Sediment comes from the physical and chemical breakdown of existing rocks, the accumulation of biological remains, minerals precipitating out of water, volcanic eruptions, and even cosmic dust falling from space. Most of the sediment on Earth’s surface originates from weathering, the slow destruction of rock by water, ice, wind, and chemical reactions. Rivers alone deliver an estimated 19.1 billion metric tons of sediment to the oceans every year, making them the single largest transport system for moving broken-down rock material across the planet.
Weathering: How Rocks Break Apart
Every grain of sand and speck of mud started as part of a larger rock. Weathering is the process that breaks those rocks down, and it works in two fundamentally different ways: mechanical and chemical.
Mechanical weathering physically shatters rock without changing its mineral composition. The most powerful example is ice wedging. Water seeps into cracks in rock, freezes, and expands by about 9%. That expansion pries the crack wider, and after thousands of freeze-thaw cycles, solid rock splits into fragments. Another form, called sheeting, happens when deeply buried rock is exposed at the surface through erosion. Freed from the immense pressure above it, the rock expands and peels apart in layers like an onion. Plant roots, burrowing animals, and even foot traffic also physically break rock into progressively smaller pieces.
Chemical weathering goes deeper, actually dissolving or transforming the minerals inside rock. When water reacts with feldspar, one of the most common minerals in Earth’s crust, it converts it into clay. Limestone dissolves almost entirely when exposed to slightly acidic rainwater, a process that carves caves and sinkholes. Iron-rich minerals like olivine react with oxygen in water or air to form iron oxide (essentially rust), which weakens the rock’s structure and produces the red and orange staining you see on many cliff faces. These chemical reactions don’t just shrink rocks. They create entirely new minerals that become part of the sediment supply.
How Sediment Gets Moved
Once weathering produces loose material, something has to carry it away. The four main transport agents are water, wind, ice, and gravity, and each one sorts sediment differently.
Rivers are the dominant movers. Fast-flowing mountain streams can tumble boulders and cobbles downstream, while slow, meandering lowland rivers carry fine silt and clay suspended in the water column. By the time a river reaches the ocean, it has sorted its sediment load by size and weight, depositing heavier particles first and carrying the lightest ones farthest.
Wind picks up where rivers leave off, especially in arid regions. Research on the Indus River basin and Thar Desert shows that monsoon winds recycle sediment from river deltas hundreds of kilometers back into the desert interior, building dune fields from material that originally washed downstream. Wind-blown sediment tends to be fine-grained and well-sorted because air simply can’t carry heavy particles very far.
Glaciers are extraordinarily effective at grinding rock. As a glacier moves, rocks frozen into its base scrape the bedrock beneath it like sandpaper, producing an ultra-fine powder called rock flour. This material gives glacial meltwater rivers their distinctive milky blue-green color. Glacial sediment is poorly sorted, meaning it contains everything from clay-sized particles to house-sized boulders, all dumped together when the ice melts.
Gravity works on its own through landslides, rockfalls, and slow downhill soil creep. It also works in combination with the other agents, pulling sediment-laden water downhill and driving underwater avalanches called turbidity currents on the ocean floor.
Biological Sediment
Not all sediment starts as rock. A significant portion of the material blanketing the ocean floor comes from the remains of living organisms. Diatoms, microscopic algae that build their cell walls out of silica, are major primary producers in modern oceans. When they die, their tiny glass-like shells sink and accumulate on the seafloor. Before diatoms rose to ecological dominance, radiolarians (single-celled organisms that also build silica skeletons) were the main contributors of siliceous sediment to the deep ocean.
Coccolithophores, another group of microscopic marine algae, construct their shells from calcium carbonate. Their accumulated remains form thick deposits of chalk. The White Cliffs of Dover, for instance, are essentially compressed coccolithophore shells. Foraminifera, tiny shelled protists, contribute similarly. In parts of the deep ocean far from any continent, biological ooze made almost entirely of these microscopic shells is the dominant sediment type.
Chemical Precipitation
Some sediment forms without any weathering or biological activity at all. When dissolved minerals in water become too concentrated to stay in solution, they crystallize out and settle as sediment. This process is most visible in evaporating bodies of water. As a shallow sea or salt lake shrinks, the water becomes increasingly mineral-rich until minerals begin to precipitate in a predictable sequence.
Gypsum crystallizes first, forming where wind-mixed surface water interacts with denser, more concentrated water below. As evaporation continues, halite (rock salt) precipitates. The Permian Basin in Texas and New Mexico preserves a dramatic example: the Castile formation is largely gypsum rock, while the overlying Salado formation is composed primarily of halite, recording a progressively drying ancient sea. These evaporite deposits can be hundreds of meters thick.
Volcanic Contributions
Explosive volcanic eruptions inject enormous quantities of fragmented rock and glass into the atmosphere. This volcanic ash, collectively called tephra, ranges from particles finer than silt to chunks the size of gravel. Fine ash (particles smaller than 63 micrometers) can stay airborne for days or weeks, traveling thousands of kilometers before settling. In practice, though, ash particles tend to clump together into aggregates that fall out of the atmosphere much faster, often within a day.
These aggregates create a sorting effect that works in both directions simultaneously. Fine ash gets pulled down prematurely by clumping onto larger particles, while coarse ash cores coated in fine particles can actually be rafted farther from the volcano than their size would normally allow. Once deposited, volcanic ash becomes a recognizable layer in the sediment record, and over geologic time, it weathers into nutrient-rich clay soils.
Cosmic Dust
Earth constantly sweeps up material from space. About 43 metric tons of cosmic dust enter the atmosphere every day, with estimates ranging from 29 to 57 tons. Roughly 80% of this material originates from short-period comets with orbits of less than 20 years. Most of it arrives as micrometeorites, particles so small they slow down in the upper atmosphere without burning up completely. They drift down and eventually settle on land and sea surfaces, becoming part of the sediment. While this is a trivial amount compared to river sediment, over millions of years cosmic dust accumulates measurably, especially in deep-ocean sediments far from land where other sediment sources are scarce.
How Humans Have Changed the Picture
Human activity has dramatically altered both the production and movement of sediment. Across North America, continent-wide rates of sediment accumulation in river valleys were broadly stable for about 40,000 years, then increased tenfold during the rapid expansion of agriculture and river modification that followed European colonization. Soil tillage, deforestation, construction, forestry, and ranching all accelerate erosion, increasing the amount of sediment washing into rivers.
At the same time, dam construction traps sediment that would otherwise reach the coast. This creates a paradox: more sediment is being produced upstream, but less of it arrives at the ocean. The agricultural land-use expansion that began around 1700 and peaked in 1960 was the primary driver of increased erosion, with sediment retention behind dams playing a secondary role. The net effect is that many river deltas and coastlines are now starved of the sediment they need to maintain themselves, even as upland erosion accelerates.
Sediment Size and Classification
Geologists classify sediment primarily by particle size, using a scale established in 1922 that groups all sediment into four main categories: clay (the finest), silt, sand, and gravel. Gravel is further divided into granules, pebbles, cobbles, and boulders. This size-based system matters because particle size controls how sediment behaves. Clay stays suspended in still water for hours or days, while sand settles in seconds. Gravel requires fast-moving water or ice to transport it at all. The size of sediment grains at any location tells you a great deal about the energy of the environment that deposited them: fine mud means calm water, coarse gravel means powerful currents or glacial dumping.