Sediment is a fundamental material in Earth science, representing the loose, solid material that forms the building blocks of most landscapes and geological formations. It is composed of particles derived from the breakdown of older rock, organic matter, or chemical precipitates. This material is defined by its ability to be transported and deposited in a new location by natural forces like water, wind, or ice. Understanding sediment is essential because it forms layers that record environmental changes and Earth’s history over vast timescales.
Defining Sediment and Its Components
Sediment is classified based on the physical properties of its individual grains, specifically their size and composition. The size of these particles spans an enormous range, from massive boulders down to microscopic clay particles. Common size categories include gravel (particles larger than two millimeters) and sand (ranging from two millimeters down to about one-sixteenth of a millimeter). Finer particles include silt and clay, with clay measuring less than one two-hundred-fifty-sixth of a millimeter.
The source material can be mineral fragments, such as quartz grains, or fragments of pre-existing rocks. Sediment can also originate from organic remains, like shells and plant matter, or from chemicals that precipitate directly out of water. Physical properties, such as the shape and size distribution of the particles, provide geologists with clues about the sediment’s history. For instance, well-rounded grains suggest long transport, while angular fragments indicate movement over a shorter distance.
The Dynamic Process of Sediment Formation
The creation of sediment is a multi-step process beginning with the breakdown of source material through weathering. Weathering can be physical, like the fracturing of rock by freezing water, or chemical, such as the dissolution of minerals by acidic rain. This process transforms massive rock formations into smaller, loose fragments or dissolved ions. Once created, these particles are subjected to erosion, which involves their removal and initiation of transport.
Sediment is carried away by agents like flowing water, wind, glacial ice, or gravity. The energy of the transporting agent determines the maximum size of particles that can be moved; for example, fast-moving water carries gravel, while wind carries only fine sand and dust. The final stage is deposition, which occurs when the transporting medium loses enough energy to hold the sediment in motion. As a river slows down, it drops its sediment load, forming distinct layers as the particles settle, a process known as sedimentation.
Primary Scientific Categories of Sediment
Scientists classify sediment into three main categories based on the material’s origin. Clastic, or detrital, sediment is the most common type, consisting of physical fragments of pre-existing rocks and minerals. These fragments result from mechanical weathering and erosion, and they are classified primarily by grain size, such as sand, silt, or gravel.
Chemical sediment forms through the precipitation of dissolved mineral ions from water solutions. This often occurs when water evaporates, leaving behind dissolved solids like rock salt or gypsum. Biogenic sediment is composed of the remains of living organisms, such as shells and skeletal fragments made of calcium carbonate, forming deposits like chalk. Plant matter accumulating in swamps can also form biogenic sediment, which eventually leads to the creation of coal.
From Sediment to Sedimentary Rock
Loose sediment layers accumulated through deposition undergo a final transformation into solid sedimentary rock via a process called lithification. This conversion involves two primary mechanisms working together under the weight of overlying layers. The first step is compaction, where pressure from accumulating sediment reduces the volume of the layers beneath. This pressure forces out water trapped in the pore spaces, squeezing the particles closer together.
Following compaction, cementation takes place. Dissolved minerals carried in the remaining pore water precipitate and crystallize in the spaces between the grains. These newly formed minerals, often including silica or calcium carbonate, act as a natural glue. This binds the loose sediment particles into a cohesive, solid rock mass, resulting in common sedimentary rocks like sandstone or shale.