• when an object is changed temporarily or
permanently due to applied force.
Deformation of rocks
• Within the Earth rocks are continually being
subjected to forces that tend to bend them,
twist them, or fracture them. When rocks
bend, twist or fracture we say that they
deform (change shape or size). The forces that
cause deformation of rock are referred to as
stresses (Force/unit area). So, to understand
rock deformation we must first explore these
forces or stresses.
Stress and Strain
• Stress is a force applied over an area. One
type of stress that we are all used to is a
uniform stress, called pressure. A uniform
stress is a stress wherein the forces act equally
from all directions. In the Earth the pressure
due to the weight of overlying rocks is a
uniform stress, and is sometimes referred to
as confining stress.
Stress and Strain
• If stress is not equal from all directions then
we say that the stress is a differential stress.
Three kinds of differential stress occur.
• When rocks deform they are said to
strain. A strain is a change in size,
shape, or volume of a material.
. the ability of an object to resist
• compressive strength
• tensile strength
• shear strength
FACTORS AFFECTING on ROCK DEFORMATION
• Strain rate
• Rock composition
• 1 Rocks near the surface, where pressures are
low, usually behave like brittle solids. Called
2 At depth, where pressures are high, rocks
show ductile behavior. Called ductile
Rocks deform permanently in two ways: brittle
deformation and ductile deformation.
1 Rocks near the surface, where temperatures
are low, usually behave like brittle solids.
2 At depth, where temperatures are high,
rocks show ductile behavior.
High Strain Rate forms a Brittle
Low Strain Rate forms a Ductile
Hard composition of rocks (like
granite) caused Brittle Deformation.
Soft composition of rocks (like
marble) caused Ductile Deformation
TYPES OF DEFORMATION
• 1 ELASTIC DEFORMATION
• STRESS is applied, released and rock returns to
• original shape and volume.
• Material is not DEFORMED.
• If stressed to a point where the material doesn’t resume
• original shape and volume, the rock is deformed.
• Point at which this occurs is the ELASTIC LIMIT.
• Pressure and temperature when stress is applied also
• determine how rocks deform at the elastic limit.
2 PLASTIC or DUCTILE DEFORMATION
Rocks subjected to stress at temperature and
pressure tend to , resulting from.
An irreversible change in shape and/or volume
without the rock breaking.
Rocks go beyond the elastic limit.
Atoms move about and adjust without the bonds
Atoms move from areas of high stress to those
Kinds of differential stress
• Tensional stress (or extensional stress),
which stretches rock;
• Compression stress, which squeezes rock;
• Shear stress, which result in slippage and
Stages of Deformation
• When a rock is subjected to increasing stress it
passes through 3 successive stages of
• Elastic Deformation
• Ductile Deformation
• -- wherein the strain is irreversible.
it is when something breaks from its original
shape and cannot return to its original
• -- wherein the strain is reversible.
The temporary change in length, volume, or
shape produced in an elastic substance by a
stress that is less than the elastic limit of the
geologic,fault rock stress exceeding the rock