Earth’s surface is constantly changing through geological processes that reshape landforms. Among the most fundamental are erosion and deposition, which collectively sculpt the landscapes we observe. These actions represent a cycle of breakdown, transport, and accumulation of Earth materials.
What is Erosion?
Erosion involves the wearing away of Earth’s surface and the subsequent transportation of loosened materials. This process is distinct from weathering, which breaks down rock without moving it. Various natural agents contribute to erosion.
Water is an agent of erosion. Raindrops dislodge soil particles, leading to splash erosion and sheet erosion (where a thin layer of soil is removed uniformly). Runoff carves small channels called rills, which deepen into larger gullies. Rivers and streams erode by dissolving minerals and carrying away particles, carving V-shaped valleys. Ocean waves erode coastlines through hydraulic pressure, impact, and abrasion.
Wind also sculpts the land, particularly in dry regions. It erodes through deflation, the lifting of loose, fine particles from the surface. Wind also causes abrasion, where windborne sand and dust abrade rock surfaces.
Glaciers, massive bodies of ice that move slowly, are erosional forces. They erode through plucking, where meltwater seeps into rock cracks, freezes, expands, and dislodges rock pieces. Glaciers also cause abrasion, as embedded rock fragments scrape and grind against the bedrock.
Gravity directly influences erosion by pulling soil and rock downslope in mass wasting events. Landslides, mudslides, and rockfalls are examples of rapid downhill movement. Slower movements like slump (sudden block movement) and creep (very slow movement) also occur. Gravity also indirectly drives other erosional agents by pulling water and ice downhill.
What is Deposition?
Deposition is the geological process where eroded sediments settle in a new location. This occurs when the transporting agent, such as wind, water, or ice, loses sufficient energy to carry its load. Larger, heavier particles are deposited first as energy decreases, while finer particles settle later.
Water is a primary agent of deposition, dropping its sediment load as its velocity decreases. Rivers deposit materials when they slow down, forming features like alluvial fans or deltas where a river empties into a larger body of water. Floodwaters deposit fine sediments across floodplains, enriching the soil and building natural levees. Along coastlines, waves deposit sand to form beaches as their energy lessens.
Wind deposition occurs when wind encounters an obstacle or its speed decreases, causing airborne particles to settle. This process forms sand dunes, which build up when wind-carried sand is blocked and accumulates. Fine silt and clay particles can also be deposited by wind, forming fertile loess deposits.
Glaciers deposit material as they melt and retreat. These deposits, known as glacial till, are unsorted mixtures of various particle sizes. Moraines, which are ridges of debris, are features formed by glacial deposition. Large rocks transported by glaciers are called glacial erratics.
How They Differ
Erosion and deposition represent opposing yet interconnected phases of Earth’s surface modification. Erosion is about the removal and transport of material. It involves forces that break down and carry away landforms. Deposition, in contrast, adds or accumulates material in a new location.
While erosion takes away, deposition builds up. Erosion involves agents with sufficient energy to move particles, such as fast-flowing water or strong winds. Conversely, deposition happens when these agents lose their energy. These processes are sequential, with erosion occurring first to transport material, followed by deposition when it settles.
Shaping Our Planet
The continuous interplay of erosion and deposition shapes the diverse features of our planet’s surface. They work over geological timescales to create and modify landscapes. The results are evident in numerous landforms.
Water erosion carves deep canyons as rivers cut through rock layers. Conversely, river sediment deposition forms fertile floodplains and deltas. The meandering curves of rivers and features like oxbow lakes also result from combined erosion on outer bends and deposition on inner bends.
Wind deposition forms sand dunes in deserts and coastal areas, where sand accumulates into mounds. These dunes vary in size and shape depending on wind direction and sand supply. Glacial erosion carves U-shaped valleys, distinct from V-shaped river valleys. Melting glaciers leave moraines, ridges of rock and sediment marking the former ice extent. Underground, groundwater erodes soluble rock to form caves, then deposits minerals to create stalactites and stalagmites.