Skip to Content

What is it called when sediment is dropped and comes to rest a erosion weathering deposition compaction?

The process by which sediment is dropped and comes to rest is called deposition. Deposition occurs when sediment that has been transported by wind, water, ice, or gravity comes to settle on the Earth’s surface. This often occurs when the transporting agent loses energy and can no longer carry the sediment load. The sediment then drops out of the water column or air and accumulates. Deposition leads to the accumulation and building up of layers of sediment over time. It is a key process in the formation of sedimentary rocks.

Sediment transport and deposition are fundamental processes that shape the Earth’s surface. Sediments like sand, silt, and clay are eroded from rocks and soils and transported by agents like rivers, ocean currents, wind, and glaciers. When the transporting agent loses energy, the sediment load drops out in a process called deposition. Over long time periods, episodes of repeated deposition lead to the accumulation of sediment in layers. These sedimentary layers record information about past environments and are also important reservoirs of water, oil, gas, metals, and minerals.

Four main processes affect sediment during transport and deposition:

  • Erosion – The removal and entrainment of sediment
  • Transportation – The movement of sediment by wind, water, ice, or gravity
  • Deposition – The dropping and accumulation of sediment
  • Compaction – The pressing down and hardening of sediment once deposited

Of these four mechanisms, deposition is the process that leads to settled sediment accumulating on the Earth’s surface. The end result of deposition over long periods of time is the formation of sedimentary rock layers.

What Controls Deposition?

Several factors control when and where deposition occurs:

  • Energy levels – Transporting agents like rivers and wind must expend energy to keep sediment entrained and moving. When energy levels drop, such as when a river enters a lake or ocean, deposition occurs.
  • Sediment load – Heavier sediment loads are more difficult for transporting agents to carry and get deposited sooner.
  • Sediment grain size – Larger grain sizes like sand require more energy to transport and deposit first.
  • Obstacles and obstructions – Any obstacle or barrier the reduces the velocity of the transporting agent will encourage deposition.
  • Topography – Deposition tends to occur in topographic lows and depressions where transporting energies decrease.

In a river system, for example, deposition tends to occur when the river enters a lake or ocean, where the lack of motion allows sediment to settle out. Heavier sediments like sand and pebbles drop out first, closer to the river mouth, while silts and clays may travel farther out into deeper, calmer waters before settling.

Types of Deposition

Deposition can occur in different environments depending on the transporting medium:

Fluvial Deposition

Fluvial deposition occurs in river systems. As river currents slow down, bedload sediments like gravel, sand, and silt are deposited first, building up channel, point bar, and floodplain deposits. When rivers flood, fine silts and clays may be deposited across floodplains. Deltas form where rivers enter lakes and oceans and drop their sediment loads.

Glacial Deposition

Glaciers carry large amounts of sediment scraped up as they move. This debris is dropped when the ice melts, leaving deposits like moraines, eskers, drumlins, and outwash plains.

Aeolian Deposition

Wind can carry large amounts of fine silt and sand-sized sediment. Deposition occurs when wind speeds decrease, such as when blowing sand hits an obstacle. Aeolian deposits include sand dunes, loess deposits, and sand sheets.

Lacustrine Deposition

In lake settings, bottom sediments like mud and organic ooze accumulate from settling particles. Near shore lake sediments are coarser, while fine silts accumulate in deeper waters.

Marine Deposition

Ocean deposition results in accumulations like beach sand, submarine fans, and abyssal plain clays. Heavier sand settles out closer to shore, while fine clays deposit far offshore.

Volcanic Deposition

Volcanic eruptions eject tephra and ash particles that settle through the air and accumulate in layers downwind of volcanoes.

Depositional Environments

Characteristic depositional environments form in different geographic settings. These include:

  • Alluvial fans – Formed where streams exit canyons out into a valley, dropping coarse sediment loads.
  • Deltas – Formed at a river’s mouth, consist of overlapping layers of sand, silt, and clay.
  • Point bars – Sand and gravel deposits that accumulate on the inside bends of meandering stream channels.
  • Beaches – Wave-deposited sand and gravel along coastlines.
  • Dunes – Mounds of wind-blown sand that accumulate around obstacles and topographic depressions.
  • Loess – Windblown deposits of silt that accumulate downwind of deserts and glacial outwash plains.
  • Deep sea fans – Fan-shaped submarine deposits of turbidity currents carrying sediment into the ocean.

Sedimentary Structures

The process of deposition under different conditions leads to distinct sedimentary structures preserved in the geologic record. These include:

  • Cross-bedding – Inclined layers indicating deposition by flowing water.
  • Graded bedding – Layers that transition from coarse to fine upwards, reflecting deposition from decelerating flows.
  • Ripple marks – Small, ridge-like beds formed by wind or water.
  • Mudcracks – Cracks from the dehydration and shrinking of muddy sediment.
  • Raindrop impressions – Small indentations in deposited sediment from falling raindrops.

These sedimentary structures provide clues about past depositional environments and the direction of sediment transport.

Depositional Basins

Deposition occurs within sedimentary basins – large-scale structural depressions in the Earth’s crust where sediment accumulates over time. Important types of continental and marine sedimentary basins include:

Continental Basins

  • Foreland basins – Formed adjacent to rising mountain chains where flexural subsidence occurs.
  • Rift basins – Formed by extension and crustal thinning, often accumulate thick sequences of terrestrial sediments.
  • Cratonic basins – Broad shallow basins located on stable continental interiors.

Marine Basins

  • Passive margins – Trailing edge margins of continents where subsidence occurs seaward.
  • Active margins – Convergent and transform plate boundaries with high rates of sediment input.
  • Cratonic basins – Broad shallow basins located on stable continental interiors.

These large sedimentary basins preserve sequences of layered strata reflecting their depositional history over geological timescales.

The Rock Record

Over long time periods, episodes of repeated deposition lead to the accumulation of sediment in layers. Compaction and cementation gradually transform these sediment layers into sedimentary rock strata. Important sedimentary rock types formed by deposition include:

  • Shale – Fine clay and silt-sized particles that settled slowly from suspension.
  • Sandstone – Sand-sized grains cemented together.
  • Conglomerate – Rounded pebble and gravel cemented together.
  • Limestone – Precipitated calcium carbonate ooze and skeletal fragments.
  • Coal – Accumulated and buried organic matter.

The sedimentary rock record provides an archive of Earth’s past depositional environments andclimate history. Sedimentary structures and sequences are studied to reconstruct ancient landscapes, paleoenvironments, and habitat conditions.

Fossil Preservation

Sediment deposition also plays a key role in the preservation of fossils. Rapid burial in accumulating sediment protects organic remains from decay and DAMAGE. Mineral-rich waters permeating the sediments precipitate minerals that petrify and preserve bone and shell. As a result, sedimentary strata host a rich record of past life embedded and entombed within the layers. Fossils provide critical insights into evolutionary patterns and environmental conditions over geological history.

Economic Deposits

Depositional environments concentrate valuable mineral resources and fossil fuels. Examples include:

  • Placer gold, tin, tungsten in stream channel and beach sand deposits.
  • Iron ore in banded iron formations precipitated from seawater.
  • Uranium in sandstones that adsorb uranium from oxygenated groundwater.
  • Coal and petroleum in terrestrial swamps and marine sediments, respectively.
  • Potash evaporites deposited in restricted marine basins.

Understanding the geological context of depositional environments guides exploration for these important economic resources.

Conclusion

Deposition is a fundamental geological process that leads to the accumulation of sediments and shaping of the Earth’s surface topography over long timescales. As sediment transporting agents like rivers and glaciers lose energy, they deposit their sediment loads in characteristic environments shaped by the interacting factors of topography, sediment supply, grain size, and transport dynamics. Repeated episodes of deposition stack up layers of sediment that lithify into sedimentary rocks. These sedimentary layers and structures record past depositional environments and climate conditions. Deposition also plays a vital role in fossil preservation and the concentration of mineral resources. Studying patterns of sediment transport and deposition provides key insights into Earth’s environmental history and guides resource exploration.

Depositional Agent Example Deposits
Rivers Alluvial fans, deltas, floodplains
Glaciers Moraines, drumlins, outwash plains
Wind Dunes, loess deposits
Lakes Lacustrine silts and clays
Oceans Beaches, submarine fans
Volcanoes Ash and tephra deposits
Sedimentary Rock Depositional Environment
Shale Low-energy lakes and marine basins
Sandstone River channels, beaches, dunes
Conglomerate High-energy alluvial fans and braided rivers
Limestone Warm, shallow marine environments
Coal Swamp and mire environments