The movement of soil across the landscape is a dynamic geological process known as sediment transport. This process links erosion, the detachment and removal of soil particles, with deposition, the final resting place of that material. Soil constantly moves, shaping the Earth’s surface over time. This movement is a fundamental component of the rock cycle, influencing soil formation, landforms, and nutrient distribution. The energy required to move soil originates from several major natural forces.
Transport Driven by Water
Water is the most dominant agent of soil transport globally, moving material from the smallest raindrop splash to the largest river flow. The process begins with splash erosion, where the impact of raindrops striking bare soil dislodges particles. This initial detachment makes the soil particles available for subsequent transport by surface runoff.
As water flows across the land in a thin sheet, it causes sheet erosion, uniformly removing the fine topsoil layer. When the flow concentrates into small channels, it creates rill erosion, which can deepen and expand into larger gullies, moving significant volumes of sediment. Once the soil enters a concentrated flow, like a stream or river, it is carried in three distinct ways, referred to as sediment loads.
The dissolved load consists of ions and chemical compounds completely dissolved during weathering. These dissolved materials travel with the water flow and only settle out if concentration changes or through biological activity. The suspended load is made up of fine particles, such as silt and clay, which are kept aloft by the water’s turbulence. These particles give rivers their muddy appearance and settle out only when flow velocity decreases significantly.
The coarsest materials are moved as bedload, which consists of larger particles like sand, gravel, and cobbles that remain in close contact with the channel floor. Bedload moves via two primary mechanics: traction and saltation. Traction is the rolling and sliding of the heaviest particles along the bottom. Saltation involves medium-sized grains that bounce or hop along the bed, lifted briefly by the force of the water before gravity quickly returns them to the bottom.
Transport Driven by Wind
Wind transport, or aeolian transport, is effective in arid and semi-arid regions or any area where vegetation cover is sparse. The amount of soil moved depends heavily on the wind velocity and the size of the soil particles. Soil particles are moved by wind through three distinct, size-dependent modes.
The largest particles, typically between 0.5 millimeters and 2 millimeters in diameter, are moved by surface creep. These particles are too heavy for the wind to lift, so they are pushed and rolled along the ground surface. Surface creep generally accounts for the smallest percentage of total soil movement by wind, usually between 5% and 25%.
Saltation is the most significant mode of wind transport, often accounting for 50% to 90% of the total soil movement. This process involves medium-sized particles (0.1 to 0.5 millimeters) which bounce or hop along the surface. When a saltating particle lands, its impact can dislodge additional particles, setting them into motion or launching finer particles into the air.
Suspension involves the finest particles, less than 0.1 millimeters, which are lifted high into the atmosphere. Once airborne, these dust particles can be carried hundreds or thousands of miles from their origin. Suspension contributes to large-scale dust storms and the deposition of fine-grained soil deposits called loess.
Transport Driven by Gravity and Ice
The force of gravity is a constant driver of soil movement on sloped terrain, in a process termed mass wasting. This movement is distinct because the soil is not entrained in a fluid medium, though water saturation often acts as a trigger. The slowest form is soil creep, a nearly continuous downslope movement occurring at rates of only millimeters per year, often caused by freeze-thaw cycles. More rapid mass wasting events include landslides, slumps, and debris flows, which move massive amounts of rock and soil downslope quickly. The resulting soil deposits, known as colluvium, are typically found at the base of slopes and cliffs.
Ice, in the form of glaciers, is another agent responsible for moving enormous volumes of soil and rock over long distances. Glaciers are powerful erosional agents, scraping and scouring the landscape as they move. The material they carry includes everything from fine clay to massive boulders, embedded directly within the ice.
When a glacier melts, it deposits this material, which is characteristically unsorted and unstratified. This mixture of rock and soil is known as glacial till. The lack of sorting in till, unlike materials carried by water or wind, indicates the deposit was transported and dropped directly by the ice.