Ch. 9: Structural Geology

Question 1

Undeformed

Answer 1

• Undeformed: Horizontal beds, spherical sand grains, no folds or faults

Question 2

deformed

Answer 2

Deformed: Tilted beds, metamorphism, folding and

faulting

Question 3

Stress

Answer 3

Causes Deformation (STRAIN)
• STRESS is the force applied per unit area across an area:
– A large force per area results in much deformation
– A small force per area results in little deformation

Question 4

STRAIN

Answer 4

STRAIN is the shape change caused by STRESS
Changes in shape may be caused by:
– Stretching: Pulling apart
– Shortening: Squeezing
together
– Shear: Sliding past

Question 5

Deformation

Answer 5

Deformation creates geologic structures
– JOINTS: Fractures that have no offset
– FAULTS: Fractures that are offset
– FOLDS: Layers that are bent by slow plastic flower

Question 6

Deformation types

Answer 6

BRITTLE deformation – Rocks break by
fracturing
– Brittle deformation occurs in the shallow crust
DUCTILE deformation – Rocks deform by flow and folding
– Ductile deformation occurs deeper in the crust, where
rocks are hot
enough to flow

Question 7

Strike

Answer 7

orientation of imaginary horizontal line on

tilted plane

Question 8

Dip

Answer 8

Angle that surface inclined from the horizontal

Question 9

Brittle structures:

Answer 9

Joints: Planar rock fractures without offset
FAULTS
• Planar fractures offset by movement across the break
• Sudden movements along faults cause earthquakes
• Faults vary by type of stress and crustal level

Question 10

Faults

Answer 10

On a dipping fault, the blocks are classified as either
the…
– Hanging wall block (above the fault)
– Footwall block (below the fault)

Question 11

DIP-SLIP FAULTS

Answer 11

DIP-SLIP FAULTS
• Sliding is parallel to fault-plane dip
– blocks move up or down the slope of the fault
• Two kinds of dip-slip fault depend on relative motion:
– NORMAL fault – Hanging wall moves down
– REVERSE Fault – Hanging wall moves up
• THRUST fault (a low-angle reverse fault)

Question 12

STRIKE-SLIP FAULTS

Answer 12

STRIKE-SLIP FAULTS
• Fault motion is parallel to the strike of the fault
• Classified by the relative sense of motion. To find this…
– Right lateral – Opposite block moves to observer’s
right
– Left lateral – Opposite block moves to observer’s left

Question 13

Tectonic Stresses and Deformation

Answer 13

Compression: Squeezing convergent
– Crust thickens
– Thrust faults
• Extension: Pull-apart divergent
– Crust thins
– Normal faults
• Shear: Blocks slide past transform
one another
– Crust neither
thickened or thinned
– Strike-slip faults

Question 14

Ductile structures

Answer 14

Dome – Fold with appearance of an
overturned bowl
Erode to expose older rocks in
center and younger rocks outside
Basin – Fold shaped like a bowl
Erode to expose younger rocks in
center and older outside

Question 15

Fold

Answer 15

FOLDS
• Layered rocks may be deformed into curves called folds
• Parts of a fold:
– HINGE: Portion of maximum curvature on a fold
– LIMB: Less curved “sides” of a fold
– AXIAL PLANE: Imaginary surface defined by
connecting hinges of successively layers of fold

Question 16

ANTICLINES

Answer 16

Anticlines: Limbs dip away from center, oldest strata

exposed in center, younger in limbs

Question 17

SYNCLINES

Answer 17

Synclines: Limbs dip toward center, younger strata exposed in center, older in limbs

Question 18

Horizontal fold

Answer 18

has a horizontal hinge

Question 19

Plunging fold

Answer 19

noses formed as folded layers are eroded; viewed on earth’s surface
has a hinge that is tilted, plunges into the ground