Module 4 - Deformation of Rocks Flashcards
1
Q
What is a bed?
A
A unit of sedimentation, which can vary considerably in thickness
2
Q
What is a bedding plane?
A
It marks the break between the beds. It represents a break in sedimentation, a change in the composition or grain size, or a change in the colour of sediments
3
Q
What is the dip?
A
It is the maximum inclination of a bed measured from the horizontal using a clinometer
4
Q
What is a strike?
A
The direction at right angles to the dip. There is zero dip in this direction. It’s measured from North using a compass
5
Q
What is the apparent dip?
A
A dip that is measured to be less than the maximum inclination.
6
Q
What are several beds known as?
A
Strata
7
Q
How will beds be different from each other?
A
- colour - changes in shade or major colour change
- grain size - fine clay / coarse pebbles / fine sand / medium sand
- sorting - well sorted - poorly sorted
- composition - contain different amounts of minerals
- bed thickness - thin (laminations) / thick (massive)
8
Q
How are beds often tilted or folded?
A
By the Earths movements so that they are no longer horizontal. It can help show where a bed is younger
9
Q
When is the term stress applied?
A
It is used to describe the forces applied to a rock by Earths movements or the weight of overlying rocks
10
Q
When is the term strain used?
A
It is used to describe the deformation caused by the applied forces
11
Q
How can you calculate strain?
A
Strain = the change of length of line / the original length of the line
It’s the change in volume or shape
12
Q
What is stress like in competent rocks?
A
They stay at the same thickness when they are deformed and react in a brittle way. Examples include sandstone, limestone and most igneous rocks
13
Q
What is stress like in incompetent rocks?
A
It will vary in thickness when they are deformed, as they behave in a plastic way. Examples include mudstones and shales
14
Q
What is the definition of stress?
A
The force applied to rocks
15
Q
What are the factors affecting stress and strain?
A
•the higher the temperature, the more plastic like the rock will be. Rocks will fold not fault when hot but when cold will act brittle and therefore fracture
•the strength of the rock increases
under pressure
•time can determine the type of deformation
-short = brittle
-long = plastic like
16
Q
Competent rock signs
A
- brittle
- joints/faults formed
- little or no plastic deformation before rupture
17
Q
Incompetent rock signs
A
- plastic/ductile
- rocks folds easily
- show cleavage and evidence of plastic flow
18
Q
Effect of increased temperature?
A
Decreases the strength of the rock
19
Q
Effect of increasing confining pressure?
A
Raises the strength of the rock
20
Q
What do tensional forces often result in?
A
The fracturing of rocks and crystal extension. If the forces result in a displacement of the rocks on either side of the fracture plane, then the structure is usually described as a fault. If there is no displacement it’s classed as a joint
21
Q
What do compressional forces often result in?
A
The fracturing or folding of rocks. When cold they tend to fracture and faults are formed. If they are warm they will more likely fold. Either way there is a shortening of the Earths crust
22
Q
What do shear forces often result in?
A
Deformation of rocks in one plane, usually horizontally these may result in faults or folds
23
Q
How can you use fossils or ooliths to measure strain?
A
You can use once symmetrical fossils or ooliths. Measure the amount and direction of deformation.
24
Q
What is tension?
A
The force trying to pull rocks apart - the Earths crust will be lengthened
25
What is compression?
The force trying to push rocks together - the Earths crust will be shortened
26
What are shear forces?
The forces which act along a plane in the rock and promote sliding along that plane
27
What are competent rocks?
They are strong and brittle and tend to joint and fault
28
What are incompetent rocks?
They are weak and plastic and tend to fold and develop cleavage
29
What do unconformities represent?
A break in time - a period in which no sediment is deposited
30
When do angular unconformities occur?
Where the rocks above the unconformity have a different dip, and possibly strike to the rocks beneath the plane of the unconformity
31
How does an unconformity form?
1. Deposition if bed rock with oldest at the bottom and youngest at the top
2. Beds of rock are tilted by the earth movements as a result of compressive forces folding the rocks
3. Erosion of older bed rocks
4. Deposition of younger overlying bed rock. The eroded surface of the older rock is the unconformity.
5. Uplift and erosion expose the unconformity
32
What is a joint?
A fracture in competent rocks along which no observable movement has occurred
33
Which forces cause joints and what can these forces result in?
Tensional forces - can result in folding, cooling or unloading of rocks. They give rocks a blocky appearance
34
What are tectonic joints?
They form as a result of folding and cause tension joints parallel to the axial plane and cross joints on the limbs
35
What type of joints are caused by folding?
Tectonic forces - they are caused by the tension when they are folded. Compressive forces fold the rock and cause the rock to be under tension. The rocks fracture due to the outer surface being stretched more than the inner surface
36
What are the two common types of tectonic joints?
Tension joints - parallel to the axial plane trace of the fold
Cross joints - at an angle to the axial plane trace of the fold
37
How do cooling joints form?
As a result of contraction on cooling of igneous rocks
38
How are cooling joints caused?
By tensional forces that are formed when an igneous rock cools and contracts. The joints are perpendicular to the cooling surfaces of thick basalt lava flows, dykes and sills and so are often vertical. As the rock cools and crystallises it shrinks slightly are results in hexagonal like columns
39
What are unloading joints like?
Often horizontal as well as vertical and form as a result of lower pressure near to the surface
40
Where do unloading joints often form?
In granite, an igneous rock cooled deep below the surface where pressure is high. The weight of the overlying rocks compressed the rock (load pressure) when the granite is exposed the lack of load pressure from overlying rocks allows them to expand. They form roughly parallel to the Earths surface
41
What is a fault?
A fracture in a rock along which there has been an observable amount of displacement. They are very common and range from a few centimeters to hundred of kilometers. They are formed by tension, compression or shear forces
42
How can faults be described?
* Fault plane
* Throw
* Fault dip
* Hanging wall
* Foot wall
* Upthrow
* Downthrow
43
What is a fault plane?
The surface along which movement takes place
44
What is throw?
The amount of vertical displacement measured vertically between the top of the same bed seen on both sides of the fault
45
What is a fault dip?
The angle between the fault plane and the horizontal
46
What is the Hanging wall?
The side that lies above the fault plane
47
What is the footwall?
The side that lies below the fault plane
48
What is upthrow?
The side of the fault that had moved up
49
What is the downthrow?
The side of the fault that had moved down
50
What are the two types of Fault?
Dip slip faults - large vertical movement
| Strike slip faults - large horizontal movement
51
What are the two types of dip slip faults?
Normal faults and reverse faults
52
What is a normal fault?
The crust is being stretched and close to the surface, the rocks are cold and brittle, they fracture to form a normal fault. The hanging wall is the down throw side. This is the result of tensional forces. The Down throw side of the fault sinks due to gravity
53
What is a graben?
Where two normal faults face each other, that is dip towards each other, then a graben or Rift Valley. The graben is the area that forms the downthrow between the two faults
54
Where is a Horst formed?
Where two normal faults face away from each other, that is dip away from each other. The Horst is the block uplifted between two faults
55
How are reverse faults formed?
Formed from compressional forces. In a reverse Fault the hanging is the upthrow side. They can be recognized from the overlap in the strata created by Fault movement.
56
What are thrust faults?
A type of reverse Fault where the dip is less than 45 degrees. Formed from compressional forces and often associated with major fold mountain systems and have displacements that can be measured in km.
57
What are strike slip faults?
They are faults where the fault plane is vertical and the movement along the fault is horizontal. Often large scale structures with large displacements.
58
What are the two main types of strike slip faults?
Tear and transform faults
59
What is a tear Fault?
The result of shearing forces applied to the rocks.
60
How can you describe a tear Fault?
With the terms dextral and sinistral. They are reflected in the sense of movement along the fault plane.
61
What does it mean if the fault is dextral?
If the block of the rock on the opposite side has moved to the right
62
When is the fault described as sinistral?
The rock opposite has moved to the left
63
What are transform faults associated with?
They are often associated with Plate margins (conservative) they are often at right angles to a mid ocean ridge. They are the results of different rates of movement within a plate and allow the rigid plates to adjust for these differences in the rate of movement
64
What are slickensides?
They are scratch marks found on fault planes. They are left from the grinding of pieces of rocks along the fault plane as the Fault moves. They show the direction of the last movement along the fault plane
65
What is fault breccia?
It's a type of rock found along Fault planes and consists of fragments broken from the rocks on either side of the fault plane. They are large and angular and made of hard, competent rock
66
What is the definition of slickensides?
The polishing and striations found on a fault plane indicating the direction of the relative movement
67
What is the definition of fault breccia?
It is composed of fragments produced by rocks fracturing during faulting
68
What is a fold?
A flexible in rocks, where there is a change in the amount of dip of a bed. They are all produced by compressive forces acting horizontally and so usually formed at convergent plate margins
69
What can happen if the stress is applied slowly and continuously over a long period of time?
Hard, Brittle rock may fold and not break.
70
What are the two categories of folds?
* upfold - anticlimax or an anti form. The dips are pointing outwards. Anticline if you know the age of the rocks
* downfold - synclinal or a synform with dips pointing inwards. Syncline if the age of the rocks are known
71
What is a fold limb?
The section of a fold between 1 hinge and the next, forming the side of a fold
72
What is a hinge?
The line along which there is a change in the amount and/or direction of dip, forming the most sharply curved part of the fold
73
What is a crest?
The highest point of a folded bed
74
What is a trough?
The lowest point of a folded bed
75
What is an axial plane?
A plane that joins the hinges of all the beds, it bisects the fold
76
What is an axial plane trace?
The outcrop of the axial plane at the Earths surface
77
What is a plunge?
The angle of dip of the gold axis from the horizontal
78
What is an antiform?
An upward closing fold
79
What is a synform?
A downward closing fold
80
How can you describe folds?
```
Gentle - 180-120 degrees
Open - 120-70 degrees
Closed - 70-30 degrees
Tight - angle less than 30 degrees
Upright - the axial plane is vertical
Inclined - the axial plane dips
Symmetrical - symmetrical about the axial plane
Asymmetrical - not symmetrical about the axial plane
```
81
What is a dome/basin?
A circular or oval geological feature that dips in all directions.
82
What's the difference between a done and a basin?
Domes have beds that dip outwards in all directions and basins have beds that dip inwards in all directions
83
What would happen if the top of the dome is eroded away?
The result will be a series of concentric strata with the oldest rocks exposed at the centre
84
Where is the youngest rock in a basin?
In the centre
85
Where is the youngest rock in a dome?
The outer side
86
What are over folds and how did they form?
They have both fold limbs dipping in the same direction but by different amounts. They form due to compressional forces
87
What are the limbs like in an overfold?
One is the correct way up whilst the other is inverted. This can be identified by observing way up structures in the rocks.
88
What method can you use to determine which way up the fold limb is?
Graded bedding. The heaviest particles are at the bottom of the bed
89
What are recumbent folds?
They are a step further than overfolds. They are formed from compressive forces, in this case, the axial plane of the gold are close to horizontal and always less than 30 degrees
90
What type of rock usually creates recumbent folds?
Incompetent or plastic rocks able to absorb high levels of stress. They are generally features of fold mountains
91
How can you recognize a recumbent fold?
Both the limbs and the axial plane will be at very low angles and one limb of the recumbent fold is inverted
92
What are nappes?
They are some of the worlds largest tectonic features. They are huge recumbent folds that have broken along thrust planes. Movement along the thrust plane is horizontal and part of the fold has been moved forward along the thrust.
93
What are Isoclinal folds?
They are parallel limbs that are nearly vertical and very tight folds. The axial planes are usually vertical
94
What is cleavage?
A feature of rocks containing Platy minerals (clay minerals and mica with long thin shapes). When they align, they form a series of planes along which the rock tends to split
95
What is the definition of cleavage?
A plane of weakness in a rock along which the rock will split
96
What type of rock does cleavage form in?
Incompetent sedimentary rocks such as shale or clay made from clay minerals
97
What does incompetent mean?
That the rocks are plastic and so not fracture
98
Where does cleavage form in terms of pressure?
Perpendicular to the maximum pressure that forms a fold, so it's at an angle to the bedding planes
99
What's the origin of cleavage?
1. The rocks containing clay mineral are heated and compressed so that the minerals begin to recrystalise
2. They usually recrystalise as micas or minerals with a distinctive platy form
3. The recrystalisation causes the rock to metamorphose into slate
4. They usually form in growing stress environments and so form in the direction of the least resistance
5. The platy minerals all form parallel to each other
100
How can you recognize bedding in photos or diagrams?
Difference in colour or composition between beds, grain size between the beds, structures within the beds and along the bedding planes
Occurs in all rock types
101
How can you recognize jointing in photos or diagrams?
Fractures where there is no displacement, perpendicular to bedding, possible mineralization along the joints
Only in competent rocks
102
How can you recognize cleavage in photos or diagrams?
Rocks split easily along parallel planes making slates, they are often parallel to the axial plane of the folds, the planes are often close together (mm)
They only develop in incompetent rocks, such as shales
103
How can you use cross cutting relationships to deuce the age relationship?
1. The first event is the lower sequence of rocks. The pebbly sandstone is laid down first followed by limestone and shale
2. Beds are folded into a synclinal fold
3. The area is eroded leaving an uneven top
4. The conglomerate was laid down horizontally on top of the eroded sandstone
5. The sandstone (youngest rock) was laid down
6. The area was tilted to the southeast, followed by more erosion
The order of events can help to determine the order of the rocks and which event came first
104
How can you identify folds on maps?
* if the dip arrows point onwards, the youngest bed are in the centre - syncline
* if they point outwards the oldest rock is in the centre - anticline
105
How can you recognize strike-slip faults from outcrop patterns on maps?
* if the axial plane of the folds is displaced, horizontal movement must have taken place - also true if a vertical dyke is displaced by the fault
* if the outcrop width of the beds on either side of the fault is the same, no vertical movement has taken place
106
How can you determine upthrow and downthrow of faults on maps?
* downthrow side - younger rocks (usually)
* if a fault displaces the axial plane trace of a syncline, the width of the centre bed of the outcrop will be wider on the downthrow side
* if a fault displaces the axial plane trace of an anticline, the width of the centre bed of the outcrop will be narrower on the downthrow side
