What Is Sediment?
QUICK ANSWER
Sediment is loose particles of weathered rock, organic matter, or precipitated minerals deposited by water, wind, ice, or gravity. Sediment ranges from large boulders to tiny clay particles. Over time, accumulated sediment can compress and cement into sedimentary rock. Sediment is essential for soil formation, ecosystems, and the geological record.
Sediment is the loose particles that move across Earth's surface, eventually settling into layers that may become rocks. From sand on a beach to mud in a lake, sediment is a fundamental geological material. Understanding sediment reveals how Earth's surface is continuously recycled and how the geological record forms layer by layer over time.
What is sediment made of?
Sediment can be made of several different materials. Most common is clastic sediment, particles of pre-existing rocks that have been broken down by weathering. Chemical sediment forms when minerals precipitate out of water solutions, like salt deposits from evaporating saltwater. Biogenic sediment consists of organic material from once-living organisms, like shell fragments forming carbonate sediment or plant material forming peat. Many natural sediments mix these types. The composition determines properties like color, density, and how the sediment will behave during transport and eventual lithification.
How is sediment classified?
Sediment is classified primarily by particle size. Boulders are over 256 mm across. Cobbles are 64-256 mm. Pebbles are 4-64 mm. Granules are 2-4 mm. Sand is 0.0625-2 mm. Silt is 0.004-0.0625 mm. Clay is finer than 0.004 mm. The size of sediment particles tells geologists about the energy of the transporting medium: high-energy flows carry larger particles, while only low-energy environments allow clay to settle. The classification by size is universal, though specific size boundaries vary slightly between different classification systems.
How does sediment move?
Sediment is transported by several agents. Water (rivers, ocean waves, ocean currents) is the most common transport medium, carrying sediment in solution, suspension, or as bedload along the bottom. Wind transports sand and dust, sometimes across entire ocean basins. Glaciers carry everything from clay-sized particles to massive boulders embedded in ice. Gravity moves sediment through landslides, debris flows, and slow soil creep. The distance sediment can be transported varies enormously: clay can travel thousands of miles in ocean currents; large boulders may move only feet during catastrophic events. Sediment usually deposits when transport energy drops.
What happens to sediment over time?
Accumulated sediment can undergo several transformations over time. Compaction happens as more sediment accumulates above, squeezing out water and reducing pore space. Cementation occurs as dissolved minerals precipitate between sediment grains, gluing them together. Together, compaction and cementation lithify sediment into solid sedimentary rock. Some sediment becomes soil through weathering and biological activity. Buried sediment can be exposed again by uplift and erosion. Marine sediment can be subducted into the mantle. The fate of sediment depends on where it deposits and the geological history that follows.
Sediment is loose particles of weathered rock, organic material, or precipitated minerals deposited by water, wind, ice, or gravity. Classified mainly by particle size (from clay to boulders), sediment is transported by various agents and eventually undergoes compaction and cementation to form sedimentary rocks. Sediment is essential for soil formation, ecosystems, and recording Earth's history in layered rock sequences.
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