Basic concepts of stress and strain important to understand geologic deformation., Slides of Geology

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Guided by

Dr.p.Madesh Professor

Presented

by

Pagadala Arun Gopi 1st Sem Geology DOS in Earth Science, University of Mysore Center for Advanced Studies in Precambrian Geology

Contents:

Introducti

onStres

sStrai

Deformati n

onConcept of stress and strain

ellipsoidsStress

ellipsoidStrain

ation between stress and strain ellipsoid ellipsoid

Stress and strain

ellipsoids

Stress on a rock body is defined as a force applied over an area to cause the deformation. Stress:

Tensional

compression^ compression

Shear^ Shear

Stress on a rock body is defined as a force applied over an area to cause the deformation. Stress:

Tensional^ Tensional

compression

Shear^ Shear

Stress on a rock body is defined as a force applied over an area to cause the deformation.

Stress :

Tensional^ Tensional

compression^ compression

Shear^ Shear

Stress acting on a plane

Normal stress

Shear stress

Shear stress

Strain: Strain may be defined as response of rock to the stress, which can be depends on temperature pressure and mineralogical composition of rock.

Concept of stress and strain ellipsoids:

As the measurement of deformation certain initial body form is required to be known. Geologist deals with the rocks and minerals, so the amount of change in the original shape of the mineral is measure of deformation. But since the minerals are of mixed shape the concept of stress and strain ellipsoids put forth.

σ σ Stress ellipsoid: Stress ellipsoid or ellipse describes the state of stress at a point in a rock. Greatest stress Minimum stress

Strain ellipsoid: It is the convenient way of visualising deformation is to imagine the change in shape of imaginary sphere or circle. A A' B B'

Strain ellipsoid: It is the convenient way of visualising deformation is to imagine the change in shape of imaginary sphere or circle. S 1

S

2

S

3)

Strain ellipsoid - flinns diagram The Flinn diagram is a graphical representation used to plot finite strain ellipsoids. The Flinn diagram describes two main types of strain ellipsoids, cigar and pancake. S 1

S

2

S

3)

a = S

/S

2

b = S

2

/S

3

K = a-1)/b-1)

Stress and strain ellipsoids– Rupture

The position of the fracture planes and their orientation in field, are very important, because only from such data the direction of actual deformative forces , the orientation of stress and strain ellipsoids can be derived.

σ σ σ3)

S

3) S 2

Tensional fractures