Brittle and plastic/ductile behavior
Ductile
Brittle
Deformation of rocks - Stress and Strain
• Stress = force applied within a body – in geology we talk about
stress rather than forces on rocks
• Any material subjected to stress begins to deform → strain
• Elastic strain is recovered when stress is lifted
• Higher stress may induce a permanent deformation plastic/ductile
• If the stress acting is greater than the strength of the material
(ultimate tensile strength) the material will break Brittle behavior
Stress configurations generate strain
• Lithostatic stress
is the same in all
directions
All types of
stress produce
• Differential stress Strain
is stronger in
certain directions
Strain of rocks is caused by stress
Compression Tension Shear
causes stretching Shear causes
causes shortening and thinning side to side
and thickening distortion
Factors influencing Deformation
• Temperature: High temperature → ductile deformation
• Rock type: certain rocks break others deform
• Time: Long time to apply stress → ductile deformation
• Confining pressure: High pressure → ductile deformation
Ductile deformation: fold structures
During crustal deformation,
rocks are often bent into a
series of wave-like
undulations called folds.
Most folds result from
compression stress that
shortens and thickens the
crust.
This happens over million
of years, in the meantime,
weathering works!
Elements and types of folds
to describe the folds, based on orientation of the limbs (side of the
folds) the axial surface and the axis, the hinge or zone of the
curvature
Ductile deformation: fold structures
• General classification of folds is based on the orientation of
the curvature: up → Anticline, down → Syncline
Depending on the orientation of the axial plane and the limbs, there are many
additional classification of folds, for this class we go for anticline and syncline
Labrador
Sideling Hill, MD - Photo courtesy of Brennan T. Jordan, Department of Earth Sciences, University of South Dakota
http://www.usd.edu/~Brennan.Jordan/
Zagros Mountains - Photo courtesy of J. T. Daniels
http://disc.gsfc.nasa.gov/geomorphology/GEO_2/GEO_PLATE_T-42.shtml
Crete, Greece
How fold end/begin: plunging folds
Folds are limited deformation of the lithosphere, there is a
begining and an end of the fold, they are called the fold plunge
Sheep Mountain anticline.
Sheep Mountain in Wyoming
Folds always happen in groups: examples
Valley and Ridge province
Valley and Ridge folds
Shenandoah Valley & Massanutten Mt.
Breaking rocks
• Rocks break when their tensile strength is not
enough to respond to the stress
– They may break during elastic deformation or
during plastic deformation
Joints
• Joint = a fracture with no movement.
• Joints always occur in ~ parallel groups
Columnar joints for in lava flows (basaltic to
intermediate in composition) as it solidifies
• Joints in lava flows form a typical columnar
patterns
• Joints can fom in layers of rocks
Faults
• Fractures in rocks, along which
appreciable displacement has taken
place.
• Faults have orientation or strike with
respect to direction to North, and an Dip angle
inclination, indicated as a dip angle
• Classified by the relative movement,
of hanging wall and foot wall.
• Faults are classified based on the
relative motion of the footwall,
whether is it along the dip or along
the orientation or strike
Dip Slip fault: Normal fault
• Stress configuration is tension
• Resulting strain is extension
• Hanging wall moves down with respect to footwall
Dip slip Fault: Reverse Fault
• Compression Stress
• Contraction/shortening deformation (strain)
• Hanging wall moves up with respect to foot wall
Outcrop showing a vertical rock face cut by a reverse fault
Dip slip Fault: Thrust Fault
• A thrust fault is a reverse fault surface has a low angle with
respect to the Earth’s surface
Strike-slip Fault
• Dominant displacement is horizontal and parallel to the strike
of the fault
• Types of strike-slip faults
– Right-lateral—As you face the fault, the opposite
side of the fault moves to the right.
– Left-lateral—As you face the fault, the opposite side
of the fault moves to the left.
Aerial View of Strike-Slip faults in China
Animation of fault
Click on image for a summary animation of the types of faults
Compression, shortening and mountain building
• How mountain ranges are built: compression stress causes
the lithosphere to “pile up” in heaps of ductile and brittle
deformed rocks
Measuring the orientation of rock structures
• Geologist use strike and dip measurements to figure out the
direction of orientation and the angle at which the rocks dip
with respect to the horizontal.
– The geologist identifies the rock and measure the
orientation using a specialized compass
The Flatirons mountain, CO - photo by Jesse Varner
Geologic Maps
– The map is prepared by locating the position of outcrops (sites where
bedrock is exposed at the surface) on a topographic map systems
(GPSs).
– Types of rocks have coded colors and symbols explained in a map
key
– Special symbols are used to indicate geologic structures like strike
and dip
Strike
and dip
symbol
A geologic map of the US
This map shows different colors for diverse rocks.
Colors are coded to represent rocks of certain type and age, for example,
turquoise and green are sedimentary rocks from the time of dinosaurs called
the Mesozoic ( from 250 to 65 Million year ago)
Geologic map of Fairfax, with GMU campus
