineteenth-century work by Gilbert and others on rivers, advanced through Hjulström's 1935 erosion–deposition diagram and Bagnold's mid-century studies of wind and water transport, and continues in modern process-based geomorphology and Earth-surface modeling.

Key figures

• Filip Hjulström
• Ralph Bagnold
• Grove Karl Gilbert

Seminal works

• hjulstrom1935

Frequently asked questions

What is the difference between weathering and erosion?

Weathering is the breakdown of rock where it sits, without significant movement, while erosion is the removal and transport of the broken-down material away from its source.