2.19 Stress and strain

Question 1

What is meant by the term stress in a geological context?

Answer 1

the force applied to rocks

Question 2

What is meant by the term strain?

Answer 2

a change in the shape or volume of a body as a result of applied stress

Question 3

What is a competent rock?

Answer 3

rocks that are strong and brittle and tend to joint and fault e.g. sandstones, limestones and most igneous rocks

Question 4

What is an incompetent rock?

Answer 4

a rock that is weak and plastic and tend to fold and develop cleavage e.g. mudstones and shales

Question 5

What are the three types of force that can be experienced by rocks?

Answer 5

• tensional
• compressional
• shear

Question 6

What are the three factors that will affect the rate of strain in a rock?

Answer 6

• temperature
• confining pressure
• time

Question 7

How does temperature affect the amount of strain?

Answer 7

at higher temperatures rocks become more plastic - at high temperatures rocks will fold, not fault, even if the original rock is competent

Question 8

How does confining pressure affect the amount of strain?

Answer 8

the strength of rocks increases with confining pressure - rocks at depth are more difficult to deform

Question 9

How does time affect the strain?

Answer 9

• if the pressure is applied for a short period of time then the rocks may behave in a brittle manner
• pressure applied over an extended period of time can result in plastic deformation

Question 10

What is meant by the term differential stress?

Answer 10

differential stress is when forces applied are not equal in all direction

Question 11

What term is given to the direction of greatest stress?

Answer 11

stress max (σ max)

Question 12

What term is given to the direction of minimum stress?

Answer 12

stress min (σ min)

Question 13

What are cooling joints?

Answer 13

• they are caused by tensional forces set up in an igneous rock as it cools steadily and contracts
• the joints are perpendicular to the cooling surfaces, so will often be vertical
• they are often evenly spaced cooling centres producing a series of column-like structures, which are polygonal in cross-sections

Question 14

What is an unloading joint?

Answer 14

• igneous rocks cool deep below the surface where the pressure is high - the mass of the overlying rocks ‘compressed’ the rock (the load pressure)
• when the rock is exposed at the surface as a result of uplift and erosion, the lack of load pressure from overlying rocks allows them to expand
• joints form roughly parallel to the Earth’s surface

Question 15

What is slaty cleavage?

Answer 15

• occurs in rocks containing platy minerals (usually clays and micas)
• under stress the minerals align forming a series of planes along which the rock tends to split

Question 16

How can you identify bedding planes in outcrops/photographs (as distinct from joints or cleavage)?

Answer 16

• differences in colour or composition between beds
• differences in grain size between beds
• structures within the beds and along the bedding planes
• form in all types of sedimentary rock

Question 17

How can you identify jointing in outcrops/photographs (as distinct from beds or cleavage)?

Answer 17

• fractures along which there is no displacement
• often perpendicular to bedding
• possible mineralisation along the joints
• only develops in competent rocks such as sandstones or limestones

Question 18

How can you identify cleavage in outcrops/photographs (as distinct from beds or joints)?

Answer 18

• the rock splits easily along parallel planes making slates
• often parallel to the axial plane of folds
• planes very close together - a few mm
• only develops in incompetent rocks such as shales, which contain platy clay minerals and micas