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Rock Deformation
 We have learned in earlier sections that the lithospheric plates are in motion. Where the plates
collide,rocks are compressed.
 Where the plates move away from each other,rocks are pulled apart. The volume and shape of
the rocks are evidently affected.
 The term ''deformation'' refers to changes in volume and shape of rocks as they are squeezed
by compressional forces or streched by tensional forces.
 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 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.
If stress is not equal from all directions then we say that the stress is a differential stress. Three
kinds
of differential stress occur.
1. Tensional stress (or extensional stress), which stretches rock;
2. Compressional stress, which squeezes rock; and
3. Shear stress, which result in slippage and translation.
When rocks deform they are said to strain. A strain is a change in size, shape, or volume of a
material.
2 WAYS TO DEFORM:
 FOLD
A fold can be defined as a bend in rock that is the response to compressional forces. Folds are
most visible in rocks that contain layering.
 PARTS:
1. The sides of a fold are called limbs.
2. The limbs intersect at the tightest part of the fold, called the hinge.
3. A line connecting all points on the hinge is called the fold axis.
4. The angle that the fold axis makes with a horizontal line is called the plunge of the fold.
5. An imaginary plane that includes the fold axis and divides the fold as symmetrically as possible
is called the axial plane of the fold.
TYPES OF FOLDS
1. MONOCLINE
2. ANTICLINE
3. SYNCLINE
 FRACTURE
As we have discussed previously, brittle rocks tend to fracture when placed under a high
enough stress. Such fracturing, while it does produce irregular cracks in the rock, sometimes
produces planar features that provide evidence of the stresses acting at the time of formation
of the cracks. Two major types of more or less planar fractures can occur: joints and faults.
1. FAULTS
-Faults occur when brittle rocks fracture and there is an offset along the fracture. When the offset is
small, the displacement can be easily measured, but sometimes the displacement is so large that it is
difficult to measure.
Types of Faults
As we found out in our discussion of earthquakes, faults can be divided into several different types
depending on the direction of relative displacement. Since faults are planar features, the concept of
strike and dip also applies, and thus the strike and dip of a fault plane can be measured. One division
of faults is between dip-slip faults, where the displacement is measured along the dip direction of the
fault, and strike-slip faults where the displacement is horizontal, parallel to the strike of the fault.
Recall the following types of faults:
Dip Slip Faults - Dip slip faults are faults that have an inclined fault plane and along which the
relative displacement or offset has occurred along the dip direction. Note that in looking at the
displacement on any fault we don't know which side actually moved or if both sides moved, all we
can determine is the relative sense of motion.
A. Normal Faults - are faults that result from horizontal tensional stresses in brittle rocks and where
the hanging-wall block has moved down relative to the footwall block.
B. Reverse Faults - are faults that result from horizontal compressional stresses in brittle rocks,
where the hanging-wall block has moved up relative the footwall block.
C. Thrust Fault - is a special case of a reverse fault where the dip of the fault is less than 45o.
Thrust faults can have considerable displacement, measuring hundreds of kilometers, and can result
in older strata overlying younger strata.
Strike Slip Faults - are faults where the relative motion on the fault has taken place along a
horizontal direction. Such faults result from shear stresses acting in the crust. Strike slip faults can be
of two varieties, depending on the sense of displacement.
A. To an observer standing on one side of the fault and looking across the fault, if the block on the
other side has moved to the left, we say that the fault is a left-lateral strike-slip fault.
B. If the block on the other side has moved to the right, we say that the fault is a right-lateral strike-
slip fault. The famous San Andreas Fault in California is an example of a right-lateral strike-slip
fault. Displacements on the San Andreas fault are estimated at over 600 km.
Joints
As we learned in our discussion of physical weathering, joints are fractures in rock that show no
slippage or offset along the fracture. Joints are usually planar features, so their orientation can be
described as a strike and dip. They form from as a result of extensional stress acting on brittle rock.
Such stresses can be induced by cooling of rock (volume decreases as temperature decreases) or by
relief of pressure as rock is eroded above thus removing weight.
Provide pathways for water and thus pathways for chemical weathering attack on rocks. If new
minerals are precipitated from water flowing in the joints, this will form a vein. Many veins observed
in rock are mostly either quartz or calcite, but can contain rare minerals like gold and silver. These
aspects will be discussed in more detail when we talk about valuable minerals from the earth in a
couple of weeks.
Because joints provide access of water to rock, rates of weathering and/or erosion are usually higher
along joints and this can lead to differential erosion.
From an engineering point of view, joints are important structures to understand. Since they are
zones of weakness, their presence is critical when building anything from dams to highways. For
dams, the water could leak out through the joints leading to dam failure. For highways the joints
may separate and cause rock falls and landslides.

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