Deposition

Erosion-Deposition Deposition is the final step in an erosional-depositional system. Rock particles that are picked up by an erosional agent will be deposited somewhere else. Agents of erosion   agents of deposition When the carrying power of a transporting agent decreases, some of the particles carried by the agent will be dropped and deposited. This process is called sedimentation.

Sedimentation Sediments are composed of rock fragments of all sizes. Running water is the principal eroding agent. Most sedimentation happens in deep lakes, seas, or oceans. Streams and rivers slow down and lose their carrying power.

Precipitation Dissolved ionic materials are released by the process of precipitation. Precipitation here refers to the process by which dissolved minerals precipitate, or fall out of solution, when liquid water evaporates. Evaporation usually requires quiet water. Precipitation produces salt (halite), gypsum, and other soluble rock deposits (limestone, dolostone).

When wind, waves, glaciers, and gravity lose their carrying power, sediments are deposited on the surface of the land.

Factors Affecting Deposition Particle size Particle shape Particle density Velocity of the transporting agent

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Particle Size The settling rate of particles is determined by the particle size. Larger heavier particles settle more quickly than smaller lighter particles. When the carrying power of the transporting agent diminishes, heavy particles settle out. In running water, very small particles remain in suspension for long periods of time. Colloidal particles (<10-4mm diameter) can remain suspended indefinitely. Colloids often end up deposited in the ocean.

Graded Bedding When a mixture of sediment sizes settles in a quiet medium (still water or air) sorting into layers takes place. The largest rocks are found at the bottom and finest at the top. Most sediments deposited in water form horizontal layers called beds. Graded bedding shows vertical sorting within the bed.

Repeated depositions show a series of beds.

Cross Bedding Winds and fast-moving rivers produce cross-bedding. Cross bedding is the result of sediment being deposited at an angle to the horizontal. This occurs because of sudden decreases in the velocity and carrying power of the medium.

Particle Shape If all other factors are equal, The shape of a particle may determine its settling rate Smooth, round particles settle more quickly than angular, flatter particles. Rounder particles have less friction with the water than flatter particles.

Particle Density The density of particles also influences the rates at which sediments settle out of running water and wind. If particles are the same size and shape, higher density particles will settle faster than lower density particles. The particles will form layers of rocks at different densities, with the densest on the bottom.

Gold Density 19.3 g/cc

Velocity of Transporting Medium The velocity of the transporting medium determines when the sediments will be released and sedimentation can occur. As the velocity of the sediment-laden flow decreases, there is a loss of carrying power, and the larger, heavier, denser particles settle out first.

Horizontal Sorting As a stream or river enters a large body of water, its velocity decreases as the distance from its mouth increases. This produces horizontal sorting. Larger, denser sediments settle out first, smaller less dense particles are carried further.

Most precipitation of dissolved sediments occurs far from shore When precipitation occurs near the shore, it provides the cement for the formation of sedimentary rocks.

Particle Velocity The velocity of particles carried by a transporting medium is not the same as the velocity of the fluid (air or water). Particles in solution move at the same velocity as the stream, as do colloids suspended in water. Most particles more slowly than the stream – the largest and the densest are the slowest. Rocks rolling or bouncing along the bottom move much slower than the stream.

Glacial Deposition Sorting in a quiet, solid medium, like ice, is more complicated than in a fluid medium. Glacial deposits of gravel, boulders, and sand are unsorted (all mixed together) – no sorting or bedding. There are two types of bedding: till and outwash.

Glacial Till

Outwash Plain Outwash plains contain material deposited by glacial meltwater. Outwash usually shows horizontal sorting.

Erosional-Depositional Systems A river is a combination of continuous erosion and deposition. Both processes occur throughout its entire length. An erosional-depositional system combines the erosional process, the transporting agents, and the process of deposition. All of the agents of erosion – running water, wind, waves, and ice – produce erosional-depositional systems.

Energy Relationships in E-R Systems Energy transformations between potential and kinetic energy occur in E-R systems. A river has its greatest potential energy at its source, where the river begins. As a river flows towards its mouth, where it ends, the potential energy decreases, being continuously transformed into kinetic energy.

Erosion occurs during the PE to KE transformation, transporting rocks and sediments downstream. Whenever there is a loss of kinetic energy, rocks and sediments carried by the river are deposited. Some kinetic energy is lost to friction between the flowing water and the channel walls. When the slope is the steepest, the stream has the greatest velocity and greatest KE. As the slope becomes less steep, the velocity and KE decrease. When the river reaches base level (usually ocean) the potential energy of the river is zero. In all erosional-depositional systems, the total energy within the system is decreasing.

Dominant Processes in E-D Systems Either erosion or deposition can predominate at any point in an E-D system. The gradient may determine which is the dominant process. Erosion is usually dominant where there is a steep slope and the river is fast. When the slope becomes gentile, the depositional process is dominant.

Meanders At the source of a river and most of its length, erosion>>deposition. Deposition>>erosion at the mouth of a river and on the inside of curves (meanders).

The velocity of a river is greater on the outside of a curve than on the inside. The erosional process dominates on the outside of a meander The depositional process is dominant at the inside of a meander – the water is flowing the slowest.

Life History of a Stream A stream passes through a series of stages, each having certain characteristics.

Youthful Stage Youthful stage has poor drainage and is characterized by lakes, waterfalls, and rapids. Stream flow is rapid and down cutting >> side cutting. V-shaped valleys are a common characteristic of young rivers.

Mature Stage Eventually slopes become less steep because they are eroded away. There is less erosion and more deposition in mature rivers. A flood plain forms between the stream and the steep walls of the valley. Little or no rapids, waterfalls left, smooth gradient

Old Rivers As flood plain widens, meandering becomes more common. During flood stage, meanders can be cut off, creating oxbow lakes. When the valley floor becomes wider than the river meanders can fill, the stream has reached old age.

Natural Levees Natural levees are broad low ridges that form during flooding. Flood waters slow down and drop sediment when overflowing their banks.

Streams getting “younger” If the gradient of a stream is steepened, the stream takes on some younger characteristics. The stream increases in velocity and cuts downward more than sideways.

Erosional-Depositional Interfaces Interfaces (boundaries between erosion and deposition can occur throughout the length of a stream. They are common at the mouth of a stream They are common when streams change direction They are common when there is a change in slope

Dynamic Equilibrium of a Stream Whenever there is a balance between erosion and deposition… Rate of erosion = rate of deposition Dynamic equilibrium can be found near the mouth of a river and near the middle of curves in the river channel.

Erosional-Depositional Changes The erosional and depositional processes produce characteristic observable changes on the surface of the land. Two major forms of deposition that owe their existence to stream flow are the delta and the alluvial fan.

Deltas Deltas are deposits of sediment formed at the mouth of a river. The river loses its carrying power as it enters a quiet body of water.

Alluvial Fan Alluvial fans are deposits of sediment formed where the velocity of a river slows as it flows out of mountains onto flat land.

How do we know water once flowed on Mars?

Lab 35: Models of Erosional-Depositional Systems Objective: you will analyze patterns of erosion and deposition using a model of a stream table.  

Relation of Slope to Stream Formation Prepare the stream table by placing the sand-soil mix at the closed end and the drain hose end open as a lake basin. Leave about 18 inches free for the lake. The drain hose should be placed in a bucket to collect water. Draw a picture below of your initial design. Start with the table flat. Let the water run for one minute at a medium flow. Draw what you see below. Raise the stream table using textbooks. More textbooks will result in a deeper stream and move more sand. Draw what you see below.

 initial    flat  raised

Freestyling Landscapes Time to mess around: rebuild your landscape, drawing it in the box below. Then try one or more of the following: a) add several small rocks to the stream flow; note their effects b) put in several sweeping curves in your stream bed c) place a solid object across the stream to form a dam; not any changes in stream patterns d) move the water source to one side and establish a new source. Not how the stream channel adjusts to this change. e) to observe patterns of deposition, add different colored sands to the source. Observe the pattern of downstream particle movement and deposition in the lake. Turn off the water supply. Cut a vertical section through the delta to reveal any pattern in the deposition. f) draw a picture of your modified stream table, including obstacles to stream flow.

before   after