Sedimentation

Sediments are loose Earth materials such as sand that accumulate on the land surface, in river and lakebeds, and on the ocean floor. Sediments form by weathering of rock. They then erode from the site of weathering and are transported by wind, water, ice, and mass wasting, all operating under the influence of gravity. Eventually sediment settles out and accumulates after transport; this process is known as deposition. Sedimentation is a general term for the processes of erosion, transport, and deposition. Sedimentology is the study of sediments and sedimentation.

There are three basic types of sediment: rock fragments, or clastic sediments; mineral deposits, or chemical sediments; and rock fragments and organic matter, or organic sediments. Dissolved minerals form by weathering rocks exposed at the earth's surface. Organic matter is derived from the decaying remains of plants and animals.

Clastic and chemical sediments form during weathering of bedrock or pre-existing sediment by both physical and chemical processes. Organic sediments are also produced by a combination of physical and chemical weathering. Physical (or mechanical) weathering—the disintegration of Earth materials—is generally caused by abrasion or fracturing, such as the striking of one pebble against another in a river or stream bed, or the cracking of a rock by expanding ice. Physical weathering produces clastic and organic sediment.

Chemical weathering, or the decay and dissolution of Earth materials, is caused by a variety of processes. However, it results primarily from various interactions between water and rock material. Chemical weathering may alter the mineral content of a rock by either adding or removing certain chemical components. Some mineral by-products of chemical weathering are dissolved by water and transported below ground or to an ocean or lake in solution. Later, these dissolved minerals may precipitate out, forming deposits on the roof of a cave (as stalactites), or the ocean floor. Chemical weathering produces clastic, chemical, and organic sediments.

Erosion and transport of sediments from the site of weathering are caused by one or more of the following agents: gravity, wind, water, or ice. When gravity acts alone to move a body of sediment or rock, this is known as mass wasting. When the forces of wind, water, or ice act to erode sediment, they always do so under the influence of gravity.

Large volumes of sediment, ranging in size from mud to boulders, can move downslope due to gravity, a process called mass wasting. Rock falls, landslides, and mudflows are common types of mass wasting. If you have ever seen large boulders on a roadway you have seen the results of a rock fall. Rock falls occur when rocks in a cliff face are loosened by weathering, break loose, and roll and bounce downslope. Landslides consist of rapid downslope movement of a mass of rock or soil, and require that little or no water be present. Mudflows occur when a hillside composed of fine-grained material becomes nearly saturated by heavy rainfall. The water helps lubricate the sediment, and a lobe of mud quickly moves downslope. Other types of mass wasting include slump, creep, and subsidence.

Water is the most effective agent of transport, even in the desert. When you think of water erosion, you probably think of erosion mainly by stream water, which is channelized. However, water also erodes when it flows over a lawn or down the street, in what is known as sheet flow. Even when water simply falls from the sky and hits the ground in droplets, it erodes the surface. The less vegetation that is present, the more water erodes - as droplets, in sheets, or as channelized flow.

Wind is an important agent of erosion only where little or no vegetation is present. For this reason, deserts are well known for their wind erosion. However, as mentioned above, even in the desert, infrequent, but powerful rainstorms are still the most important agent of erosion. This is because relatively few areas of the world have strong prevailing winds with little vegetation, and because wind can rarely move particles larger than sand or small pebbles.

Ice in glaciers is very effective at eroding and transporting material of all sizes. Glaciers can move boulders as large as a house hundreds of miles.

Generally, erosive agents remove sediments from the site of weathering in one of three ways: impact of the agent, abrasion (both types of mechanical erosion, or corrasion), or corrosion (chemical erosion). The mere impact of wind, water, and ice erodes sediments; for example, flowing water exerts a force on sediments causing them to be swept away. The eroded sediments may already be loose, or they may be torn away from the rock surface by the force of the water. If the flow is strong enough, clay, silt, sand, and even gravel, can be eroded in this way.

Sediments come in all shapes and sizes. Sediment sizes are classified by separating them into a number of groups, based on metric measurements, and naming them using common terms and size modifiers. The terms, in order of decreasing size, are boulder (>256 mm), cobble (256–64 mm), pebble (64–2 mm), sand (2-1/16 mm), silt (1/16–1/256 mm), and clay (<1/256 mm). The modifiers in decreasing size order, are very coarse, coarse, medium, fine, and very fine. For example, sand is sediment that ranges in size from 2 millimeters to 1/16 mm. Very coarse sand ranges from 2 mm to 1 mm; coarse from 1 mm to 1/2 mm; medium from 1/2 mm to 1/4 mm; fine from 1/4 mm to 1/8 mm; and very fine from 1/8 mm to 1/16 mm. Unfortunately, the entire classification is not as consistent as the terminology for sand - not every group includes size modifiers.

When particles are eroded and transported by wind, water, or ice, they become part of the transport medium's sediment load. There are three categories of load that may be transported by an erosion agent: dissolved load, suspended load, and bedload. Wind is not capable of dissolving minerals, and so it does not transport any dissolved load. The dissolved load in water and ice is not visible; to be deposited, it must be chemically precipitated.

Sediment can be suspended in wind, water, or ice. Suspended sediment is what makes stream water look dirty after a rainstorm and what makes a windstorm dusty. Suspended sediment is sediment that is not continuously in contact with the underlying surface (a stream bed or the desert floor) and so is suspended within the medium of transport. Generally, the smallest particles of sediment are likely to be suspended; occasionally sand is suspended by powerful winds and pebbles are suspended by floodwaters. However, because ice is a solid, virtually any size sediment can be part of the suspended sediment load of a glacier.

Bedload consists of the larger sediment that is only sporadically transported. Bedload remains in almost continuous contact with the bottom, and moves by rolling, skipping, or sliding along the bottom. Pebbles on a riverbed or beach are examples of bedload. Wind, water, and ice can all transport bedload, however, the size of sediment in the bedload varies greatly among these three transport agents.

Because of the low density of air, wind only rarely moves bedload coarser than fine sand. Some streams transport pebbles and coarser sediment only during floods, while other streams may, on a daily basis, transport all but boulders with ease.

Floodwater greatly increases the power of streams. For example, many streams can move boulders during flooding. Flooding also may cause large sections of a riverbank to be washed into the water and become part of its load. Bank erosion during flood events by a combination of abrasion, hydraulic impact, and mass wasting is often a significant source of a stream's load. Ice in glaciers, because it is a solid, can transport virtually any size material if the ice is sufficiently thick and the slope is steep.

For a particular agent of transport, its ability to move coarse sediments as either bedload or suspended load is dependant on its velocity. The higher the velocity, the coarser the load.

Transport of sediments causes them to become rounder as their irregular edges are removed by both abrasion and corrosion. Beach sand becomes highly rounded due to its endless rolling and bouncing in the surf. Of the agents of transport, wind is most effective at mechanically rounding (abrading) clastic sediments, or clasts. Its low density does not provide much of a "cushion" between the grains as they strike one another.

Sorting, or separation of clasts into similar sizes, also happens during sediment transport. Sorting occurs because the size of the grains that a medium of transport can move is limited by the medium's velocity and density. For example, in a stream on a particular day, water flow may only be strong enough to transport grains that are finer than medium-grained sand. So all clasts on the surface of the streambed that are equal to or larger than medium sand will be left behind. The sediment, therefore, becomes sorted. The easiest place to recognize this phenomenon is at the beach. Beach sand is very well sorted because coarser grains are only rarely transported up the beach face by the approaching waves, and finer material is suspended and carried away by the surf.

Ice is the poorest sorter of sediment. Glaciers can transport almost any size sediment easily, and when ice flow slows down or stops the sediment is not deposited due to the density of the ice. As a result, sediments deposited directly by ice when it melts are usually very poorly sorted. Significant sorting only occurs in glacial sediments that are subsequently transported by meltwater from the glacier. Wind, on the other hand, is the best sorter of sediment because it can usually only transport sediment that ranges in size from sand to clay. Occasional variation in wind speed during transport serves to further sort out these sediment sizes.

When the velocity (force) of the transport medium is insufficient to move a clastic (or organic) sediment particle it is deposited. When velocity decreases in wind or water, larger sediments are deposited first. Sediments that were part of the suspended load will drop out and become part of the bedload. If velocity continues to drop, nearly all bedload movement will cease, and only clay and the finest silt will be left suspended. In still water, even the clay will be deposited, over the next day or so, based on size—from largest clay particles to the smallest.

During its trip from outcrop to ocean, a typical sediment grain may be deposited, temporarily, thousands of times. However, when the transport medium's velocity increases again, these deposits will again be eroded and transported. Surprisingly, when compacted fine-grained clay deposits are subjected to stream erosion, they are nearly as difficult to erode as pebbles and boulders. Because the tiny clay particles are electrostatically attracted to one another, they resist erosion as well as much coarser grains. This is significant, for example, when comparing the erodibility of stream bank materials—clay soils in a river bank are fairly resistant to erosion, whereas sandy soils are not.

Eventually the sediment will reach a final resting place where it remains long enough to be buried by other sediments. This is known as the sediment's depositional environment.

Unlike clastic and organic sediment, chemical sediment cannot simply be deposited by a decrease in water velocity. Chemical sediment must crystallize from the solution; that is, it must be precipitated. A common way for precipitation to occur is by evaporation. As water evaporates from the surface, if it is not replaced by water from another source (rainfall or a stream) any dissolved minerals in the water will become more concentrated until they begin to precipitate out of the water and accumulate on the bottom. This often occurs in the desert in what are known as saltpans or lakes. It may also occur along the sea coast in a salt marsh.

Another mechanism that triggers mineral precipitation is a change in water temperature. When ocean waters with different temperatures mix, the end result may be seawater in which the concentration of dissolved minerals is higher than can be held in solution at that water temperature, and minerals will precipitate. For most minerals, their tendency to precipitate increases with decreasing water temperature. However, for some minerals, calcite (calcium carbonate) for example, the reverse is true.

Minerals may also be forced to precipitate by the biological activity of certain organisms. For example, when algae remove carbon dioxide from water the acidity of the water decreases, promoting the precipitation of calcite. Some marine organisms use this reaction, or similar chemical reactions, to promote mineral precipitation and use the minerals to form their skeletons. Clams, snails, hard corals, sea urchins, and a large variety of other marine organisms form their exoskeletons by manipulating water chemistry in this way.

Landscapes form and constantly change due to weathering and sedimentation. The area where sediment accumulates and is later buried by other sediments is known as its depositional environment. There are many large-scale, or regional, environments of deposition, as well as hundreds of smaller subenvironments within these regions. For example, rivers are regional depositional environments. Some span distances of hundreds of miles and contain a large number of subenvironments, such as channels, backswamps, floodplains, abandoned channels, and sand bars. These depositional subenvironments can also be thought of as depositional landforms, that is, land-forms produced by deposition rather than erosion.

Erosion, weathering, and sedimentation constantly work together to reshape the earth's surface. These are natural processes that sometimes require us to adapt and adjust to changes in our environment. However, too many people and too much disturbance of the land surface can drastically increase sedimentation rates, leading to significant increases in the frequency and severity of certain natural disasters. For example, disturbance by construction and related land development is sometimes a contributing factor in the mudflows and landslides that occur in certain areas of California. The resulting damage can be costly both in terms of money and lives.

The world's rivers carry as much as 24 million tons of sediment to the ocean each year. About two-thirds of this may be directly related to human activity, which greatly accelerates the natural rate of erosion. This causes rapid loss of fertile topsoil, which leads to decreased crop productivity.

Increased sedimentation also causes increased size and frequency of flooding. As stream channels are filled in, the capacity of the channel decreases. As a result, streams flood more rapidly during a rainstorm, as well as more often, and they drain less quickly after flooding. Likewise, sedimentation can become a major problem on dammed rivers. Sediment accumulates in the lake created by the dam rather than moving farther downstream and accumulating in a delta. Over time, trapped sediment reduces the size of the lake and the useful life of the dam. In areas that are forested, lakes formed by dams are not as susceptible to this problem. Sedimentation is not as great due to interception of rainfall by the trees and underbrush.

Vegetative cover also prevents soil from washing into streams by holding the soil in place. Without vegetation, erosion rates can increase significantly. Human activity that disturbs the natural landscape and increases sediment loads to streams also disturbs aquatic ecosystems. Many state and local governments are now developing regulations concerning erosion and sedimentation resulting from private and commercial development.