Crags, Cracks, and Crumples:
Crustal Deformation and Mountain Building
Prepared by

Ronald Parker
Earlham College Departme...
Mountains
 Attractive landscape features for humans.
 Provide beautiful scenery.
 Refuge from the mundane.
 Inspire po...
Mountain Belts
 Mountains frequently occur in elongate, linear belts.
 Mountains are constructed by tectonic plate inter...
Mountains
 Mountain building involves…
 Structural deformation.
 Jointing.
 Faulting.
 Folding.
 Partial melting.
 ...
Orogenic Belts
 Mountains are born and have a finite lifespan.
 Young mountains are high, steep, and growing upward.
 M...
Crustal Deformation
 Orogenesis causes deformation, consisting of…
 Bending.
 Breaking.
 Tilting.
 Squashing.
 Stret...
Deformation
 Deformation strain creates geologic structures.
 Joints – Fractures that have no offset.
 Folds – Layers t...
Deformation
 Deformed terrane passes into undeformed terrane.
 Undeformed (unstrained).

Horizontal beds.
Spherical sa...
Deformation
 Deformed terrane passes into undeformed terrane.
 Deformed (strained).

Tilted beds.
Metamorphic alterati...
Deformation
 Deformation results in one or all of the following...
 Translation – Change in location.
 Rotation – Chang...
Strain
 Changes in shape caused by deformation.
 Stretching – Pulling apart.
 Shortening – Compressing, squeezing.
 Sh...
Deformation Types
 Two major types: brittle and ductile.
 Brittle deformation – Rocks break by fracturing.

Brittle def...
Deformation Types
 Two major types: brittle and ductile.
 Ductile deformation – Rocks deform by flow and folding.

Duct...
Causes of Deformation
 Strain is the result of deformation, but what causes it?
 Caused by force acting on rock, a pheno...
Causes of Deformation
 Types of stress:
 Compressional – Squeezing.
 Tensional – Pulling apart.
 Shear – Sliding past....
Stress
 Pressure – Object feels the same stress on all sides.

Earth: Portrait of a Planet, 2nd edition, by Stephen Marsh...
Stress
 Compression – Squeezing (greater stress in 1 direction).
 Tends to thicken material.

Earth: Portrait of a Plane...
Stress
 Extension – Pull apart (greater stress in 1 direction).
 Tends to thin material.

Earth: Portrait of a Planet, 2...
Stress
 Shear – Blocks of rock sliding past one another.
 Crust is neither thickened or thinned.

Earth: Portrait of a P...
Geologic Structures
 Geometric features created by deformation.
 Folds, faults, joints, etc.
 Often preserve informatio...
Measuring Structures
 Dip is always…
 Perpendicular to strike.
 Measured downslope.

 Linear structures measure simila...
Joints
 Planar rock fractures without offset.
 Result from tensional tectonic stresses.
 Systematic joints occur in par...
Faults
 Planar fractures offset by movement across the break.
 Faults are abundant and occur at a variety of scales.
 F...
Faults
 Faults may offset large blocks of Earth.
 The amount of offset is a measure called displacement.
 The San Andre...
Fault Movement
 The direction of relative block motion…
 Reflects the dominant type of crustal stress.
 Defines the typ...
Fault Classification
 Fault plane orientation.
 Vertical.
 Horizontal.
 Dipping.

 Relative motion of the offset bloc...
Fault Orientation
 On a dipping fault, the blocks are classified as the…
 Hanging wall block (above the fault), and the....
Recognizing Faults
 Continuous layers in rock are displaced across a fault.
 Faults may juxtapose different kinds of roc...
Recognizing Faults
 Brittle faults can be distinguished from ductile faults.
 Brittle fault motion results in shattered ...
Recognizing Faults
 Ductile faults can be distinguished from brittle faults.
 Ductile fault motion results in plasticall...
Dip-Slip Faults
 Sliding is parallel to fault plane dip.
 Thus, blocks move up or down the slope of the fault.
 Two kin...
Normal Fault
 Hanging wall moves down relative to the footwall.
 Accommodate crustal extension (pulling apart).
 The fa...
Normal Faults
Reverse and Thrust Faults
 Hanging wall moves up the footwall.
 Reverse faults – Fault dip is steeper than 35o.
 Thrust...
Thrust Faults
 Bring old rocks up and over younger rocks.
 Common at the leading edge of orogenic deformation.
 Can tra...
Strike-Slip Faults
 Fault motion is parallel to the strike of the fault.
 Classified by the relative sense of motion. To...
Fault Systems
 Faults commonly occur in groups called fault systems.
 Due to regional stresses that create many similar ...
Fault Systems
 Normal fault systems.
 Fault blocks slide away from one another.
 Fault dips often decrease with depth, ...
Folds
 Layered rocks may be deformed into curves called folds.
 Folds occur in a variety of shapes, sizes, and geometrie...
Folds
 Folds often occur in a series.
 Folding may result in extremely complex geometries.
 Orogenic settings produce l...
Fold Identification
 Anticline – Arch-like fold; limbs dip away from the hinge.
 Syncline – Trough-like fold; limbs dip ...
Fold Identification
 Monocline – A fold like a carpet draped over a stairstep.
 Generated by blind faults in the basemen...
Fold Identification
 Folds are described by the severity of folding.
 Open fold – Has a large angle between limbs.
 Tig...
Fold Identification
 Folds are described by hinge geometry.
 Plunging fold – Has a hinge that is tilted.
 Non-plunging ...
Fold Identification
 Erosion of plunging folds can create zig-zag outcrops.

Earth: Portrait of a Planet, 2nd edition, by...
Fold Identification
 Folds are described by their 3-dimensional shape.
 Dome – Fold with appearance of an overturned bow...
Forming Folds
 Folds develop in two ways.
 Flexural folds – Layers slip as stratified rocks are bent.
 Analogous to she...
Forming Folds
 Folds develop in two ways.
 Flow folds – Form by ductile flow of hot, soft rock.

Earth: Portrait of a Pl...
Forming Folds
 Horizontal compression causes rocks to buckle.
 Shear causes rocks to smear out.

Earth: Portrait of a Pl...
Tectonic Foliation
 Foliation develops via compressional deformation.
 Flattening – Develops perpendicular to shortening...
Tectonic Foliation
 Foliation can develop as the result of shearing.
 Foliation created as ductile rock is smeared.
 Sh...
Orogenesis and Rock Genesis
 Orogenic events create many kinds of rocks.
 Igneous rocks – Intrusive and extrusive.

Sub...
Orogenesis and Rock Genesis
 Orogenic events create many kinds of rocks.
 Sedimentary rocks – Weathering and erosion.

...
Uplift
 Construction of mountains requires substantial uplift.
 Mt. Everest (8.85 km above sea level).
 Comprised of ma...
Crustal Roots
 High mountains are supported by thickened lithosphere.
 Thickening is caused by collisional orogenesis.
...
Isostacy
 Surface elevation represents a balance between forces.
 Gravitational attraction – Pushes plate into the mantl...
This concludes the
Chapter 11
Crags, Cracks, and Crumples:
Crustal Deformation and Mountain Building

LECTURE OUTLINE

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Op ch11 lecture_earth3, Orogeny
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Orogeny refers to forces and events leading to a large structural deformation of the Earth's lithosphere (crust and uppermost mantle) due to the engagement of tectonic plates. Response to such engagement results in the formation of long tracts of highly deformed rock called orogens or orogenic belts. The word "orogeny" comes from the Greek (oros for "mountain" plus genesis for "creation" or "origin"),[1] and it is the primary mechanism by which mountains are built on continents. Orogens develop while a continental plate is crumpled and is pushed upwards to form mountain ranges, and involve a great range of geological processes collectively calledorogenesis.

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Op ch11 lecture_earth3, Orogeny

  1. 1. Chapter 11 Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building LECTURE OUTLINE earth Portrait of a Planet Third Edition ©2008 W. W. Norton & Company, Inc. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  2. 2. Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building Prepared by Ronald Parker Earlham College Department of Geosciences Richmond, Indiana
  3. 3. Mountains  Attractive landscape features for humans.  Provide beautiful scenery.  Refuge from the mundane.  Inspire poetry and art.  Mountains provide vivid evidence of tectonic activity.  They embody…  Uplift.  Deformation.  Metamorphism. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  4. 4. Mountain Belts  Mountains frequently occur in elongate, linear belts.  Mountains are constructed by tectonic plate interactions in a process called orogenesis. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  5. 5. Mountains  Mountain building involves…  Structural deformation.  Jointing.  Faulting.  Folding.  Partial melting.  Foliation.  Metamorphism.  Glaciation.  Erosion.  Sedimentation.  Constructive processes build mountains up; destructive processes tear them back down again. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  6. 6. Orogenic Belts  Mountains are born and have a finite lifespan.  Young mountains are high, steep, and growing upward.  Middle-aged mountains are dissected by erosion.  Old mountains are deeply eroded and often buried.  Ancient orogenic belts are found in continental interiors.  Orogenic continental crust is too buoyant to subduct.  Hence, if it escapes erosion it is usually preserved. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  7. 7. Crustal Deformation  Orogenesis causes deformation, consisting of…  Bending.  Breaking.  Tilting.  Squashing.  Stretching.  Shearing.  Deformation is a force applied to rock.  Changes in shape via deformation are called strain.  The study of deformation is called structural geology. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  8. 8. Deformation  Deformation strain creates geologic structures.  Joints – Fractures that have no offset.  Folds – Layers that are bent by slow plastic flow.  Faults – Fractures that are offset.  Foliation – Planar metamorphic fabric. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  9. 9. Deformation  Deformed terrane passes into undeformed terrane.  Undeformed (unstrained). Horizontal beds. Spherical sand grains. Flat-lying clays. No folds or faults. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  10. 10. Deformation  Deformed terrane passes into undeformed terrane.  Deformed (strained). Tilted beds. Metamorphic alteration. Sand = Quartzite. Clay = Slate, phyllite, schist, or gneiss. Folding and faulting. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  11. 11. Deformation  Deformation results in one or all of the following...  Translation – Change in location.  Rotation – Change in orientation.  Distortion – Change in shape.  Deformation is often easy to see. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  12. 12. Strain  Changes in shape caused by deformation.  Stretching – Pulling apart.  Shortening – Compressing, squeezing.  Shear – Sliding past.  Elastic strain – Reversible change in shape.  Permanent strain – Irreversible change in shape.  Two types of permanent strain: brittle and ductile. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  13. 13. Deformation Types  Two major types: brittle and ductile.  Brittle deformation – Rocks break by fracturing. Brittle deformation occurs in the shallow crust.  A transition between the two occurs at 10-15 km depth. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  14. 14. Deformation Types  Two major types: brittle and ductile.  Ductile deformation – Rocks deform by flow and folding. Ductile deformation occurs in the deeper crust.  A transition between the two occurs at 10-15 km depth. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  15. 15. Causes of Deformation  Strain is the result of deformation, but what causes it?  Caused by force acting on rock, a phenomenon called stress.  Stress is the force applied across an area.  A large force per area results in much deformation.  A small force per area results in little deformation. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  16. 16. Causes of Deformation  Types of stress:  Compressional – Squeezing.  Tensional – Pulling apart.  Shear – Sliding past.  Tectonic collision produces horizontal compression.  Large scale.  Most common type of deformation. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  17. 17. Stress  Pressure – Object feels the same stress on all sides. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  18. 18. Stress  Compression – Squeezing (greater stress in 1 direction).  Tends to thicken material. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  19. 19. Stress  Extension – Pull apart (greater stress in 1 direction).  Tends to thin material. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  20. 20. Stress  Shear – Blocks of rock sliding past one another.  Crust is neither thickened or thinned. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  21. 21. Geologic Structures  Geometric features created by deformation.  Folds, faults, joints, etc.  Often preserve information about stress fields.  3-D structural orientation is described by strike and dip.  Strike – Horizontal intersection with a tilted surface.  Dip – Angle of surface down from the horizontal. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  22. 22. Measuring Structures  Dip is always…  Perpendicular to strike.  Measured downslope.  Linear structures measure similar properties.  Bearing (compass direction).  Plunge – Angle from the horizontal.  Strike and dip measurements are common. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  23. 23. Joints  Planar rock fractures without offset.  Result from tensional tectonic stresses.  Systematic joints occur in parallel sets.  Minerals can fill joints to form veins.  Joints control weathering of rock. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  24. 24. Faults  Planar fractures offset by movement across the break.  Faults are abundant and occur at a variety of scales.  Faults may be active or inactive.  Sudden movements along faults cause earthquakes.  Faults vary by type of stress and crustal level.  Compression.  Tension.  Shear.  Brittle (shallow).  Ductile (deep). Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  25. 25. Faults  Faults may offset large blocks of Earth.  The amount of offset is a measure called displacement.  The San Andreas (below) - Displacement of 100s of kms.  The recently developed stream is offset ~ 100m. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  26. 26. Fault Movement  The direction of relative block motion…  Reflects the dominant type of crustal stress.  Defines the type of fault.  All motion is relative.  To help visualize fault motion… Imagine that one block is stationary (fixed in place). Then, imagine that faulting moves the other block. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  27. 27. Fault Classification  Fault plane orientation.  Vertical.  Horizontal.  Dipping.  Relative motion of the offset blocks.  Dip slip – Blocks move parallel to fault plane dip.  Strike slip – Blocks move parallel to fault plane strike.  Oblique slip – Combination of dip-slip and strike-slip. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  28. 28. Fault Orientation  On a dipping fault, the blocks are classified as the…  Hanging wall block (above the fault), and the...  Footwall block (below the fault).  Standing in a tunnel excavated along the fault…  Your head is near the hanging wall block.  You are standing on the foot wall block. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  29. 29. Recognizing Faults  Continuous layers in rock are displaced across a fault.  Faults may juxtapose different kinds of rock.  Scarps may form where faults intersect the surface.  Fault friction motion may bend rocks into drag folds.  Fault-broken rocks may be more easily eroded.  Minerals may grow on fault surfaces due to fluid flow. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  30. 30. Recognizing Faults  Brittle faults can be distinguished from ductile faults.  Brittle fault motion results in shattered and crushed rock. Fault breccia – Fault zone preserving broken fragments of rock. Fault gouge – Fault zone preserving pulverized, powdered rock. Slickensides – Surface polished by fault motion. Slip lineations – Linear grooves on a fault preserving direction. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  31. 31. Recognizing Faults  Ductile faults can be distinguished from brittle faults.  Ductile fault motion results in plastically deformed rocks. Rocks do not break; instead, they are intensely sheared. Rocks from ductile shear zones are called mylonites. Mylonites typify detachment faults in collisional orogens. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  32. 32. Dip-Slip Faults  Sliding is parallel to fault plane dip.  Thus, blocks move up or down the slope of the fault.  Two kinds of dip-slip fault depend on relative motion.  Reverse fault – Hanging wall moves up. Thrust fault (a special type of reverse fault).  Normal fault – Hanging wall moves down. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  33. 33. Normal Fault  Hanging wall moves down relative to the footwall.  Accommodate crustal extension (pulling apart).  The fault below shows displacement and drag folding. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  34. 34. Normal Faults
  35. 35. Reverse and Thrust Faults  Hanging wall moves up the footwall.  Reverse faults – Fault dip is steeper than 35o.  Thrust faults – Fault dip is less than 35o.  Accommodate crustal shortening (compression). Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  36. 36. Thrust Faults  Bring old rocks up and over younger rocks.  Common at the leading edge of orogenic deformation.  Can transport thrust sheets 100s of kilometers.  Act to shorten and thicken mountain belts. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  37. 37. Strike-Slip Faults  Fault motion is parallel to the strike of the fault.  Classified by the relative sense of motion. To find this…  Imagine standing on one block looking across the fault.  Which way does the opposite block move?  Right lateral – Opposite block moves to observer’s right.  Left lateral – Opposite block moves to observer’s left.  Large strike-slip faults may slice the entire lithosphere. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  38. 38. Fault Systems  Faults commonly occur in groups called fault systems.  Due to regional stresses that create many similar faults.  May diverge from a common horizontal detachment fault.  Thrust fault systems.  Stack fault blocks on top of one another.  Act to shorten and thicken the crust.  Result from horizontal compression. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  39. 39. Fault Systems  Normal fault systems.  Fault blocks slide away from one another.  Fault dips often decrease with depth, joining a detachment.  Blocks rotate on faults and create half-graben basins.  Act to stretch and thin the crust.  Result from horizontal extension (pull-apart) stress. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  40. 40. Folds  Layered rocks may be deformed into curves called folds.  Folds occur in a variety of shapes, sizes, and geometries.  A special terminology is used to describe folds.  Hinge – Portion of maximum curvature on a fold.  Limb – Less-curved “sides” of a fold  Axial plane – Imaginary surface defined by connecting hinges of successively nested folds. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  41. 41. Folds  Folds often occur in a series.  Folding may result in extremely complex geometries.  Orogenic settings produce large volumes of folded rock.  Deformed rock often experiences multiple events. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  42. 42. Fold Identification  Anticline – Arch-like fold; limbs dip away from the hinge.  Syncline – Trough-like fold; limbs dip toward the hinge.  Anticlines and synclines frequently alternate in series. Anticline Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  43. 43. Fold Identification  Monocline – A fold like a carpet draped over a stairstep.  Generated by blind faults in the basement rock.  These faults do not cut through to the surface.  Instead, displacement folds overlying sedimentary cover. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  44. 44. Fold Identification  Folds are described by the severity of folding.  Open fold – Has a large angle between limbs.  Tight fold – Has a small angle between limbs. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  45. 45. Fold Identification  Folds are described by hinge geometry.  Plunging fold – Has a hinge that is tilted.  Non-plunging fold – Has a horizontal hinge. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  46. 46. Fold Identification  Erosion of plunging folds can create zig-zag outcrops. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  47. 47. Fold Identification  Folds are described by their 3-dimensional shape.  Dome – Fold with appearance of an overturned bowl. Erode to expose old rocks in center; younger rocks outside. Result from crustal upwarping.  Basin – Fold shaped like a bowl. Erode to expose young rocks in center; older outside. Result from crustal subsidence.  Domes and basins result from vertical crustal motions. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  48. 48. Forming Folds  Folds develop in two ways.  Flexural folds – Layers slip as stratified rocks are bent.  Analogous to shear as a deck of cards is bent. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  49. 49. Forming Folds  Folds develop in two ways.  Flow folds – Form by ductile flow of hot, soft rock. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  50. 50. Forming Folds  Horizontal compression causes rocks to buckle.  Shear causes rocks to smear out. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  51. 51. Tectonic Foliation  Foliation develops via compressional deformation.  Flattening – Develops perpendicular to shortening strain.  Sand grains flatten and elongate; clays reorient.  Foliation parallels fold axial planes. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  52. 52. Tectonic Foliation  Foliation can develop as the result of shearing.  Foliation created as ductile rock is smeared.  Shear foliation is not perpendicular to compression.  Rocks that are sheared have a distinctive appearance. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  53. 53. Orogenesis and Rock Genesis  Orogenic events create many kinds of rocks.  Igneous rocks – Intrusive and extrusive. Subduction-related volcanic arc. Rift-related decompressional melting.  Metamorphic rocks – Regional and contact. Igneous intrusion. Deep burial. Horizontal compression. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  54. 54. Orogenesis and Rock Genesis  Orogenic events create many kinds of rocks.  Sedimentary rocks – Weathering and erosion. Erosional debris is shed to adjacent regions. Sediments accumulate in basins created by crustal flexure. Sediments can preserve evidence of mountains eroded away. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  55. 55. Uplift  Construction of mountains requires substantial uplift.  Mt. Everest (8.85 km above sea level).  Comprised of marine sediments (formed below sea level).  Lofty mountains are supported by a thickened crust. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  56. 56. Crustal Roots  High mountains are supported by thickened lithosphere.  Thickening is caused by collisional orogenesis.  Average continental crust – 35 to 40 km thick.  Beneath orogenic belts – 50 to 70 km thick.  This thickened crust helps buoy the mountains upward. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  57. 57. Isostacy  Surface elevation represents a balance between forces.  Gravitational attraction – Pushes plate into the mantle.  Buoyancy – Causes plate to float higher on the mantle.  The term isostatic equilibrium describes this balance.  Isostacy is compensated after a disturbance.  Adding weight pushes the lithosphere down.  Removing weight causes isostatic rebound.  Compensation is slow, requiring asthenospheric flow. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
  58. 58. This concludes the Chapter 11 Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building LECTURE OUTLINE earth Portrait of a Planet Third Edition ©2008 W. W. Norton & Company, Inc. Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

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