Geology Chapter 10: Rock Deformation

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Questions and Answers

What primarily determines whether a rock will undergo ductile or brittle deformation?

  • The age of the rock and its geological history
  • The temperature and pressure conditions at depth (correct)
  • The mineral composition and strength of the rock
  • The time over which forces are applied (correct)

At what depth is ductile deformation more likely to occur?

  • At any depth depending on the mineral composition
  • At medium depths with moderate pressure
  • At great depths where temperature and pressure are higher (correct)
  • At shallow depths where rocks are cooler

Which type of rocks is favored for brittle behavior?

  • Igneous rocks such as granite and basalt (correct)
  • Sedimentary rocks with a high clay content
  • Metamorphic rocks with strong internal bonds
  • Nonsilicate rich rocks like shale

What characterizes a monocline?

<p>It features a single limb formed by vertical displacement along a fault. (D)</p> Signup and view all the answers

Which statement is true regarding the application of force on rocks?

<p>Brittle deformation occurs with rapid application of forces. (D)</p> Signup and view all the answers

In terms of geological formations, what distinguishes a dome from a basin?

<p>A dome exhibits circular upwarping, placing the oldest strata inside. (A)</p> Signup and view all the answers

What is the first step in mapping rock structure?

<p>Identifying the rock type at a surface exposure (D)</p> Signup and view all the answers

Which type of fault is defined by movement parallel to the dip of the fault surface?

<p>Dip-slip fault (A)</p> Signup and view all the answers

What is the distinction between a normal fault and a reverse fault?

<p>A normal fault has downward movement of the hanging wall, while a reverse fault has upward movement. (C)</p> Signup and view all the answers

What occurs in subduction zones?

<p>One tectonic plate is forced beneath another plate. (A)</p> Signup and view all the answers

What characterizes a reverse fault?

<p>The hanging wall moves up relative to the footwall. (B)</p> Signup and view all the answers

What geological feature results from two normal faults?

<p>A horst (A), A graben (C)</p> Signup and view all the answers

Which of the following is true regarding normal faults?

<p>The hanging wall moves down relative to the footwall. (C)</p> Signup and view all the answers

Which statement accurately describes a fault scarp?

<p>It represents the line of the fault on the surface. (B)</p> Signup and view all the answers

In which settings can normal faults commonly be found?

<p>At spreading centers (B)</p> Signup and view all the answers

What type of fault involves horizontal movement with no vertical displacement?

<p>Strike-slip fault (B)</p> Signup and view all the answers

Which process leads to the formation of columnar joints in igneous rock?

<p>Cooling and shrinkage of the rock (B)</p> Signup and view all the answers

How do joints enhance the process of chemical weathering?

<p>By providing pathways for water and chemicals (D)</p> Signup and view all the answers

What is a significant consequence of seepage along joints in a dam structure?

<p>Risk of failure and erosion (C)</p> Signup and view all the answers

Which of the following best describes the formation of exfoliation domes?

<p>Occurring due to unloading of overlying rock (C)</p> Signup and view all the answers

What is the relationship between strike and dip in geological formations?

<p>Dip is always perpendicular to the strike. (A)</p> Signup and view all the answers

In a geologic map, the strike reported as N60oE indicates what?

<p>The strike is 60 degrees east of north. (A)</p> Signup and view all the answers

What characterizes an anticline in geological terms?

<p>Older rocks are found on the inside. (A)</p> Signup and view all the answers

Which of the following describes a syncline?

<p>It is characterized by a trough of downfolded layers. (C)</p> Signup and view all the answers

What defines a plunging anticline?

<p>Its hinge line dips into the ground. (C)</p> Signup and view all the answers

What is the primary force responsible for the formation of anticlines and synclines?

<p>Compressional forces (D)</p> Signup and view all the answers

Which part of a folded rock refers to the imaginary surface that divides the fold as symmetrically as possible?

<p>Axial plane (C)</p> Signup and view all the answers

What features differentiate symmetrical anticlines from asymmetrical ones?

<p>Symmetrical anticlines have limbs at the same angle. (A)</p> Signup and view all the answers

What is the main focus of structural geology?

<p>The study of the deformation of rocks (D)</p> Signup and view all the answers

Which type of stress occurs when forces push rock together and reduce its volume?

<p>Compressive stress (B)</p> Signup and view all the answers

What happens to a rock experiencing elastic deformation when the stress is released?

<p>It rebounds to its original shape (D)</p> Signup and view all the answers

Which type of fault is characterized by the rock above the fault plane moving down relative to the rock below?

<p>Normal fault (B)</p> Signup and view all the answers

How does the orientation of folds and faults contribute to economic geology?

<p>It can trap oil and gas deposits. (B)</p> Signup and view all the answers

What type of deformation occurs when a rock bends or flows without breaking?

<p>Ductile deformation (A)</p> Signup and view all the answers

What is the term for the amount of force acting on the rock causing deformation?

<p>Stress (A)</p> Signup and view all the answers

Which of the following describes shear stress?

<p>Forces push toward each other but parallel (A)</p> Signup and view all the answers

Flashcards

Stress

The amount of force acting on a rock.

Strain

The change in shape or volume of a rock due to stress.

Compressional Stress

Forces push rocks together, shortening the rock.

Tensional Stress

Forces pull rocks apart, elongating the rock.

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Shear Stress

Forces push rocks parallel to one another, causing twisting.

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Elastic Deformation

Temporary shape change in a rock due to stress that is not permanent; rock returns to original shape.

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Ductile Deformation

Permanent shape change in a rock due to stress without breaking; flowing or bending.

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Brittle Deformation

Fracturing or breaking of a rock due to stress.

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Depth's Influence on Deformation

Rocks behave differently at different depths. Near the surface, rocks are cooler and experience less pressure, leading to brittle deformation (fracturing). At greater depths, higher pressure and temperature make rocks more pliable, resulting in ductile deformation (folding).

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Rock Type and Deformation

The composition of a rock influences its tendency to deform. Silicate-rich rocks (like granite and basalt) are strong and more likely to fracture (brittle). Non-silicate rich rocks (like shale) are weaker and more likely to fold (ductile).

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Time and Deformation

The speed at which forces act on a rock determines its behavior. Rapid forces lead to brittle deformation (fracturing), while slow, sustained forces result in ductile deformation (folding).

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What is an Outcrop?

An outcrop is a visible exposure of rock at the Earth's surface. It provides a window into the rock layers and structures beneath the ground.

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How to Map Rock Structure

To map rock structure, we need to identify outcrops, determine their rock types, measure the strike and dip of folds, faults, and layers, and plot this information on a geologic map.

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Dip

The angle of inclination of the rock surface from the horizontal plane. Think of it as the path a ball would take rolling down the rock face. Dip is always perpendicular to the strike. It's reported as an angle and direction, for example: 30oE means the dip is 30 degrees in the eastward direction.

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Geologic Map

A map that shows the strike and dip of each rock structure, with different colors representing different rock formations. This map helps us understand the shape and orientation of structures beneath the ground.

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Limb (of a fold)

The two sides of a folded rock layer. Think of them as the slopes of the fold.

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Hinge Line (Axis)

A line drawn along the points of maximum curvature of each layer in a folded rock. It represents the line of greatest bending.

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Axial Plane

An imaginary surface that divides a fold as symmetrically as possible. It's like the plane of symmetry of the fold.

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Plunge (of a fold)

The angle of incline of the hinge line (axis) of a folded rock. It measures how much the axis dips into the Earth.

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Hogback

Steeply inclined, angular ridges formed when folded strata (rock layers) are resistant to weathering and erosion. They form outcrops.

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Hanging Wall

The rock surface located above a fault.

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Foot Wall

The rock surface located below a fault.

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Reverse Fault

A type of fault where the hanging wall moves upward relative to the footwall, compressing the crust.

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Normal Fault

A type of fault where the hanging wall moves downward relative to the footwall, stretching the crust.

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Fault Scarp

A cliff-like feature on the surface that forms along the line of a fault.

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What are subduction zones?

Subduction zones are areas where tectonic plates collide, with one plate diving beneath the other. This process creates deep trenches, volcanic arcs, and earthquakes.

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What is a plunging fold?

A plunging fold is a fold where the hinge line (the line of maximum curvature) dips below the horizontal.

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What is an axial trace?

The axial trace of a fold is the line of intersection between the fold's hinge line and the Earth's surface.

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What is a monocline?

A monocline is a fold with only one limb, usually caused by vertical displacement along a fault.

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What are domes and basins?

Domes are circular upwarps where the oldest rocks are in the center, while basins are circular downwarps where the youngest rocks are in the center.

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Basin and Range Province

A region in the southwestern United States characterized by alternating mountain ranges and valleys formed by extensional forces pulling the crust apart.

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Strike-Slip Fault

A type of fault where the movement is horizontal and parallel to the strike (the line of intersection between the fault plane and the Earth's surface). It doesn't involve vertical displacement.

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Joint

A fracture in rock where there is no noticeable displacement. Essentially, it's a crack with no movement across the crack.

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Columnar Joints

Elongated, pillar-like structures formed in cooling igneous rock due to shrinkage fractures. These structures are perpendicular to the cooling direction.

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Study Notes

Chapter 10: Deformation of Rocks

  • Rocks deform in response to stress, resulting in folding, flowing, and fracturing.
  • Folds and faults are geologic structures.
  • Structural geology studies the deformation of rocks.
  • Stress is the amount of force acting on a rock (tectonic forces).
  • Strain is the shape or volume changes caused by stress (deformation).

Types of Differential Stress

  • Compressional stress: Forces push rocks together, shortening the rock body and reducing its volume.
  • Tensional stress (extensional stress): Forces pull rocks apart, elongating the rock body.
  • Shear stress: Forces push rocks toward each other but parallel to one another.

Types of Deformation

  • Elastic deformation: A temporary change in shape in response to stress. The rock returns to its original shape when the stress is released.
  • Ductile deformation: The rock bends or flows, changing its shape without breaking. This change is permanent.
  • Brittle deformation (brittle failure): The rock fractures and breaks into separate pieces. This change is permanent.

Factors Affecting Ductile or Brittle Deformation

  • Depth: At shallower depths, rocks are cooler and under lower pressure, favoring brittle deformation. Deeper, hotter rocks under high pressure favor ductile deformation.
  • Rock type: Silicate-rich rocks (e.g., granite, basalt) tend to be brittle, while non-silicate rocks (e.g., shale) are more ductile due to weaker internal bonds.
  • Time: Quickly applied forces trigger brittle deformation, while slowly applied forces create ductile deformation.

Mapping Rock Structure

  • Locate outcrops (surface exposures of rocks) to identify rock types.
  • Measure the strike and dip of folds, faults, and rock layers.
  • Plot the strike, dip, and rock type data on a geologic map.

Strike

  • A line produced by the intersection of a horizontal plane on a rock surface.
  • Determined by the path a ball would take if rolled on the rock surface.
  • Reported as an angle relative to north.

Dip

  • The angle of inclination of a rock surface from the horizontal plane.
  • Perpendicular to the strike, representing the path a ball would roll down a rock face.
  • Reported as an angle and a direction (e.g., 30° East).

Parts of a Folded Rock

  • Limb: The two sides of the fold.
  • Hinge line (axis): A line drawn along the points of maximum curvature of each layer.
  • Axial plane: An imaginary surface that divides the fold as symmetrically as possible.

Types of Folds

  • Anticline: Arching of geologic layers with older rocks inside the fold.
  • Syncline: Trough, downfolded layers with older rocks on the outside of the fold.
  • Monocline: A fold with only one limb. Produced by vertical displacement along a fault rather than compression.
  • Dome: Circular upwarping; oldest strata within.
  • Basin: Circular downwarping; youngest strata within.

Faults

  • Faults are fractures with appreciable displacement.
  • Dip-slip faults: Movement parallel to the dip of the fault surface, creating vertical displacement.
    • Normal fault: Hanging wall moves down relative to the footwall (extensional forces).
    • Reverse fault: Hanging wall moves up relative to the footwall (compressional forces).
  • Strike-slip fault: Horizontal movement parallel to the strike.

Features of Faulted Terrain

  • Horst: Uplifted block bounded by two normal faults.
  • Graben: Down-dropped block bounded by two normal faults.
  • Fault-block mountains are produced in normally faulted terrain.

Joints

  • Joints are fractures with no appreciable displacement.
  • Common examples include columnar joints (cooling igneous rocks) and sheeting joints (unloading overlying rock).
  • Important because they enhance chemical weathering, influence stream directions,, and can affect construction projects (like dam failures).

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