Deformation of Rocks

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A bed is a unit of sedimentation, which can vary considerably in thickness.

A bedding plane marks the break between beds.

Beds will be different from each other in a variety of ways:

colour: small changes in shade or major differences (e.g. between a red bed and a green bed)

grain size: grains can vary between a fine clay and coarse pebbles or there could be minor changes betweem a fine sand and medium sand

grain shape: will vary between angular and rounded

sorting: will vary between well sorted and poorly sorted

composition: each bed will contain different amounts of minerals, such as quartz, mica or clay and the cement or matrix will vary

bed thickness: some beds will be very thin, called laminations, or they can be thick, known as massive

Beds are often tilted or folded by the Earth's movements, so that they are no longer horizontal. Dip is measured as the maximum angle between a horizontal line and the dipping bedding place.

Stress is the force applied to rocks.

Strain is the term used to describe the deformation caused by the applied forces. The strain is a change in volme or shape - or both.

Rocks behave in different ways to stress, depending on their physical properties:

competent rocks stay the same thickness when they are deformed and react in a brittle way. Sandstones, limestones and most igneous rocks are competent

incompetent rocks will vary in thickness when they are deformed, as they behave in a plastic way. Mudstones and shales are typical incompetent rocks.

Factors Affecting Stress and Strain

Three main factors which will affect the stresses applied and the resulting strains. All these factors operate together:

the higher the temperature, the more plastic the rocks will be. At high temperatures the rocks will fold, not fault, even if the original rock is competent. Cold rocks will behave as brittle materials and therefore fracture

the strength of the rocks increases with confining pressure and therefore rocks are depth are more difficult to deform. The weight of the overlying rocks causes the confining pressure

time is crucial in determining the type of deformation that occurs. If the pressure is applied for a short time, then the rocks may behave in a brittle manner, whilst pressure applied over an extended period of time can result in plastic deformation.

Relationship between Forces and Geological Structures

Tensional forces usually reslt in the fracturing of rocks and crustal extension. If the forces result in a displacement of the rocks on either side of the fracture plane, then the structure is described as a fault. If there is no displacement, the structure is described as a joint.

Compressional forces can result in either the fracturing or folding or rocks. When the rocks are cold they will tend to fracture and faults are formed. If the rocks are warm then they are more likely to fold. In both cases, the deformation will result in a shortening of the Earth's crust.

Shear forces result in the deformation of rocks in one plane usually horizontally. These may result in faults or folds.

Using Fossils and Ooliths to Measure Strain

In order to measure the amount of strain a rock has undergone, we can use bilaterally symmeratrical fossils or ooliths. Undeformed fossils are used to establish the original shape and then the amount and direction of deformation can be analysed. Ooliths were originally spherical in shape and after deformation they become ellipsoid.

Beds and Bedding Planes

Stress and Strain