Geologic Structures
Folds on the Island of Crete
Geologic Structures
• Geologic structures are dynamically-produced patterns or
arrangements of rock or sediment that result from, and
give information about, forces within the Earth
Structures are
produced as rocks
change shape and
orientation in response
to applied stress
Namibia, Africa
Types of Stress
Stress = force per unit area (that which causes rocks to deform)
Strain = deformation in response to stress
a) Compressive
• Rocks are squeezed
• Results in shortening or flattening
b) Tensile
• Rocks are pulled apart
• Results in stretching and elongation
a
c) Shear
• Rocks are sheared
b
• Results in a sort of smearing
Geologic structures are indicative of the type of
stress and its rate of application, as well as the
physical properties of the rocks or sediments
c
Stress and Strain
The type of rock, the temperature and pressure, and the rate of
stress all influence how a rock will accommodate strain
When stressed, rocks can behave with:
a) Elastic strain
Compressional waves (p waves) in an earthquake
• Occurs when material recovers its
initial shape after the stress is
reduced or removed
b) Ductile (Plastic) strain
Ductile strain
• Occurs when the rock bends but
doesn’t break
c) Brittle strain
• Occurs when the rock breaks rather
than bending
Geologic structures are indicative of the type of
stress and its rate of application, as well as the
physical properties of the rocks or sediments
Brittle strain
Responses to Stress
a
a) Folding
• Produced by
compressive stress
on ductile rocks
b) Faulting
• Produced by any
type of stress on
brittle rocks
Or both!
b
Responses to Stress
c) Tilted beds
• Related to faulting and
folding
• Deformation can
change the orientation
of rock layers
Cocked Hat mountain, Brooks Range, AK
Danxia Landform, China
Responses to Stress
d) Jointing
• Can form in response
to any type of stress or
as a result of cooling
• Fractures in a rock
along which no
movement has
occurred
• Often promotes
weathering since joints
provide access for
water
Giants Causeway, County Antrim, Northern Ireland
Folds
Wavelike bends in a layered rock
Formed by COMPRESSIVE STRESS
Axial Plane =
plane of symmetry
Fold axis (hinge
line) = surface
trace of axial
plane
Fold limb
STRESS
Monocline
Syncline
STRESS
Anticline
Looks like an “A”
Overturned
anticline and syncline
Fold limb
Folds
What kind of fold is this?
Folds
Wavelike bends in a layered rock
Formed by COMPRESSIVE STRESS
Monocline
Syncline
Anticline
Looks like an “A”
Overturned
anticline and syncline
Folds
What kind of fold is this?
Folds
Wavelike bends in a layered rock
Formed by COMPRESSIVE STRESS
Monocline
Syncline
Anticline
Looks like an “A”
Overturned
anticline and syncline
Folds
What kind of fold is this?
Folds
Wavelike bends in a layered rock
Formed by COMPRESSIVE STRESS
Monocline
Syncline
Anticline
Looks like an “A”
Overturned
anticline and syncline
Folds
What kind of fold is this?
Folds
Wavelike bends in a layered rock
Formed by COMPRESSIVE STRESS
Monocline
Syncline
Anticline
Looks like an “A”
Overturned
anticline and syncline
Folds
What kind of fold is this?
Folds
Wavelike bends in a layered rock
Formed by COMPRESSIVE STRESS
Monocline
Syncline
Anticline
Looks like an “A”
Overturned
anticline and syncline
Folds
What kind of fold is this?
Folds
Wavelike bends in a layered rock
Formed by COMPRESSIVE STRESS
Monocline
Syncline
Anticline
Looks like an “A”
Overturned
anticline and syncline
Faults
Fractures along which movement has occurred
Formed by ANY KIND of STRESS
Hanging
wall
Hanging
wall
Footwall
Footwall
Types of Faults
Faults are considered
“active” if movement has
occurred along them within
the last 11,000 years.
• Dip-slip faults have movement
parallel to the dip of the fault plane
– In normal faults, the hanging-wall
block has moved down relative to the
footwall block
– In reverse faults, the hanging-wall
block has moved up relative to the
footwall block
Types of Faults
• Strike-slip faults have movement that is
predominantly horizontal and parallel to
the strike of the fault plane
– A viewer looking across to the other side of a
right-lateral strike-slip fault would observe it
to be offset to their right
– A viewer looking across to the other side of a
left-lateral strike-slip fault would observe it
to be offset to their left
Right-lateral San Andreas Fault
Types of Faults
• Dip-slip faults have movement
parallel to the dip of the fault plane
– Fault blocks, bounded by normal
faults, that drop down or are uplifted
are known as grabens and horsts,
respectively
• Grabens associated with divergent plate
boundaries are called rifts
– Thrust faults are reverse faults with
dip angles less than 30° from
horizontal
Faults
What kind of faults? Which is the hanging wall? The footwall?
Right or left lateral?
Normal faults, upper right side is the hanging wall
Right lateral, strike slip fault
Thrust fault (reverse fault)
Graben, normal faulting
Reverse Fault
Left lateral, strike slip fault
What does she want to know? How
to keep track? Or the organization
of it by the Borough? They haven’t
announced this year’s yet, but it
should start in May I believe
(although this year they could
probably start in April). How’s the
binding coming?
Orientation of
Geologic Structures
• Geologic structures are most obvious
in deformed sedimentary rocks
• Tilted beds, joints, and faults are planar
features whose orientation is described
by their strike and dip
– Strike is the compass direction of a line
formed by the intersection of an inclined
plane with a horizontal plane
– Dip is the direction and angle from
horizontal in which a plane is oriented
WHAT KIND OF ROCK IS THIS??
Rock Identification Review
Karri Sicard & Trish Gallagher
Alaska Division of Geological & Geophysical Surveys (DGGS)
Sedimentary, Igneous, or Metamorphic?

Key vocabulary
 Clasts
– Broken pieces of rock or
minerals
 Amorphous – you can see no crystals or
clasts
 Examples:
obsidian—volcanic glass, chert—
silicious ooze
 Crystals
– another word for mineral, all
different shapes
 Matrix – the background that the
minerals or rocks are in
Lets Practice!
ROCK
TYPE
Flow Chart
Sedimentary, Igneous, or Metamorphic?
It's an Igneous Rock if



It has a bubbly, frothy or glassy texture
It is fine-grained, uniform and hard
It contains bubble-like cavities that may be lined with minerals
It’s a Sedimentary Rock if



It is soft enough to be scratched by a knife
It is obviously made up of different particles
It contains fossils, bedded layers, rounded clasts
It’s a Metamorphic Rock if




It has a fine, uniform texture
It contains obvious streaks or clumps of minerals
It is made mostly of quartz or calcite
It contains minerals like garnet
Identifying Sedimentary Rocks
Igneous
Rocks
Based on
mineral
ID!
Mafic
vs.
Felsic
Metamorphic Rocks