Chapter 9: Geologic Structures & Mountain Building

Question 1

Orogens

Answer 1

Mountain belts (from the greek words oros, meaning mountain, and genisis meaning formation).

Question 2

Orogeny

Answer 2

A mountain building event which may last for tens of millions of years.

Question 3

Mountain Building

Answer 3

the process of forming a mountain belt

Question 4

uplift

Answer 4

the vertical rise of the land surface and the rock beneath which also causes deformation

Question 5

deformation

Answer 5

rocks that bend, break, or flow as a result of mountain building/uplift.

Question 6

geologic structures

Answer 6

features produced by deformation including

• joints (cracks)
• faults (fractures where rock slide past each other)
• folds (bends, curves, or wrinkles of rock layers)
• foliation (a fabric or layering in rock)

Question 7

changes caused by deformation

Answer 7

• change in location (displacement)
• change in orientation (rotation)
• change in shape (distortion/strain)

Question 8

types of strain

Answer 8

• stretching (layer becomes longer)
• shortening
• shear strain (movement of one part of a rock body past another so that angles between features in the rock change)

Question 9

types of strain

Answer 9

• brittle: many bonds break and stay broken, leading to the formation of a permanent fracture across which material no longer connects.
• plastic/ductile: some bonds break, but new ones quickly form. this way the atoms within grains rearrange and the grains change shape without permanent cracks forming.
• elastic

Question 10

conditions where plastic deformation takes place

Answer 10

• warm rock
• great pressure
• slow rate of change
• variety of rock composition

Question 11

conditions where brittle deformation takes place

Answer 11

• cooler temperatures
• closer to the earth’s surface (less pressure)
• sudden change in shape (marble bench example)
• rock composition also contributes to whether the rock is more brittle/plastic

Question 12

stress

Answer 12

the force applied per unit area of the plane; by using the word stress, we emphasize that the consequences of applying a force depend not only on the amount of force but also on the area over which the force acts. Three types

• compression
• tension
• shear

Question 13

compression

Answer 13

takes place when a rock is squeezed together

Question 14

tension

Answer 14

occurs when the opposite ends of an object are pulled in opposite directions

Question 15

shear stress

Answer 15

when one surface of an object slides relative to another

Question 16

pressure

Answer 16

occurs when an object is subjected to the same stress on all sides

Question 17

joints

Answer 17

natural cracks along which the rock bed broke and separated into separate pieces during brittle deformation, not the same as faults (faults are larger scale and moved or are active)

Question 18

vein

Answer 18

a mineral filled crack where groundwater or hydrothermal fluids seep through cracks in rocks and precipitate out of the earth/groundwater

Question 19

faults

Answer 19

a fracture on which sliding occurs and slip events, or faulting, can generate earthquakes.

Question 20

strike line

Answer 20

deviation (in degrees) from north

- perpendicular to the dip

Question 21

dip

Answer 21

a deviation (in degrees) from a flat, horizontal plane; slope of the plane

• perpendicular to the strike
• 0 degrees of dip mean that the bed is horizontal, 90 degrees of dip mean that the bed is vertical

Question 22

fault displacement

Answer 22

the amount of movement that takes place across a fault

Question 23

dip faults

Answer 23

Angled

• movement parallels the dip line
• hanging wall block slides up or down relative to the footwall block

Question 24

types of dip faults

Answer 24

• reverse: hanging wall block slides up the fault (compression)
• thrust: a reverse fault with a gentle dip (less than 30 degrees, happens at continental collision)
• normal: hanging wall block slides down (accommodates stretching)

Question 25

strike-slip faults

Answer 25

Straight down into the crust; movement parallel to the fault If you are facing the fault and looking across it: - if the block on the far side slipped to your left, the fault is a left-lateral strike-slip fault. - if the block slipped to the right, the fault is a right-lateral strike-slip fault

Question 26

oblique slip faults

Answer 26

Also at an angle, but movement occurs diagonally across the fault plane (scrubby)

Question 27

fault scarp

Answer 27

the "step" like structure created from dip-slip and oblique-slip faults. In other words: a planar feature formed by off-set at the surface by fault movement

Question 28

fault breccia

Answer 28

shattered, angular fragments of rock from brittle conditions at faults

Question 29

fold

Answer 29

a curve in the shape of rock layer; ductile deformation structures

Question 30

parts of a fold

Answer 30

- limbs; the sides of the fold that have less curvature - hinge: a line along which the curvature of the fold is greatest - axial surface: an imaginary plane that contains the hinge lines of successive layers and effectively divides the fold into two halves

Question 31

types of folds

Answer 31

- anticlines, syncline, monoclines - non-plunging and plunging folds - domes and basins

Question 32

anticlines

Answer 32

folds with an arch-like shape: limbs dip away from the hinge

Question 33

synclines

Answer 33

folds with a trough-like shape; limbs dip "toward" the hinge

Question 34

monoclines

Answer 34

looks like a carpet draped over a stair-step

Question 35

plunging vs. non-plunging folds

Answer 35

plunging: folds that do not have a horizontal hinge; the hinge is tilted/plunges into the surface of the earth non-plunging: hinge is horizontal/flat

Question 36

domes vs basins

Answer 36

both ciruclar dome: a fold with the shape of an overturned bowl; oldest layer at the center basin: a bowl shaped (right-side up) fold; youngest layer at the center

Question 37

Stress causes _____, _______ results in ________.

Answer 37

Stress causes strain, strain results in structures.

Question 38

mylonite

Answer 38

cohesive foliated rock; produced by tectonic reduction of grain size through crystal plastic processes in restricted areas of intense deformation.

Question 39

triangular facets

Answer 39

occur at normal faults (extension) and at mountain fronts; mountains move up, valley move down

Question 40

what are salt "tectonics"?

Answer 40

- any type of deformation involving salt - salt beds become buried by other rock beds - salt is less dense, behaves buoyantly, rises toward the surface - intrudes overlying rocks creating faults and folds

Question 41

diapir

Answer 41

a domed rock formation in which a core of rock has moved upward to pierce the overlying strata