Chapter 9

Question 1

Structural geology

Answer 1

Study of the deformation pf rocks (shape, volume, position, orientation)

Question 2

Stress

Answer 2

Force per unit area. Compressional (convergent plate boundaries, shortening, folding), tensional (divergent plate boundaries, extension, faulting) or shear (transform plate boundaries)

Question 3

Strain

Answer 3

Change in shape of a rock when stress is applied.

Question 4

Elastic behavior

Answer 4

Material returns to its original shape when stress is removed. Occurs up to a maximum value of strain, called the elastic limit. (rocks = 0.001)

Question 5

Plastic behavior

Answer 5

Material does not return to its original shape when stress is removed, behaves as a very viscous liquid.

Question 6

Primary structures

Answer 6

Happen during rock formation (strata, cross bedding, ripple marks, mud-cracks, graded beds)

Question 7

Secondary structures

Answer 7

Happen after rock is formed (folds, faults, fractures)

Question 8

Differential stress

Answer 8

When the force is not the same in each direction

Question 9

Brittle deformation

Answer 9

Elastic deformation followed by limited plastic deformation. Lower temperatures/pressures (near surface) and high strain rates (fast deformation). Crystalline igneous rocks more susceptible, forms joints and faults.

Question 10

Ductile Deformation

Answer 10

Plastic strain at constant stress. Higher temperatures/pressures (deeper/close to magma) and low strain rates (slow deformation). Weaker and layered (sedimentary & metamorphic) rocks, forms folds.

Question 11

Strike direction

Answer 11

Intersection of a horizontal plane with an inclined plane.

Question 12

Dip angle

Answer 12

Maximum angle of an inclined plane.

Question 13

Anticline fold

Answer 13

Concave downwards (arch)

Question 14

Syncline fold

Answer 14

Concave upwards (saucer)

Question 15

Axial plane

Answer 15

Axis of symmetry with limb on each side

Question 16

Axis

Answer 16

Intersection of axial plane and a particular rock layer.

Question 17

Plunge

Answer 17

Difference in angle between horizontal and axis

Question 18

Folds

Answer 18

Geologic structure that is formed by layers or beds of rock being bent. Visible by outcrops or seismic reflection.

Question 19

Upright folds

Answer 19

Symmetric, vertical axial plane, limbs dip at same angle in opposite directions

Question 20

Inclined folds

Answer 20

Asymmetric, axial plane not vertical, limbs dip at different angles in opposite directions

Question 21

Overturned folds

Answer 21

Asymmetric, axial plane not vertical, limbs dip at different angles in same direction

Question 22

Joints

Answer 22

Fractures along which no displacement has occurred, occurs in igneous and sedimentary rocks

Question 23

Faults

Answer 23

Fractures along which displacement has occurred. Classified by strike, dip, slip.

Question 24

Dip-slip faults

Answer 24

Slip is parallel to dip. Hanging wall located above, footwall located below.

Question 25

Normal fault

Answer 25

Formed by tension, hanging wall moves down relative to footwall

Question 26

Reverse fault

Answer 26

Formed by compression, hanging wall moves up relative to footwall

Question 27

Strike-slip fault

Answer 27

Slip is parallel to strike. Right lateral (formed by motion to right) and left lateral (formed by motion to left)

Question 28

Structural traps

Answer 28

Formed by folding and faulting of rocks.

Question 29

Stratigraphic traps

Answer 29

Formed by the sequence of deposition that placed an impermeable cap rock above a porous reservoir layer.

Question 30

Thrust fault

Answer 30

Special case of a reverse fault, dip less than 45°.