Sediments are solid materials formed from the breakdown of pre-existing rocks, minerals, or organic matter. These particles are transported by natural forces like wind, water, or ice, eventually settling and accumulating. Sediments are ubiquitous, forming beach sand, lakebed mud, and wind-borne dust. Understanding sediments is fundamental to deciphering Earth’s past environments and geological processes, providing a physical record of the planet’s history and influencing landscapes.
Grouping by Particle Size
Sediments are primarily categorized by the size of their individual particles, a fundamental characteristic influencing their transport and deposition. The smallest particles are clay, which measures less than 0.0039 millimeters in diameter. These microscopic particles are so fine they can remain suspended in water for extended periods, forming compact layers when deposited.
Slightly larger than clay is silt, with diameters ranging from 0.0039 to 0.0625 millimeters. Silt feels smooth when wet but gritty when dry, and is commonly found in riverbeds and lake bottoms. Sand particles are larger, 0.0625 to 2 millimeters, easily visible and gritty. Examples include sand on beaches or in desert dunes, which can be moved by both wind and water.
Particles larger than 2 millimeters are broadly classified as gravel. This category includes granules (2-4 mm), pebbles (4-64 mm), cobbles (64-256 mm), and boulders (over 256 mm). These larger sediments typically require stronger currents, such as fast-flowing rivers or glacial ice, for transport. Particle size plays a significant role in how sediments behave, with finer sediments often carried farther and deposited in calmer environments, while coarser sediments are found closer to their source.
Categorizing by Formation and Composition
Beyond particle size, sediments are also classified based on their origin and the materials they are composed of. One major category is clastic sediments, which form from the weathering and erosion of pre-existing rocks. Common examples include sandstones, formed from sand grains, and shales, which originate from compacted clay and silt particles.
Another type is chemical sediments, which form when minerals precipitate directly out of water solutions without biological involvement. For instance, limestone can form from calcite precipitation, and rock salt (halite) forms from the evaporation of saline water bodies. These sediments often indicate past environments where water was abundant and mineral-rich.
Biogenic sediments originate from the remains of living organisms. This category includes materials like shell fragments from marine creatures that form certain types of limestone. Plant matter that accumulates and compacts over geological time can form coal. Diatomaceous earth, another example, consists of the fossilized remains of diatoms. These sediments provide direct evidence of past life and biological activity in specific environments.
Understanding Sediments by Texture
The texture of sediments provides additional insights into their history, including how far they have traveled and the energy of the environment where they were deposited. Sorting describes the uniformity of particle sizes within a sediment sample. Well-sorted sediments, like beach sand, have very similar sizes, indicating prolonged transport and deposition by a consistent energy source. Conversely, poorly-sorted sediments contain a wide range of particle sizes, often found in deposits like glacial till.
Particle roundness refers to the degree to which the sharp edges and corners of sediment grains have been worn away. Angular particles have sharp corners, suggesting minimal transport from their source rock. Sub-angular to sub-rounded particles show some wear, while rounded and well-rounded particles have smooth, abraded surfaces, indicating extensive transport over long distances or prolonged exposure to high-energy environments. For example, river pebbles are often well-rounded due to constant tumbling.
Particle shape, while less commonly used as a primary classification, also contributes to sediment texture. Grains can be spherical, platy, or irregular. The shape can influence how particles pack together and how they are transported. Analyzing these textural properties helps scientists reconstruct the past conditions of sediment transport and depositional environments.