Sediment is formed by the breakdown of existing rocks, the accumulation of biological remains, and the precipitation of dissolved minerals from water. These three pathways produce the three main categories of sediment: clastic (from rock fragments), biogenic (from living organisms), and chemical (from mineral-rich solutions). Most sediment you encounter, from beach sand to river mud, is clastic, meaning it started as larger rock that was broken down and carried to a new location.
Physical Weathering: Breaking Rock Apart
The most straightforward way sediment forms is through mechanical forces that shatter rock into smaller pieces without changing its chemical makeup. Several natural processes drive this breakdown.
Frost wedging is one of the most powerful. Water seeps into cracks in rock, and when it freezes, it expands by about 9%. That expansion forces the crack wider, allowing even more water in next time. Over repeated freeze-thaw cycles, chunks of rock split away entirely. This is why mountainous regions often have talus slopes, those fan-shaped piles of angular rock fragments at the base of steep cliffs.
Pressure release works on a larger scale. Rock that formed deep underground was once squeezed under the weight of kilometers of overlying material. When erosion strips that weight away, the rock expands and cracks. Granite, which has no built-in weak planes, cracks parallel to the exposed surface, producing the rounded, peeling-layer appearance of formations like Half Dome in Yosemite. Plant roots contribute too, threading into tiny cracks where moisture collects and gradually prying them open as the roots grow. Even burrowing animals loosen and displace material over time.
Chemical Weathering: Dissolving and Transforming Minerals
While physical weathering chips rock into smaller versions of itself, chemical weathering actually changes the minerals involved, weakening rock and creating entirely new substances. Three reactions do most of the work.
Oxidation is essentially rusting. Iron-bearing minerals react with oxygen, becoming softer and more crumbly. You can see this wherever rock has turned reddish-brown: that color is oxidized iron. Carbonation happens when rainwater absorbs carbon dioxide from the atmosphere, forming a weak acid. That acid slowly dissolves minerals like calcite, which is why limestone caves and sinkholes exist. Hydrolysis occurs when water itself reacts with minerals, breaking them down into clay and releasing dissolved elements like potassium and calcium. Feldspar, one of the most common minerals in the Earth’s crust, converts to clay through hydrolysis over long periods.
Chemical weathering is why certain minerals dominate sediment while others disappear. Quartz is nearly immune to chemical breakdown, so it persists through long transport and heavy weathering. Feldspar survives moderate weathering but tends to vanish from sediments that have traveled long distances or been exposed to the elements for extended periods. The end result is that most sand is dominated by quartz, while finer sediments like mud are rich in clay minerals, the stable byproducts of chemical reactions that destroyed less resistant minerals.
How Sediment Travels
Once rock breaks into fragments, those fragments need to move before they can accumulate as sediment somewhere new. Water is the dominant transport agent. Rivers carry enormous volumes of material as bedload (particles rolling and bouncing along the bottom), suspended load (finer grains held up in the flow), and dissolved load (invisible ions in solution). Wind picks up and carries fine sand and dust, sometimes thousands of kilometers. Glaciers are uniquely powerful transporters because ice can drag everything from fine clay to house-sized boulders without sorting them by size.
In cold-climate rivers, ice itself acts as a transport agent through a process called rafting. Drifting ice lifts, drags, and carries trapped particles downstream, sometimes moving boulder-sized sediment that flowing water alone could never budge. This sculpts river channels in ways unique to ice-affected regions. The farther sediment travels, the more it gets rounded, sorted by size, and stripped of weaker minerals. A beach sand grain has typically been through far more processing than a freshly broken fragment at the base of a cliff.
Biological Sediment: Shells, Skeletons, and Organic Matter
Not all sediment comes from rock. In the ocean, a huge proportion of the seafloor is covered in biogenic ooze: accumulated remains of microscopic organisms. Tiny shelled plankton called foraminifera, along with even smaller algae called coccolithophores, produce calcium carbonate shells that rain down to the seafloor when the organisms die. Other organisms, including radiolarians, diatoms, and silicoflagellates, build skeletons out of silica instead of carbonate. Together, these biological remains make up the bulk of sediment across most of the deep ocean.
Zooplankton also package sediment more efficiently by producing fecal pellets packed with shell fragments, tiny organisms, and clay particles. These pellets sink faster than individual particles would, accelerating the delivery of material to the seafloor. In shallow water, coral fragments, broken shells from mollusks, and pieces of calcareous algae contribute to biogenic sediment as well. Many tropical beaches are made almost entirely of biological fragments rather than weathered rock.
Chemical Precipitation From Water
The third pathway creates sediment when dissolved minerals come out of solution and form solid crystals. This happens most commonly when water evaporates, concentrating the dissolved minerals until they can no longer stay in solution. Evaporite deposits form this way in shallow seas, desert lakes, and tidal flats. Gypsum precipitates first as water concentrates, followed by rock salt (halite) at higher concentrations. Permanent deposits only accumulate where the dry-season deposition outpaces wet-season dissolution.
Chemical precipitation also occurs when water chemistry shifts. Where deep, mineral-rich water mixes with shallower water, the change in conditions can trigger minerals to crystallize. Limestone can form this way, as can chert (a hard, silica-rich rock). These chemically formed sediments are distinct from clastic and biogenic types because they never existed as fragments or biological structures. They crystallize directly from the water.
Where Sediment Accumulates
Sediment collects in specific settings called depositional environments, and each one produces characteristic types of sediment. Alluvial fans form at the base of mountains where fast-moving streams suddenly spread out and dump their coarse load of gravel and sand. River floodplains accumulate fine silt and clay during periodic flooding. Deltas build where rivers empty into the sea, depositing layers of sand and silt that slope seaward. Glacial regions produce unsorted mixtures of everything from clay to boulders, called till, along with sorted gravels deposited by meltwater streams.
Beaches collect well-sorted sand, while the deep ocean floor, far from any continent, receives only the finest clay particles drifting through the water column and the constant rain of microscopic shells from above. The deep abyssal plains are blanketed in fine mud and microfossils, producing mudstone and chert over geologic time.
How Loose Sediment Becomes Rock
Loose sediment eventually transforms into solid sedimentary rock through two main processes. Compaction occurs as new layers pile on top, and the weight squeezes grains closer together, forcing out water and air. Cementation then locks the grains in place as minerals dissolved in groundwater crystallize in the spaces between particles, acting like glue. The most common natural cements are calcite, silica, iron oxides, and clay minerals. Together, compaction and cementation turn beach sand into sandstone, seafloor mud into mudstone, and shell accumulations into limestone.