Surface Processes

Rocks generally form at depth under conditions of high temperatures and pressures. The minerals which comprise the rocks are stable under these physical conditions.

Geological changes may raise the rocks towards the surface of the Earth where they experience quite different physical conditions. The minerals may be less stable under these pressures and temperatures. This instability may cause the minerals to start to break down at the Earth’s surface. The processes which cause the minerals to break down are collectively called weathering. The minerals break down in situ (the products do not move, they stay at the point of formation.)

Exercise 1

Make a list of the differences between the conditions experienced by rocks at the surface and those at depth. (See Geoscience page 79)

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Minerals which formed under conditions most like those at the surface are the most stable minerals at the surface.

Weathering

Weathering processes can be divided into two (or three) main groups.

Chemical weathering

Chemical weathering processes are those which cause the rocks and minerals to change by chemical reactions which form more stable substances.

Exercise 2

Write a sentence to describe each of the following weathering processes. (See Geoscience page 80).

Solution

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Carbonation

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Hydrolysis

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Hydration

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Oxidation

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Ion exchange

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Chelation

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Physical weathering

Physical weathering processes do not change the minerals chemically but they break the mineral grains apart from neighbouring mineral grains.

Exercise 3

Write a sentence to describe each of the following weathering processes.

Stress release

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Freeze-thaw

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Insolation

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Crystallisation of new minerals

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Biological weathering

Biological weathering is sometimes used as a separate group of processes but many of the effects of plants and animals on rocks can be classified as physical or chemical.

Exercise 4

Write a short paragraph to describe how animals and plants can contribute to weathering.

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Weathering produces solutions and mineral grains called clasts.

Weathering and climate

The rate of weathering depends on the climate – temperature, rainfall etc. Chemical weathering is most important in hot, wet climates. In cold climates the most important weathering processes are physical and in hot, dry climates there is very little weathering of any sort.

Exercise 5

Annotate the following graph which shows the relationship between weathering and climate. (See Geoscience page 81).

Erosion, transportation and deposition

Erosion, like weathering is also the breakdown of rocks and minerals but there is an important distinction: erosion involves movement. This movement is caused by some sort of agent, the main examples of agents of erosion are the wind, the sea, streams, rivers and glaciers.

Mass movement

The simplest form of movement does not require an agent – mass movement is the transport of grains under the influence of gravity. Weathering produces a clastic sediment, the mass of which may cause it to start moving downslope. This can happen quickly as in a rockfall or slump or avalanche or it can happen slowly as in creep or solifluction.

The action of rivers and streams (Fluvial processes)

Erosion by moving water happens in two ways. Firstly the movement of water across the bed has an effect, this is called hydraulic action. Secondly the sediment being transported in the river wears away the bed (this is abrasion) and the fragments themselves are ground down becoming smaller and more rounded (this is attrition).

The sediment is transported as either bedload – the coarse fragments which move close to the bed and the suspended load – finer fragments carried in the water. There is also a component carried as dissolved material.

Exercise 6

Draw a diagram to show the ways in which streams and rivers transport sediment. (See Geoscience page 84).

For each grain size there is a specific velocity at which the grains start to move, this is the entrainment velocity. This initial movement is erosion of the bed, the grains are broken away from the surrounding sediment. Experiments show that they will continue to be transported even if the velocity falls below the entrainment velocity. Eventually the velocity will fall low enough for the grains to be deposited.

Exercise 7

Annotate the graph to show how the grainsize and flow velocity control transport, erosion and deposition. This is called the Hjulstrom curve.

As the river current slows, the sediment can no longer be transported, so deposition occurs. A river channel has regions where erosion occurs and where deposition occurs.

Exercise 8

Draw a diagram to show the way in which river channels change their course. (See McLeish pages 79 & 80).

The action of the wind (Aeolian processes)

Moving air transports sediment in the same way that water currents do but because air is much less dense, it is only able to transport very fine grains. Clay or silt sized clasts can be kept in suspension by turbulent air currents and this wind blown dust can be transported over huge distances, eventually being deposited as loess. Sand sized fragments can roll, bounce or slide across the surface of the sediment.

If there are small depressions in the surface of the sediment, the wind will erode these to form deflation hollows. These stop at the water table because water between the grains provides cohesion preventing the grains from being moved by the wind. Another feature of Aeolian environments is a desert pavement. The wind selectively removes the finer fragments leaving behind larger clasts.

The wind has a “sand-blasting” erosive effect. The sand grains it carries are thrown against other rocks at very high speeds causing them to be eroded. The constant bombardment rounds the sand grains producing a very uniform texture sometimes called “millet seed” sand.

The action of the sea (marine processes)

Waves are the most important way in which the sea affects rocks. Waves are caused by the wind blowing across the surface of the sea so their height and power depends on the wind speed, the length of time the wind is blowing and the distance of open sea that the wind blows across (the fetch). When the waves break on the beach the water flows up the beach (swash) and then back down (the backwash).

Exercise 9

Draw a diagram to show the way in which waves break over a beach. (See McLeish page 82).

Waves only affect the upper part of the sea. Their effects die out rapidly with depth and the depth below which they have no effect is called wavebase.

Exercise 10

Use page 83 of McLeish to write a short account of how the sea erodes rocks at the coast.

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Waves normally strike the beach at an angle, this causes the sediment to move along the beach in a process called longshore drift.

As in rivers and streams, deposition in the sea is dependent upon the energy of the water. Generally the high energy environment of the beach shows coarse pebbles and sand. Further out onto the continental shelf, finer sands and silts give way to mud. In the deep sea, turbidity currents may deposit pebbles, sands and mud onto the abyssal plain. Between these high energy events, a constant “rain” of the skeletons of plankton build up to form deep sea sediments called oozes. Planktonic remains dissolve in the deepest water so the only sediments to accumulate are red clays, formed from dust from the atmosphere which settled onto the surface of the ocean.

The action of glaciers (glacial processes).

Some parts of the world are cold enough to be covered by sheets of ice, for example Greenland and Antarctica. Other regions have smaller features called glaciers which act as giant rivers flowing down valleys from the mountains. The rate at which claciers flow varies. They are affected by the presence of meltwater at the base which lubricates them. The meltwater can refreeze around rocks and boulders causing them to be pulled up from the ground and carried along in the base of the glacier. This process is called plucking. The captured boulders grind away at the rocks beneath producing a very fine dust called a rock flour. Rock fragments which fall onto the glacier from the mountain slopes above are transported as a moraine. This can either be in the middle of the glacier (medial), at the sides (lateral) or at the end (terminal).

When the glacial deposits material (usually as a result of climate change) the sediment left behind is highly immature – a wide range of sediment sizes from the finest rock flour to coarse boulders called erratics. This unsorted material is called till or boulder clay.

Recognising sedimentary environments

The geologist has to use the information presented in ancient sediments to work out the environment in which the rock forms. The principle we use is called Uniformitarianism. This means the “the present is the key to the past”, it is the fundamental assumption in all geology – that the processes which create and destroy rocks today are the same as those which operated in the past. So if we find sediments and structures forming in certain environments today, and we also find them in ancient rocks, we can say what environment they formed in.

Sedimentary structures

Probably the most useful features for interpreting ancient environments are sedimentary structures. These are features which form in the sediment during transport and deposition.

The most obvious sedimentary structures are beds. These are packets of sediment separated from one another by bedding planes. They form due to varying energy conditions in lots of different depositional environments. A similar effect produces laminations.

Exercise 11

Use Geoscience page 92 to distinguish between laminations and bedding.

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Currents of wind and water produce ripples and dunes. Cross-laminations show how ripples moved through time.

Exercise 12

Draw asymmetric and symmetric ripples. Explain how each one forms. Explain how aeolian dunes are different from ripples.

Some sedimentary structures are formed by exposing wet sediment to the drying effect of the sun.

Exercise 13

Sketch mudcracks (dessication cracks) and explain how they form.

Organisms also leave traces in sediment.

Exercise 14

Explain how rootlet beds form and the meaning of the term bioturbation.

Exercsie 15

Explain how rootlets and mucdcracks can be used to show that the sediment layer has not been turned upside down.

Turbidity currents flow down onto the seafloor, depositing sediment in a rapid event.

Exercise 16

Describe sole structures and graded bedding. Explain how each one formed.

Some sedimentary structures form gradually after deposition.

Exercise 17

Draw and describe load structures and flame structures.

Sediment sequences – using sedimentary logs

It is often impossible to interpret how a single bed of sedimentary rock forms without looking at the rock layers which surround it. The best way to do this is to draw a sedimentary (sometimes called graphic) log.

Exercise 18

Sketch the graphic log for a turbidity current. (See page 98 Geoscience).

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