Deformation & Mountain Building

Question 1

Stress

Answer 1

Forces acting on the surface of a material

Question 2

Deformation

Answer 2

Change in a rock’s position, shape, and/or volume from the application of stress

Question 3

Strain

Answer 3

The measure of deformation (the change in shape or volume that resulted from applied stress)

APPLYING STRESS = STRAIN

Question 4

What are the 3 main types of stress?

Answer 4

Tensional (aka Extensional), Compressional, and Shear

Question 5

Tensional Stress

Answer 5

A pulling motion that acts outward from the rock, causing it to become longer and thinner
Occurs at divergent plate boundaries and often leads to normal faults

Question 6

Compressional Stress

Answer 6

A pushing or squishing motion that acts inward on a rock, causing it to become shorter and thicker
Occurs at convergent plate boundaries and often leads to reverse faults

Question 7

Shear Stress

Answer 7

Forces acting parallel but in opposite directions, causing the rock to become tilted
Occurs at transform plate boundaries

Question 8

What are the different types of Deformation (or results of stress)? What determines which type a rock will go through?

Answer 8

Elastic, Plastic/Ductile, and Brittle
The type of deformation is controlled by the rock type, composition, temperature, pressure, and time

Question 9

Elastic Deformation

Answer 9

A temporary change in position, shape, or volume
The rock will return to its original shape once the stress is removed
Like Elastic Rebound in earthquakes

Question 10

Plastic/Ductile Deformation

Answer 10

Bending, flowing, and/or smearing of rock in a way that does not return to its original shape once stress is removed

Question 11

Brittle Deformation

Answer 11

A rock that breaks, cracks, or fractures under stress and does not return to its original shape once stress is removed

Question 12

Are there some materials that can undergo all types of deformation?

Answer 12

Yes, metal is a good example of this

Question 13

How does intensity of pressure affect deformation?

Answer 13

Some rocks are brittle at low pressure, but ductile at high pressure

Question 14

What is Strike?

Answer 14

The compass direction that the rock is facing (based on an imaginary horizontal line/plane that the geologic structure falls on)

Question 15

What is Dip?

Answer 15

The angle of the geologic feature or rock relative to the Strike (or horizontal plane)

Question 16

How do we classify rock fractures?

Answer 16

Based on the amount of movement along the fracture
Joints vs Faults

Question 17

Joints

Answer 17

Fractures with no appreciable movement
Can occur in columns due to weathering (this is not the same as cleavage) or onion-like cracks due to the expansion or uplifting of igneous rocks

Question 18

Faults

Answer 18

Fractures with significant movement
Hanging Wall vs Foot Wall

Question 19

What is Slip?

Answer 19

The direction of motion along a fault, divided into Dip-Slip and Strike-Slip

Question 20

Dip-Slip faults

Answer 20

Movement occurs vertically in line with the dip of the fault (i.e. there will be an angle)
There are two types: Normal (hanging go down) and Reverse (hanging go up)

Rarely occur in isolated instances, usually happen in a series

Question 21

Strike-Slip faults

Answer 21

Movement occurs horizontally parallel to the strike
There are two types: Right-Lateral (appears to move right) and Left-Lateral (appears to move left)

Usually occurs at transform boundaries

Question 22

What if we bend a rock instead of breaking it (ductile instead of brittle)?

Answer 22

We get folds, which have two main parts: the hinge and the axial plane

Question 23

Fold Hinge

Answer 23

The point or line that joins the two areas of greatest curvature (the very top or bottom of the curve)
Think door hinge - where the bend occurs

Question 24

Axial Plane

Answer 24

The imaginary surface or line that divides the fold into two halves along the hinge

Question 25

What are the different kinds of fold geometry?

Answer 25

- Symmetric: same angle on both sides (stress applied equally) - Asymmetric: different angles (stress applied more strongly in one direction over the other) - Horizontal: the hinge has no slope - Plunging: the hinge in dipping

Question 26

What are the types of Folds?

Answer 26

Anticline and Syncline (can be either horizontal or plunging) We also see Domes and Basins

Question 27

Anticline Folds

Answer 27

Arches upward, with the oldest rock at the center of the fold (when looking at it from the surface) Makes an angry face

Question 28

Syncline Folds

Answer 28

Arches downward, with the youngest rocks at the center Makes a happy face

Question 29

What shape does plunging anticlines and synclines make?

Answer 29

A zigzag

Question 30

What are Domes?

Answer 30

Broadly circular bulge caused by the rock being pushed upwards from below Rocks dip away from the summit like a hill Youngest layer will be the top one, unless the bulge has been weathered away, in which case, the oldest layer will be the one at the very center of the summit

Question 31

What are Basins?

Answer 31

Broadly circular bowl-shaped structure Rocks dip towards the center The youngest layer will be the one at the center of the bowl

Question 32

What are the 4 main ways to create a mountain?

Answer 32

Hot Spots, Mid-Ocean Ridges, Subduction, and Collision

Question 33

Hot Spot Mountains

Answer 33

Will be made of dark mafic rock Ex: Hawaii

Question 34

Mid-Ocean Ridge Mountains

Answer 34

Taller at center and gets deeper further away (the center is younger and less dense, further = older and more dense)

Question 35

Subduction Mountains

Answer 35

Magma is generated at subduction zones, which may build mountains and volcanoes (though they are usually smaller) Since the subducting crust will be oceanic, the lava will be mafic Ex: the Andes

Question 36

Collision Mountains

Answer 36

Categorized based on size and material colliding: continents crashing into either small crustal block or other continents

Question 37

Continent & Crustal Block Collisions

Answer 37

Produces small mountains like terranes or accreted terranes (which are just multiple crustal blocks plastered together) Ex: Western North America

Question 38

Continent & Continent Collisions

Answer 38

Produces the tallest mountains, such as the Himalayas (which are the youngest + highest and are still colliding) and the Appalachians (which used to be bigger and were originally formed during the creation of Pangea)

Question 39

Why did India remain relatively unchanged when the plate collided with Asia, while the Asian interior deformed significantly?

Answer 39

Rocks in the Asian interior were composed of younger crustal sediments, which are weaker and deform/fault much easier than the igneous rock of India