Geology: Stress and Deformation

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Questions and Answers

What does the term 'strike' refer to in structural geology?

The angle of elevation of an inclined plane.

The compass direction of a line formed by the intersection of an inclined plane and the horizontal plane. (correct)

The depth at which rocks deform.

The speed at which geological changes occur.

How is 'dip' defined in the context of geologic structures?

The total thickness of sedimentary layers.

The angle of inclination of a geological feature down from the horizontal. (correct)

The maximum angle of inclination of a rock layer.

The horizontal distance from the base to the top of a formation.

Which statement about ductile materials is correct?

They can only deform elastically at low temperatures.

They can undergo significant deformation without breaking at high temperatures. (correct)

They exhibit a small region of ductile behavior before fracture.

They do not change shape under any stress condition.

What primarily influences ductile deformation in rocks?

The presence of significant water content. Signup and view all the answers

Which type of strain is characterized by a temporary or elastic deformation under stress?

Elastic strain Signup and view all the answers

At what depth do rocks typically start to deform in a ductile manner due to increasing temperature and pressure?

15 km below the surface Signup and view all the answers

What is indicated by the 'orientation of a geologic structure'?

The spatial relationship between various rock layers and geological features. Signup and view all the answers

What is a characteristic feature of brittle materials under stress?

They have a small region of ductile behavior before fracture. Signup and view all the answers

What type of fault occurs when the block above the fault moves down relative to the block below?

Normal Fault Signup and view all the answers

Which type of strike-slip fault has the block on the opposite side moving toward the right?

Right-Lateral Strike-Slip Fault Signup and view all the answers

Which characteristic is associated with a brittle behavior in rocks?

Low temperature and low confining stress Signup and view all the answers

What is the major distinguishing factor of a thrust fault compared to other types?

It is a shallow-angle reverse fault. Signup and view all the answers

In general, what produces joints in rocks?

Brittle deformation Signup and view all the answers

Which type of fault involves movement not strictly vertical or horizontal but at an angle?

Oblique Slip Fault Signup and view all the answers

What does the direction of dip signify in geological terms?

It is perpendicular to the strike. Signup and view all the answers

What causes the formation of folds in geological materials?

Ductile deformation Signup and view all the answers

Which type of fold has limbs that incline away from the hinge and features the oldest beds at its core?

Anticline Signup and view all the answers

What is the characteristic feature of a syncline?

It is concave upwards Signup and view all the answers

What describes a monocline?

Two horizontal limbs with a shorter inclined segment Signup and view all the answers

In which type of fault does the axial plane steeply incline, causing one limb to be higher than the other?

Overturned Signup and view all the answers

What is the process by which minerals change their shape and size without altering their identity?

Recrystallization Signup and view all the answers

Which process involves transformation where one mineral changes into another with the same composition but a different crystal structure?

Phase Change Signup and view all the answers

What type of geological structure features limbs that are neither entirely horizontal nor vertical?

Monocline Signup and view all the answers

What type of deformation occurs when minerals become elongated without changing their composition?

Plastic Deformation Signup and view all the answers

Study Notes

Stress and Deformation

Stress is a force acting on a rock, often from tectonic plates shifting or the weight of overburden.
Strain is the deformation of a rock in response to stress.
Elastic Deformation: Temporary, recoverable shape change.
Ductile Deformation: Permanent, irreversible shape change.
Brittle Deformation: Causes fracturing when the rock cannot deform any further.

Types of Stress

Compressional Stress: Forces directed towards each other, causing shortening and thickening.
Tensional Stress: Forces directed away from each other, causing elongation and thinning.
Shear Stress: Forces acting parallel to each other but in opposite directions, causing twisting and sliding.

Factors Affecting Rock Deformation

Temperature: Higher temperatures favor ductile deformation, while lower temperatures favor brittle deformation.
Confining Pressure: Higher confining pressure favors ductile deformation by confining the rock and preventing fracturing.
Strain Rate: A rapid strain rate favors brittle deformation, while a slow strain rate favors ductile deformation.
Water: Water can act as a lubricant, favoring ductile deformation.

Types of Strain

Shear Strain: Occurs when a sideways force is exerted on a medium, leading to a change in angles between features.
Elastic Strain: A temporary change in shape or size due to stress, but the rock returns to its original form once the stress is removed.

Materials and Deformation

Ductile Materials: A larger region of ductile behavior before fracturing.
Brittle Materials: Have a smaller region of ductile behavior before fracturing.

Structural Geology

Structural Geology: The branch of geology concerned with the study of rock deformation.
Orientation of a Geologic Structure: Described using strike and dip
- Strike: The compass direction of the line formed by the intersection of an inclined plane and the horizontal plane.
- Dip: The angle between the inclined plane and the horizontal plane.

Joints

Natural cracks in rocks produced by brittle deformation.
Rocks on either side of a joint don't slide past one another.
Formed by tensional stress in brittle rocks.

Faults

Planar structures resulting from brittle deformation.
Involve the sliding of rocks along a fault plane.

Types of Faults

Inclined Fault: A fault plane that is inclined relative to the horizontal.
- Hanging Wall: The block of rock above the fault plane.
- Footwall: The block of rock below the fault plane.
Normal Fault: The hanging wall moves down relative to the footwall. These are formed from tensional stress.
Reverse Fault: The hanging wall moves up relative to the footwall. These are formed from compressional stress.
Thrust Fault: A type of reverse fault with a dip of 45 degrees or less, moving similarly to a steeper angle reverse fault.
Strike-Slip Fault: Blocks of rock slide past each other horizontally. Shear stress develops along the fault.
- Right-Lateral Strike-Slip Fault: The block on the opposite side moves toward the right.
- Left-Lateral Strike-Slip Fault: The block on the opposite side moves toward the left.
Oblique-Slip Fault: A combination of both dip-slip and strike-slip movement.

Folds

Produced by the deformation of ductile materials due to compressional stress.
Contortions of rock layers forming wave-like curves.

Parts of a Fold

Hinge Line/Fold Axis: The point of greatest curvature.
Limbs: The sides of the folds with the least curvature.
Axial Plane: The plane that bisects the fold and contains the fold axis.

Types of Folds

Anticline: Limbs incline away from the hinge.
- Oldest beds in the core of the fold and convex upwards.
Syncline: Limbs incline towards the hinge.
- Youngest beds in the core of the fold and concave upwards.
Monocline: A bend in a flat-lying rock layer.
- Consists of two horizontal limbs connected by a shorter inclined limb.
Overturned Fold: The axial plane is inclined, and one limb is steeper than the other.

Metamorphism

The process of change in a rock's shape, size, mineral composition, and texture without melting.
Recrystallization: Occurs when minerals change their shape and size without changing their composition.
Phase Change: One mineral transforms into another mineral with the same composition but a different crystal structure (example: quartz changing into coesite (both SiO2)).
Neocrystallization: New minerals grow and form inside the rock that are different from the original minerals.
Pressure Solution: Minerals dissolve and re-precipitate at points of higher pressure in the rock, commonly occurring at relatively low temperatures.
Plastic Deformation: Minerals may become flattened or elongated without changing their composition or crystal structure.

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Description

This quiz covers the concepts of stress and deformation in geology, including types of stress and their effects on rocks. You will learn about elastic, ductile, and brittle deformation and how temperature and pressure influence these processes.