1.5 Deformational Structures

Question 1

What law can be used to explain how rocks are deformed?

Answer 1

Hooke’s law - stress is proportional to strain

Question 2

What is stress?

Answer 2

The force applied to a rock

Question 3

What is strain?

Answer 3

The change in shape of a rock

Question 4

What will the change that will take place in a rock under stress depend on?

Answer 4

• the amount of stress applied to the rock
• the type of stress applied to the rock
• the time over which the stress is applied
• the temperature at which the stress is applied
• the type of rock stress is applied to

Question 5

What are the 3 main types of stress?

Answer 5

• tensional stress
• compressional stress
• shear stress

Question 6

What is tensional stress?

Answer 6

Force when the rock is being pulled apart

Question 7

What is compressional stress?

Answer 7

Force when the rock is being pressed together

Question 8

What is shear stress?

Answer 8

Force when the rock is being sheared by opposing movements either side

Question 9

Is elastic deformation common in rocks?

Answer 9

No

Question 10

When might elastic deformation occur in rocks?

Answer 10

With a rebound effect in earthquakes

Question 11

What is elastic deformation?

Answer 11

When an object changes shapes and then returns to its original shape

Question 12

When does brittle deformation occur?

Answer 12

When rock is cold and cracks

Question 13

What features can brittle deformation form?

Answer 13

Joints and faults

Question 14

When does plastic deformation occur?

Answer 14

When the temperature of a rock is increased and can bend to form folds

Question 15

What feature can plastic deformation form?

Answer 15

Folds

Question 16

What are competent rocks?

Answer 16

Strong rocks

Question 17

What are incompetent rocks?

Answer 17

Weak rocks

Question 18

Examples of competent rocks?

Answer 18

Granite, sandstone, limestone

Question 19

Examples of incompetent rocks?

Answer 19

Shale, mudstone

Question 20

What are faults?

Answer 20

Fractures in the upper part of the crust, where movement has taken place

Question 21

What type of deformation are faults a result of?

Answer 21

Brittle

Question 22

What is the dip of a fault?

Answer 22

The angle of the fault plane measured from the horizontal

Question 23

What is the throw or downthrow of a fault?

Answer 23

The amount of vertical movement along the fault plane

Question 24

What is the hanging wall of a fault?

Answer 24

The block positioned over the fault

Question 25

What is the footwall of a fault?

Answer 25

The block positioned below the fault

Question 26

What is a normal fault?

Answer 26

When the hanging wall is downthrown

Question 27

What is a reverse fault?

Answer 27

When the hanging wall is upthrown

Question 28

What are the types of dip slip faults?

Answer 28

Normal and reverse

Question 29

What are the types of fault?

Answer 29

Dip slip and strike slip

Question 30

Where do folds form?

Answer 30

Where rocks are taken deeper into the crust and bent and buckled by compressive forces

Question 31

What type of deformation is involved when rocks are folded?

Answer 31

Plastic or ductile

Question 32

What is an antiform?

Answer 32

A fold in the rock that folds upwards (sad face)

Question 33

What is a synform?

Answer 33

A fold in the rock that folds downwards (happy face)

Question 34

What will happen to a fold's axial plane when it is symmetrical?

Answer 34

The axial plane will be vertical

Question 35

What will happen to a fold's axial plane when it is asymmetrical?

Answer 35

The axial plane will be tilted at a lower angle

Question 36

If a symmetrical fold has resulted in an antiform, where will the greatest compressive force be coming from?

Answer 36

The side with the gentlest limb

Question 37

If a symmetrical fold has resulted in a synform, where will the greatest compressive force be coming from?

Answer 37

The side with the steepest limb

Question 38

What happens when the forces creating a synform/antiform become greater?

Answer 38

It may lead to one limb being overturned - creating overfold

Question 39

How is an overfold formed?

Answer 39

When the force on an antiform/synform becomes greater and leads to one limb being overturned`

Question 40

How can the pattern of beds on the top surface a plan diagram tell you the type of fold?

Answer 40

* narrow single band in middle → antiform | * thick single band in middle → synform

Question 41

What is a thrust fault?

Answer 41

When some folds are pushed over so much that their axial planes are almost horizontal, and may snap along the axial plane

Question 42

Why is geotechnical investigation carried out on all major construction sites?

Answer 42

To determine if the foundation material is strong enough for what is being constructed

Question 43

What should be considered when seeing if a rock is strong enough to be built on?

Answer 43

* ground surface - signs of instability? * soil - high porosity? * weathered bedrock * jointing and faults

Question 44

What might foundation material be liable to?

Answer 44

* compaction - clay affected particularly * swelling - expansion of clays and other materials when they absorb water * rotational slip or sliding - rocks prone to shearing may be affected

Question 45

Rocks susceptible to compaction, swelling and rotational slip?

Answer 45

* mudstone, siltstone and shale - compaction * clay - rotational slip * sliding and rotational slip - glacial till/boulder clay

Question 46

Ways to make slopes more stable?

Answer 46

* efficient drainage - increase stability * form loose materials into shallow slopes by planting vegetation * retaining walls - prevent rock slides * gabions - prevent rock slides * rock bolts to tie loose blocks into the firmer rock - prevent rock falls

Question 47

Ways to make tunnels more stable?

Answer 47

* creating an arch or cylinder shape to tunnel - withstand pressure * reinforcement - prevent ground swelling from below and closing tunnel * support sides - e.g. rings, arches and shotcrete to prevent collapse * good drainage - reduce pressure of water on tunnel sides

Question 48

What is a good example of how an engineering project was carried out in difficult conditions?

Answer 48

The Channel Tunnel - crossrail in London - clay and chalk

Question 49

What are the types of dam?

Answer 49

* embankment * arch * gravity * buttress

Question 50

Where are embankment dams used?

Answer 50

Where foundation is weak, but there are large volumes of material available

Question 51

How does an embankment dam resist water?

Answer 51

Sheer size of the dam resists pressure of water

Question 52

Where are arch dams constructed?

Answer 52

In narrow valleys with strong bedrock either side

Question 53

How is an arch dam kept in position against the valley walls?

Answer 53

Its curved shape

Question 54

What are gravity dams formed out of?

Answer 54

One giant concrete block

Question 55

Why is less concrete needed in a buttress dam than a gravity dam?

Answer 55

The concrete slabs are supported by the buttresses (arms projecting out of the dam) leaving a space in the centre

Question 56

What type of force are normal faults formed by?

Answer 56

Forces of tension

Question 57

What type of force are reverse faults formed by?

Answer 57

Forces of compression

Question 58

Why are normal faults more common than reverse faults?

Answer 58

Compression is more likely to cause folding than to produce (reverse) faulting