Deformation Definition:
"The term deformation refers to the distortation (strain) that is expressed in a deformed rock. This is also what a word literally means a change in form or shape."
OR
Deformation is the transformation from an initial to final geometry by means of rigid body translation, rigid body rotation strain and volume change.
It is useful to think of a rock or rock unit in terms of continum of particles.
Deformation relates the position of particles before and after the deformation history, and the position of points before and after deformation can be connected with vectors.
Types of deformation:
There are 2 types of deformation-
1.Homogeneous deformation:
In homogeneous deformation straight lines remain straight, parallel lines remain parallel, and identically shaped and oriented objects will be same after the deformation.
2. Inhomogeneous deformation:
In inhomogeneous deformation strain is distributed unevenly throughout deformed body in which straight lines and parallel lines in undeformed material become curved and non-parallel.
Strain ellipsoid:
It is a geometric representation of the 3-D strain that develops during homogeneous deformation.
To visualise a strain ellipsoid, you can imagine a sphere embedded in rocks.
- The behaviour of four material lines is illustrated for 2-D case in which a circle changes into an ellipse.
- In homogeneous strain, 2 orientations of material lines remain perpendicular before and after strain. These 2 material lines from the axes of an ellipse is called strain ellipse.
- Length of these two material lines change from initial to final stage otherwise we would not strain our initial circle.
- In 3-D case we have 3 material lines that remain perpendicular after strain and they define the axes of an ellipsoid the strain ellipsoid.
- The lines that are perpendicular before and after strain are called principal strain axes.
Deformation can be witnessed in:
1.Folds. (No plate movement occurs)
2.Faults. (Plate movement occurs)
3.Joints.
Folds:
Flat laid sedimentary and volcanic rocks are often bent into series of wave like undulation known as folds.
2 most common types of fold
- Anticline and Syncline
- Domes and Basins
Faults:
Faults are fractures in the crust along which appreciable displacement occurs .
Types of faults :
- .Dip -slip faults
- .Strike-slip faults
- .Tensional and compressional forces.
Joints :
They are the fractures along which no appreciable displacement occurs.
Types of joints:
- .Columnar joints
- .Sheeting
STRESS AND STRAIN :
These 2 terms are used to describe the type of rock deformation.
Stress:
It is a force applied to a body per unit area.
2 types
1.Uniform or confining stress :
Force on a body that is equal in all directions does not usually deform a rock but may result in change in size.
2.Differential stress :
Stress that is not equal in all directions and is caused by tectonic forces ,usually causes change in shape, but not in size.
Types of differential stress:
1.Tension - A stretching stress
Rocks have very little strength under tensional stress and break apart easily.
2.Compression - A squeezing stress
Rocks are relatively strong under compression .
3.Shear- Stress operates in opposite directions across the body.
STRAIN:
Deformation or change of shape a rock body experiences when under differential stress.
Strain is proportional to stress.
Types of strain :
1.Elastic strain:
Recoverable strain
When stress is removed object regains its original shape.
2.Plastic strain :
Permanent strain when stress exceeds the strength of the rock the rock will bend or fold.
3.Brittle strain:
Permanent strain when stress exceeds the strength of the rock the rock will break or fracture.
Elastic limit:
Limiting stress beyond which the rock cannot return to its original shape and will be permanently deformed.
Factors that influence the type of permanent strain in rocks experiencing the same amount of stress :
- Temperature
- Confining stress
- Time and strain rate
- Composition -Important in 2 ways :
- Mineral composition
- Amount of water in rock
