Sediment
Loose particles transported and deposited by natural processes.
Sediment is a naturally occurring solid material composed of loose particles that are broken down from larger rocks and minerals through weathering and erosion. These particles are then transported by wind, water, ice, or gravity to a new location where they settle and accumulate. For instance, sand and silt carried in suspension by rivers eventually deposit on the seafloor through sedimentation; if buried and compacted over time, they can lithify into sedimentary rocks such as sandstone and siltstone. The primary agents of transport are water (fluvial processes), wind (aeolian processes), and glaciers. Fluvial transport creates features like beach sands and river channel deposits, while sediment also settles in slow-moving or standing water bodies such as lakes and oceans. Aeolian processes form desert sand dunes and loess deposits, and glacial transport leaves behind moraines and till. Sediment classification is based on grain size, shape, and composition. Grain size is measured on a logarithmic scale called the Phi scale, ranging from colloids to boulders. Shape is defined by three parameters: form (or sphericity), which describes the overall shape as spherical, platy, or rodlike; roundness, which measures the sharpness of corners and edges from very angular to very rounded; and surface texture, which includes small-scale features like scratches, pits, or ridges. Composition is assessed by parent rock lithology, mineral content, and chemical makeup, though terms like "clay" can refer to both a size range and a specific mineral group. Sediment transport depends on flow strength and particle size, density, and shape, with stronger flows increasing lift and drag. The Exner equation describes the balance between sediment in transport and deposition, influencing patterns of erosion and deposition in streams. In seas, oceans, and lakes, sediment accumulates from terrigenous material (originating on land) or biological sources, such as the exoskeletons of dead organisms in mid-ocean regions.
- field
- Earth and planetary sciences
- classification_basis
- Grain size, grain shape, and composition
- grain_size_scale
- Phi scale (log base 2)
- transport_agents
- Water, wind, ice, gravity
- key_formula_proponents
- William C. Krumbein; Sneed and Folk
Lore & Background
Sediment is classified based on its grain size, grain shape, and composition. Grain size is measured on a log base 2 scale called the Phi scale, which classifies particles from colloid to boulder. The shape of particles is defined by three parameters: form (overall shape, such as spherical, platy, or rodlike), roundness (how sharp the corners are, ranging from very angular to very rounded), and surface texture (small-scale features like scratches, pits, or ridges). William C. Krumbein proposed formulas for converting measurements of a particle's long, intermediate, and short axes into a single measure of form, and an alternate measure was proposed by Sneed and Folk.
Reader's Guide
Sediment is fundamental to understanding Earth surface processes and the formation of sedimentary rocks. The Exner equation describes the overall balance between sediment in transport and sediment being deposited on the bed, showing that changes in flow power alter the ability to carry sediment, which is reflected in patterns of erosion and deposition. Sediment accumulates in seas, oceans, and lakes, and can consist of terrigenous material originating on land or biological material from the water body. Deposited sediments are the source of sedimentary rocks, which can contain fossils. Lake bed sediments that have not solidified into rock can be used to determine past climatic conditions. Key marine depositional environments include littoral sands, the continental shelf, the shelf margin, the shelf slope, and beds of estuaries. The null point theory explains how sediment deposition undergoes a hydrodynamic sorting process within the marine environment leading to a seaward fining of sediment grain size.
Did You Know?
- Sediment size is measured on a log base 2 scale called the Phi scale, from colloid to boulder.
- Surface texture of quartz grains can reveal transport history; frosted grains are characteristic of aeolian (wind) sediments.
- The Exner equation states that the rate of increase in bed elevation due to deposition is proportional to the amount of sediment that falls out of the flow.
- The null point theory explains a seaward fining of sediment grain size in marine environments.
Frequently Asked Questions
Who is Sediment?
Sediment is any collection of loose, solid particles that have been broken free from their original setting and carried to a new location by natural forces. It serves as the raw, unconsolidated building material that eventually becomes sedimentary rock once compacted and cemented together.
What are Sediment's powers and role?
Sediment is mobilized by water currents, wind gusts, glacial ice, or plain gravitational pull, then dropped wherever those agents lose energy. Water is by far the most common carrier, though aeolian (wind-driven) and glacial transport also play major roles in reshaping landscapes.
How does Sediment's story end?
The final chapter for most sediment is lithification—the slow, pressure-driven compaction and mineral cementation that locks loose grains into solid sedimentary rock. Before that endpoint, however, the same particles can be reworked, re-sorted, and re-deposited countless times.
Why is Sediment important?
Sediment is the sole source material for every sedimentary rock, which in turn preserve the geological record of past climates, organisms, and environments. It also builds coastlines, river deltas, and continental shelves, making it central to understanding how Earth's surface evolves.
How is Sediment classified?
Sediment is sorted by grain size (measured on the phi scale, a base-2 logarithmic system), grain shape, and mineral composition. The classification frameworks we rely on today were shaped by key contributors such as Krumbein, Sneed, and Folk.
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