11.1 – Rock Deformation
Transcript 11.1 – Rock Deformation
11.1 – Rock
types of stress.
Mountains provide some of the most
spectacular scenery on our planet.
It is theorized that all continents were
once mountainous masses and grow by
the addition of mountains to their
As geologists unravel the secrets of
mountain formation, they also gain a
deeper understanding of the
evolution of Earth’s continents.
Factors that influence the strength of a rock and
how it will deform include temperature,
confining pressure, rock type, and time.
Deformation is a general term that refers to all
changes in the original shape and/or size of a
Most crustal deformation occurs along plate
Stress is the force per unit area acting on a
Strain is the change in shape or volume of a
body of rock as a result of stress.
Temperature and Pressure
Rocks deform permanently in two ways:
brittle deformation and ductile
Brittle deformation is the fracturing of an object
once its strength is exceeded.
Rocks near the surface, where the temperatures
and confining pressures are low
Ductile deformation is a type of solid state flow
that produces a change in the size and shape of
an object without fracturing the object.
Rocks, where temperatures and confining
pressures are high
Mineral composition and texture of a
rock also greatly affect how it will
Rocks like granite and basalt that are
composed of minerals with strong internal
molecular bonds usually fail by brittle
Rocks that are weakly cemented, like
sedimentary rocks or metamorphic rocks,
contain zones of weakness and fail by
Forces that are unable to deform
rock when first applied may cause
rock to flow if the force is
maintained over a long period of
In nature small stresses applied
over a long period of time plays an
important role in the deformation of
Types of Stress
Rocks are exposed to many different
forces due to plate motions.
The three types of stress are tensional,
compressional and shear.
Tensional – when rocks are pulled in
Compressional – when rocks are
pushed together or squeezed.
Shear – when rocks are distorted from
Types of Stress
The three main types of folds are:
Anticline– are most commonly formed
by the upfolding, or arching, of rock
Syncline– are linear downfolds in
Found along with anticlines
Monocline – are large step-like folds in
otherwise horizontal sedimentary
The major types of faults are:
Normal– occur when the hanging wall block
moves down relative to the footwall block
Reverse– are faults in which the hanging
wall block moves up relative to the footwall
Thrust – are reverse faults with dips less
than 45 degrees
Strike-slip - are faults in which the
movement is horizontal and parallel to the
trend, or strike, of the fault surface
Copy and label the following
p. 309 Figure 2
p. 310 Figure 3
p. 312 Figure 6