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What is an eroded sediment?



Sediments are deposits formed through the erosion of rocks. Eroded materials are moved away from their 'parent material' by wind, water, ice or gravity and deposited as alluvium. The shape of the landscape affects the rate of erosion - so streams on steep land will erode faster than streams on low lying ground.

There is a general cycle of deposition and erosion over very long periods of time. Eroded sediments collect, perhaps in the sea bed, and then over time harden to form 'sedimentary rocks' (such as Limestone). These rocks may then start to become eroded again as the cycle recommences. Each time the cycle takes place, the characteristics of the sediment will change as the particle size become smaller and smaller.

The process of the breaking down of the rock is called weathering. There are several different forms of weathering.

Physical weathering: is a mechanical grinding process which may occur for instance where water flows into cracks, freezes and expands. Ice expansion like this can exert a surprising 1tonne/2.5cm2 force! This is termed 'ice shatter' and is common in temperate climates such as Britain. In warmer regions there can be equivalent processes at work. For instance the growth of salt crystals in rock cracks can prise rock fragments apart. Also the continued heating and cooling of rocks can break rock down due to related rock expansion and contraction. Naturally rocks are heavy!! So if a layer of rock is eroded away, the rocks beneath may expand as they are no longer being pushed down. This can lead to cracks in the rock appearing (quite explosively actually!)

Chemical weathering: is the other form - strongly affected by water containing dissolved chemicals which can attack rocks; chemicals like acides, chlorides, dissolved minerals and gases. These substances come from the atmoshphere, from leaching and from atmospheric pollution (like car exhaust), not to mention volcanos!

One important form of chemical weathering is oxidation. Water containing oxygen can form 'oxides' such as iron oxide, if the metal ion is present. Oxides can take on striking colours - for instance limenites are browny, haematites are red and goethites are yellow. Oxides become very stable (chemically) and the process is not easily reversible, although some actions such as those of bacteria can reverse the process.

Carbonation is the process whereby water contains carbonic acid (H2SO3). This acid attacks limetone. This effect is noteable on limestone buildings which have stood for hundreds and hundreds of years and have become heavily eroded only in the past 50 or so years as cars became common. An example is the ancient roman theatre in Amman, the capital of Jordan.

Vegetation: As well as climate, the effect of vegetation is very important in the process of erosion. Plants can bind sediments in their dense roots preventing erosion. In arid conditions where there isn't much vegetation erosion, for instance by wind, can be much worse than in temperate conditions where there is planty of vegetation. Once vegetation takes hold of a soil, ecological 'succession' takes place where the first grasses are soon joined by low shrubs and then tree saplings and eventually forest. You can therefore imagine that deforestation, for whatever reason (forest fire, or land clearance) can increase the probability of erosion taking place.

Water: Water is key to the transport of weathered rocks. Eroded grains are carried along rivers and deposited as sediment. Sediment particles look small, and they are, but they still have a mass (they have a weight). So for water to carry the particles, the water has to have a source of energy - this is given due to gravity. The 'kinetic' energy of water flowing fast down a steep slope can carry all particles, big and small. But as the river flows to the lowlands, the available energy decreases, and the river becomes able to carry only smaller and smaller particles. Particles too big are dropped to the river bed. The river therefore can 'sort' particles out by their size with the smallest ones being carried the furthest. Where rivers meet the sea there are estuaries, and estuaries are full of mud - very very fine particles that have been carried great distances. These are dropped as sediments.