Deformation and
Mountain Building
Deformation
Stress is the force applied to material that tends to change its dimensions. Strain
is the effect of stress shown by the material. Strength is the limiting stress that a
material can withstand without failing by rupture or continuous plastic flow. The
response of a rock to stress depends on the type of stress, the amount of pressure, the
temperature, the type of rock, and the length of time the rock is subjected to the stress.
Types of Stress
There are three types of stress:
1. Compressional stress - Forces are directed toward one another. Decreases the
volume of a material. Lithostatic pressure is an all-sided confining pressure
produced by burial.
2. Tensional stress - Stretching stress that tends to increase the volume of a
material.
3. Shear stress - Force is parallel, but in opposite directions. resulting in
displacement of adjacent layers along closely spaced planes.
Rock Response to Stress
Strain on a rocks may be taken as:
1. Elastic deformation - Strain is proportional to stress. Rock will return to
original volume/shape if stress is removed.
2. Plastic Deformation - Permanent deformation caused by flowing and folding
at stresses above the elastic limit at high confining pressure and/or
temperature. Warm rocks tend to deform plastically.
3. Brittle Deformation - Any rock will break if the applied stress is too great.
Rocks at or near the surface (cold, low pressure) tend to deform by brittle
rupture. Results in fracturing and faulting (rock shows differential movement
on either side of the fracture surface.
Time Factor
At a particular temperature and pressure, the response of a rock to stress is
dependent upon the type of stress and the length of time over which the stress is
applied. Slow application of stress favors plastic deformation. Rapid application of
stress favors brittle deformation.
Strength of Rocks
Different types of rock respond to stress differently. Rocks have different
strengths for different types of applied stress. Tensional strength is less than
compressional strength.
1. Material is brittle when the difference is large. Rock behaves as a brittle
material near surface where temperature and pressure are low.
2. Material is ductile when the difference is small. Rock behaves as a ductile
material before it fails at high confining pressures and/or high temperatures.
Measuring Rock Deformation
Geologists use the concept of strike and dip to describe the orientation of
deformed rock layers:
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Strike - Bearing (direction) of a horizontal line on a rock bed or structure.
Dip - The angle between the horizontal and the rock bed or structure.
Features of Plastic Deformation
Folds are produced by plastic and elastic deformation during compressive stress.
Mechanism of folding (not in book) falls in to two categories:
1. Concentric (Flexural Slip) folding - The bending of surface rock beds without
change of thickness or volume (= elastic deformation).
2. Flow Folding - Occurs in plastic rocks subjected to directed stress at high
pressures and temperatures (= plastic deformation). Thickness and volume of
rock beds can change.
Fold Nomenclature
The following terms used to describe fold parts and orientations:
1. Axial plane - Imaginary plane that intersects the crest or trough of a fold to
divide it into 2 equal portions.
2. Axis - The line formed by the intersection of the axial plane and bedding
plane.
3. Limbs - The sides or legs of a fold.
4. Plunge - The dip of the fold axis.
5. Symmetrical folds - Mirror image on either side of the axial plane.
6. Asymmetrical folds - One limb is steeper than the other.
7. Overturned folds - One limb has been tilted beyond the vertical, but both limbs
dip in the same direction.
8. Recumbent fold - Axial plane is horizontal, so fold lies on its side.
9. Isoclinal fold - Fold limbs are parallel to one another.
Types of Folds
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Monocline - Simple, step-like bends caused by elastic deformation of
otherwise horizontal sedimentary beds.
Anticline - Up-arched rock beds. Oldest rocks are exposed in the centers of
eroded anticlines. Rocks dip away from the center of the fold. A plunging
anticline has a V-shaped outcrop pattern where the V points in the direction of
plunge.
Syncline - Downarched rock beds. Youngest rocks are exposed at the centers
of eroded synclines. Rocks dip toward the center of the fold. A plunging
syncline has a V-shaped outcrop pattern where the V points away from the
direction of plunge.
Dome - Upwarped structure with a circular or elliptical outcrop pattern. Beds
dip away from center of structure. Oldest rocks at center.
Basin - Downwarped structure which has a circular or elliptical outcrop
pattern. Beds dip toward center of structure. Youngest rocks at center of basin.
Features of Brittle Deformation
Joints
Fracture along which no differential movement has taken place. Often occur in
parallel groups called joint sets. Intersecting joint sets produce a joint system. Joints
can be caused by compression or tension. Compressional stress can produce joints in
the area of a fold axis. Columnar jointing is produced by tensional stress in cooling
rock (volcanics). Sheet jointing is closely spaced jointing parallel to a rock's surface
produced by unloading (tensional stress). Weathering and erosion of jointed rocks can
produce arches.
Faults
Rock on either side of fracture move relative to one another. Classified on the
basis of the relative direction of movement. The absolute direction of movement
cannot usually be determined. Produce fault scarps (cliff formed by vertical
movement) and fault breccia (rock broken into angular blocks by shear stress along
fault). Three basic groups of faults are recognized:
1. Dip-slip faults - Displacement is primarily vertical and parallel with the dip of
the fault plane. The hanging wall (rock above the fault surface) may move up
or down relative to the footwall (rock below the fault surface).
1. Normal fault - Hanging wall moves down relative to the footwall.
Caused by tensional stress. Horst and graben is a series of normally
faulted blocks. Down-dropped block is a graben, upraised block is a
horst.
2. Reverse fault - Hanging wall moves up relative to the footwall. Caused
by compressional stress. A thrust fault is a low-angle (fault plane dips
<45 degrees) reverse fault.
1. Strike-slip faults - Faults having primarily horizontal displacement along the
strike of the fault plane. Caused by shear stress. A transform fault is a strike-
slip fault that allows lateral movement of new crust away from the mid-ocean
ridge without the relative position of ridge segments changing.
1. Right-lateral - The rock on the opposite side of the fault moves to the
right.
2. Left-Lateral - The rock on the opposite side of the fault moves to the
left.
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Oblique-slip Faults - Fault displacement has both dip-slip and strike slip
movement. Given names like left-laterial, reverse or right-lateral, normal.
Mountains
Any area of land that stands significantly higher than the surrounding country.
Mountain ranges are linear associations of peaks and ridges that are related in age and
origin. Mountain systems consist of several mountain ranges and represent linear
zones of intense deformation and crustal thickening.
Types of Mountains
Mountains can be produced in several different ways:
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Volcanic Mountains - Produced by hot spot activity. May be isolated or in a
chain.
Mountains Formed by Igneous Intrusions - Intrusion of batholith causes uplift
and erosion. Pluton forms small mountains.
Block-Fault Mountains - Produced by normal faulting in areas subjected to
tensional stress. Horst and graben blocks are produced. Horst blocks form
mountains.
Mountains formed by compression at convergent plate margins
Mountains formed by accretion of microplates