Sedimentation

Lore & Background

Sedimentation takes place when particles in suspension settle out of the fluid in which they are entrained, due to forces such as gravity, centrifugal acceleration, or electromagnetism. Settling refers to the falling of suspended particles through the liquid, while sedimentation is the final result of that process. In geology, the term is broadly applied to the entire range of processes that result in the formation of sedimentary rock, from initial erosion through sediment transport and settling to lithification, though the strict geological definition is the mechanical deposition of sediment particles from an initial suspension in air or water.

Sedimentation may pertain to objects of various sizes, ranging from large rocks in flowing water to suspensions of dust and pollen particles, cellular suspensions, and even solutions of single molecules such as proteins and peptides. The process is classified into three types: Type 1, where particles settle discretely at a constant velocity (e.g., sand and grit); Type 2, where particles flocculate and change settling velocity (e.g., alum or iron coagulation); and Type 3, or zone sedimentation, where particles at high concentration settle as a mass, creating distinct clear and sludge zones.

When particles settling from a suspension reach a hard boundary, the concentration at the boundary is opposed by diffusion, leading to sedimentation equilibrium. In estuarine environments, vegetation such as mangroves can attenuate waves or currents, promoting the settlement of suspended particles. Undesired increased transport and sedimentation of suspended material is called siltation, which can result from poor land management and high flooding frequency, and is a major source of pollution in waterways.

Reader's Guide

Sedimentation is a fundamental process in geology, chemistry, and environmental science, governing the formation of sedimentary rock and the behavior of particles in fluids. Its classification into Type 1, Type 2, and Type 3 sedimentation provides a framework for understanding how particles settle under different conditions, from discrete sand grains to flocculating colloids and high-concentration sludge. The concept of sedimentation equilibrium, where settling is opposed by diffusion, allows measurement of particle properties, such as the size of macromolecules using centrifugal force in an ultracentrifuge.

In the geologic record, most sedimentation occurred in brief depositional episodes separated by long intervals of nondeposition or erosion. The rate of sedimentation varies widely, from less than 3 millimeters per thousand years for pelagic sediment to several meters per thousand years in major river deltas, though long-term accumulation is determined more by subsidence creating accommodation space. Human activities can enhance sedimentation rates, leading to siltation, which is a major pollution source and can harm fragile ecosystems like coral reefs. Climate change also affects siltation rates. Understanding sedimentation is crucial for managing waterways, interpreting sedimentary rocks, and designing water treatment processes.

Did You Know?

• Sedimentation can involve objects ranging from large rocks to single molecules such as proteins and peptides.
• Type 1 sedimentation involves particles that settle discretely at a constant velocity, such as sand and grit.
• In estuarine environments, mangroves promote the settlement of suspended particles by attenuating waves or currents.
• The rate of sedimentation for pelagic sediment can be less than 3 millimeters per thousand years.