Geological structures: Rock mechanics 3.3.1
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|(a)||(i) the geological structures produced by rock deformation as a result of tectonic stresses (tension, compression and shear forces)
(ii) the identification, measurement and description of these geological structures on
photographs, maps, cross sections and in the field, including production of labelled field sketches
(iii) the construction of geological cross sections from geological maps
(iv) use of a compass-clinometer
||(i) how tectonic stress and strain vary due to temperature, confining pressure and time, resulting in the plastic or brittle deformation of rocks
(ii) the use of stress and strain diagrams
|To include the formation of joints, slickensides and fault breccia.
||how compressive forces can lead to the formation of a slaty cleavage.
||To include the relationship of cleavage to folds.|
The topic of rock mechanics looks at how rocks behave under stress, relating this to existing knowledge of pressure may help as both are a force per unit area. The amount of deformation a rock experiences due to stress is referred to as strain. The use of these terms is key throughout the topic. Learners have the chance to develop a wide range of skills here which will make the topic very engaging, there is opportunity for using evidence from the field and learning how it can be recorded, this could be done with photographs and maps or by carrying out field work. Constructing cross sections is may be a challenge but by building up the difficulty gradually and with learners maintaining attention to detail they are rewarded by developing a skill which has many applications.
Common misconceptions or difficulties students may have:
The visualisation in three dimensions of folds, faults and bedding planes may be difficult for some to grasp, in particular relating what is seen on a map surface compared to idealised fold structures. To help here many computer programs are available which can transfer between different views, or building 3D models could help. Learners may get mixed up between stress and strain if these terms are introduced in a vague way, relating to existing knowledge on forces may help here. Drawing cross sections is likely to be a challenge to many learners, to help learners should use great attention to detail, and think carefully about what scales mean, think about predicting what a cross section should look like before drawing it.
Conceptual links to other areas of the specification – useful ways to approach this topic to set students up for topics later in the course:
The main application of the skills in this topic are in 6.2.1 Engineering geology where learners consider measuring rock strength and how geological features effect this. Learners consider how surveys are carried out in preparation for developing a site, the terminology introduced in 3.3.1 is applied throughout. Topic 7.2.2 Oil and gas basins gives learners many opportunities to apply their understanding of cross sections, in particular looking at an arrangement of rock types and structures where a hydrocarbon deposit is likely to be found.
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