Structural Geology L03

Question 1

define pressure and stress.

Answer 1

pressure - force acting per unit area - does not have direction/vectors.

stress - amount of force exerted per unit area experienced
by a material (rock) - has direction/vector

Question 2

Explain body forces and surface forces.

Answer 2

Body Forces
* Act throughout the volume of the solid
and directly proportional to its volume or
mass.
* Depends on the densities; while densities
itself depends on pressure.

Surface Forces
* Act only on the surface area bounding
a volume and arise from the
interatomic stresses exerted from one
side of the surface to the other.
* Increase linearly with depth known as
lithospheric stress/pressure.

Question 3

differentiate normal stress and shear stress.

Answer 3

• stress vector oriented perpendicular to a
  surface is called the normal stress on that
  surface
• stress vector that acts parallel to a surface
  is referred to as the shear stress.

Question 4

Explain principle stress direction.

Answer 4

Slide no 7

Question 5

Types of stress in lithosphere.

Answer 5

• Residual stress
• lithostatic stress
• Tectonic stress
• Deviatoric stress

Question 6

what is residual stress.

Answer 6

• Stress can be locked in and preserved after the external
  force or stress field has been changed or removed - elastic strain remains after the external stress field is
  removed.

for example,
– metamorphic transformations that involve volumetric
changes,
– intrusions where magma cooling sets up stresses that are locked into the crust,
– changes in temperature and/or pressure, or
– past tectonic episodes

Question 7

What is tectonic stress.

Answer 7

stress related to plate movements and plate tectonics.

Question 8

what is Anderson’s classification on tectonic stress?

Answer 8

– normal, thrust and strike-slip regimes
– no shear stress at the Earth’s surface (shear stress cannot
occur in fluids), one of the principal stresses has to be vertical,
implying that the other two are horizontal.

Question 9

differentiate total stress and effective stress.

Answer 9

• Total stress (σ) is equal to the sum of effective stress (σ’) and pore water
  pressure (u) or,
• alternatively, effective stress is equal to total stress minus pore water
  pressure.

refer slide no. 18.

Question 10

what is mean stress and deviatoric stress?

Answer 10

• Mean stress,
  gives an average measure of stress.
• Deviatoric stress is the difference
  between the mean stress and the total stress - The deviatoric stress tensor
  represents the anisotropic
  component

Question 11

define rheology.

Answer 11

refers to the flow of matter, primarily in
the liquid state, but also as ‘soft solids’ or solids under
conditions in which they respond with plastic flow rather
than deforming elastically in response to an applied force - temperature-dependant

Question 12

differentiate elasticity, plasticity, and faulting.

Answer 12

Elasticity
-Hooke’s Law, elongation = strain
-Strain is not permanent
-Faulting = sliding/shearing, need to overcome
the frictional resistance
-earthquake -> release the shearing, rock bounce
back

Plasticity
-Strain is permanent
-Rock does not rupture by faulting, but the
surrounding rocks may permanently distort

Faulting occurred in:
-Weak material (e.g. shale) zone: less resistance
- Stronger material zone: more resistance

Question 13

Relate temperature and rock type with the rheology.

Answer 13

Higher temperature, and the elasticity gradually lose

Rheology is temperature-dependant

Plastically deform -> creep

Deeper part of the earth, tend to plastic deformation - Temperature > 70% of melting point, the rock tends to deform plastically.

Question 14

explain Mohr circle.

Answer 14

STUDY MATTT KAU TAK TAHUUU