Turning Rocks into Sediment

When you were in lower school science you studied the rock cycle. This section looks at the weathering and erosion of rocks - part of that cycle - in rather greater detail. Firstly you have to know about the pace; most processes are gradual (which means they are slow) - weathering and erosion normally take millions of years, but others such as meteorite impacts are catastrophic - they make major changes in seconds.

What the Syllabus says - and the Details

Weathering

Rock surfaces are changed by weathering processes.

Weathering is the breakdown of rocks in situ (that means where it started off) by physical, chemical and biological processes including: -

• the physical weathering of rocks by freeze/thaw producing angular rock fragments

Click this link for a quick explanatory video or

Click this link for a longer explanatory video.

On Mountains debris accumulates at the foot of slopes due to gravity. This material is called scree and is caused by the process of freeze thaw. The picture below shows scree that has accumulated beneath a cliff in this way.

• the chemical weathering of granite to produce sand, clay and soluble material

Click Link 3 for an explanatory video

• the chemical weathering of limestone by the action of weak acids producing

soluble materials. This steadily corrodes (dissolves away) the limestone along lines of weakness such as joints or bedding planes - see picture of Malham Cove inYorkshire below.

• biological weathering by plant roots which break up rocks and organic acids which dissolve them.

Erosion involves the transportation of solid weathering products by water, wind, ice or gravity. Water, ice and wind erode rocks by processes called abrasion (rocks being ground down by particles rubbing against them and attrition (grains being bashed together and broken - as shown in this video).


Rivers transport  sediment by the processes of; traction, saltation, suspension and solution.

Rivers also deposit sediment when they no longer have enough energy to carry it along. The smaller the sediment grains, the further the water can carry it before it is deposited - as shown in the diagram below.

Rock is broken down by weathering and erosion into particles of various sizes and

dissolved materials that can be incorporated into new sediments.

Using Sediment Size and Shape to Work out How it Was Transported
The grain size is useful...
The greater the transport distance the more rounded the grains tend to be.
Grain shape is useful...
Spherical grains have probably been rolled along, that suggests they arrived by river or wind.
Flat shaped grains have probably been slid along, that suggests they came by glacier.
Rounded grains (ones with all the corners knocked off) can only develop if the grains have been transported for a long time.
We can work out the energy of the environment of deposition from sediment grain shape - less angular material suggests greater energy in the environment as the grains must have been being moved around more to get their corners knocked off.
Graded bedding suggests a still water depositional area (such as a lake) being fed by a river that carried water from large storms.


Using Sediment Size and Shape to Work out the Environment of Deposition
The grain sorting is useful - smaller grains get carried further into lakes or the sea by the rivers that flow into them. If the grains are large, they were probably deposited near to the land, if they're small then the deposition could be taking place a distance off-shore in deep seas.


Using Sediment bedding to interpret the Environment of Deposition
Thick accumulations of sediment normally suggest that deposition occurred for a long time.
Thick beds of sediment suggest the arrival of sediment continued without many breaks.
Thinly bedded (laminated) sediment suggests that breaks in deposition were frequent - possibly happening in spring and autumn but not winter and summer.