Lecture 9: Deformation

Question 1

Define Deformation and where it occurs

Answer 1

any changes in the original form of a rock body subjected to stress; most deformation of earth’s crust occurs along or near margins of lithospheric plates

Question 2

What is strain?

Answer 2

The observed result of stress (revealed by structures produced by deformation)

Question 3

What is stress? (2 types)

Answer 3

1. Uniform Stress: applied to material equally in all directions
2. Differential stress: applied unequally

Question 4

What is differential stress? (3 types)

Answer 4

Compressional stress: squeezing, causes rock bodies to shorten and thicken vertically
Tensional: stretching, lengthen and thin vertically
Shear: slicing, causes rock bodies to change shape laterally

Question 5

What are the 3 types of plate boundaries?

Answer 5

Convergent plate boundary: lithospheric plates move toward one another (compressive stress)
Divergent plate boundary: plates move away from one another (tensional stress)
Transform plate boundary: move past one another (shear stress)

Question 6

What are the 2 ways in which rocks deform?

Answer 6

Brittle Deformation: rocks behave like a brittle solid and break (producing fractures)
Ductile: rocks can “flow” internally and behave in a more plastic manner (tending to bend and fold rather than breaking)

Question 7

What happens when rocks have been deformed?

Answer 7

They can be tilted or bent

Fractures are often inclined

Question 8

What is strike and dip? (Go over how to calculate it in the textbook)

Answer 8

We can recognize a dipping plane

The intersection of the dipping plane with a horizontal plane defines a line of strike

Question 9

What is a joint?

Answer 9

A brittle deformation feature: fractures along which no significant displacement has occurred; “tension” cracks by relatively gentle warping of the crust (ex dolostone)
Can also develop as cooling features (shrinkage cracks) in lava flows and other igneous rock bodies (ex basalt)

Question 10

What is a fault?

Answer 10

A brittle deformation feature: • Fractures along which significant displacement has occurred

Question 11

What are the 2 main types of faults?

Answer 11

Dip slip faults and strike slip faults

Question 12

What are dip slip faults?

Answer 12

“block” of rock above the dipping fault plane is called the “hanging wall” “block” of rock below the dipping fault plane is called the footwall

Question 13

What are the main types of dip slip faults?

Answer 13

normal fault: produced when hanging wall moves down relative to footwall
reverse fault: produced when the hanging wall moves up relative to the footwall

Question 14

Where are normal faults common?

Answer 14

at divergent plate margins where tensional stress predominates, normal faults commonly develop
bounded by normal faults are downthrown blocks called grabens
upthrown blocks called horsts

Question 15

Where are reverse faults common?

Answer 15

at convergent plate margins where compressional stress predominates, reverse faults commonly develop

Question 16

What is a strike slip fault?

Answer 16

The fault is vertical (so there is no hanging wall or foot wall) the blocks move laterally relative to one another
Strike slip faults are right lateral or left lateral depending on the relative movement of the blocks
Example: right lateral strike slip fault…. Looking at the rock from one rock.. it looks as if its moving right regardless

Question 17

What are folds?

Answer 17

A Ductile Deformation Feature: tend to warp, fold or crinkle; consider dip and strike patterns of strata exposed at the surface, we then can deduce their structure under the surface

Question 18

Describe the anatomy of a fold

Answer 18

simple symmetrical fold features:

• an axial plane that divides the fold into mirror-image “limbs”
• a fold axis that defines the direction of the crest of the fold. In a symmetrical fold, the axis is horizontal

Question 19

A symmetrical fold whose fold axis is tilted is said to….

Answer 19

“plunge”. As in “dip”, the plunge of a fold has a plunge angle (relative to the horizontal) and direction (compass direction)

Question 20

What are the main types of folds?

Answer 20

if relative ages of strata are known*, we can identify synclines and anticlines

Question 21

What is a syncline?

Answer 21

(a trough-like structure) strata dip TOWARD centre of fold with OLDEST strata in center

Question 22

What is an anticline?

Answer 22

(a arch-like structure) strata dip AWAY from centre of fold with YOUNGEST strata in centre

Question 23

What is a plunging fold?

Answer 23

When exposed in a horizontal erosion surface, arrowhead shaped patterns emerge
in syncline arrowhead pattern points AWAY from the direction of the plunge
in anticline arrowhead pattern points TOWARD the direction of the plunge

Question 24

What is a basin?

Answer 24

basins are bowl shaped with youngest strata in centre

Question 25

What is a dome?

Answer 25

domes are hump shaped with oldest strata in centre