Mountain buildings

1. February 1, 2018February 1, 2018 11
Department of Geology
University of Dhaka
Tenth Lecture
Mountain Building
S K Saha

2. Mountain Building and Geologic StructuresMountain Building and Geologic Structures

3. Can you name these mountains?Can you name these mountains?
• Andes Urals AppalachiansAndes Urals Appalachians CaledoniansCaledonians
• Great Dividing Range Himalayas Alps RockiesGreat Dividing Range Himalayas Alps Rockies
Great
Dividing
Range
Andes
Rocky
Mountains
Alps
Himalaya
s
Appalachians
Caledonian
s
Urals

4. Mountain Building (Orogenesis)Mountain Building (Orogenesis)
ProcessesProcesses
What Processes BuildWhat Processes Build
Mountains?Mountains?
• Volcanic ActivityVolcanic Activity
• Tectonic ActivityTectonic Activity
– Folding – bending ofFolding – bending of
rockrock
– Faulting – breakingFaulting – breaking
of rockof rock

5. Factors Affecting Deformation
Rock Deformation
 Factors that influence the strength of a rock
and how it will deform include temperature,
confining pressure, rock type, and time.
• Stress is the force per unit area acting on a solid.
• Strain is the change in shape or volume of a
body of rock as a result of stress.
- Most crustal deformation occurs along plate
margins.
• Deformation is a general term that refers to all changes
in the original shape and/or size of a rock body.

6. Factors Affecting Deformation
Rock Deformation
 Temperature and Pressure
• Rocks deform permanently in two ways: brittle
deformation and ductile deformation.
- Ductile deformation is a type of solid state flow
that produces a change in the size and shape of
an object without fracturing the object.
- Brittle deformation is the fracturing of an object
once its strength is exceeded.

7. Factors Affecting Deformation
Rock Deformation
 Rock Type
 Time
• Forces that are unable to deform rock when first
applied may cause rock to flow if the force is
maintained over a long period of time.
• Mineral composition and texture of a rock also
greatly affect how it will deform.

8. Rock Deformation - How Does it Occur?
– Applied stresses can deform or strain rocks until
they become contorted or fracture.
– Stresses are categorized as compressional,
tensional or shear.
– Stress is a force applied
per area
– Strain is the change in shape
that results from the stress
being applied.
– Deformation and strain
are the same thing.

9. Rock Deformation-How Does it Occur?
• Types of Strain
– Compression
– In compression the rocks are squeezed
towards one another along the same
line.
– Rock layers in compression are shortened the
rocks by folding or faulting

10. Rock Deformation-How Does it Occur?
• Types of Strain
– Tension
– In tension the forces along the same line
act in opposite directions.
– Tension lengthens the rocks or pulls
them apart.

11. Rock Deformation-How Does it Occur?
• Types of Strain
– Shear
– In shear the forces act parallel to one
another, but in opposite directions
– Deformation occurs along closely spaced
planes like the slip between cards in a
deck.

12. Rock Deformation-How Does it Occur?
• Types of Strain
– Rocks will deform elastically until they reach the
elastic limit unless the force is applied quickly.
– Elastic strain occurs if
rocks return to their
original shape when
the stress is released.
– Plastic strain occurs
when rocks fold or
fracture when stress is
applied and do not
recover their original
shape.

13. Rock Deformation-How Does it Occur?
• Types of Strain
– What determines whether a rock will bend
elastically, plastically or fracture?
– Type of stress applied
– Pressure and
temperature
– Rock type
– Length of time

14. Rock Deformation-How Does it Occur?
• Types of Strain
– Rocks will deform elastically until they reach the
elastic limit unless the force is applied quickly.
– Ductile rocks show a
great amount of plastic
strain (they bend)
before they fracture
– Brittle rocks fracture
after only a small
amount of plastic
strain.

15. Strike and Dip-The Orientation of Deformed
Rock Layers
– Principle of original horizontality states that
most rocks are originally laid down
horizontally.
– When we see rocks inclined, they have been
deformed by folding and/or fracturing.

16. Strike and Dip-The Orientation of Deformed
Rock Layers
– Strike and dip are measurements used to
describe a rock body's orientation with respect
to the horizontal.
– Strike is the
intersection of a
horizontal plane with
an inclined plane.
– Dip is the maximum
angle of an inclined
plane.

17. Deformation and Geologic Structures
• Folded Rock Layers
– Folds are layers of rock that were once
planar that are bent or crumpled.
– Folds form during compression and
undergo plastic strain.
– This occurs deep in the crust where the
rocks behave ductilely.
– There are 3 kinds of folds:
»Monoclines
»Anticlines
»Synclines

18. Types of Stress
Rock Deformation
 The three types of stresses that rocks
commonly undergo are tensional stress,
compressional stress, and shear stress.

19. Types of Stress
• Compression
– Crumpling
• Tension
– Stretching
• Shear Stress
– Grind past

20. Folds
Rock Deformation
 Anticlines
• Anticlines are most commonly formed by the upfolding, or
arching, of rock layers.
 Synclines
 Monoclines
• Monoclines are large step-like folds in otherwise horizontal
sedimentary strata.
• Synclines are linear downfolds in sedimentary strata.
• Synclines are often found in association with anticlines.

21. Anticlines and Synclines

22. Monoclines

23. Faults
Rock Deformation
 Normal Faults
• Normal faults occur when the hanging wall block moves down
relative to the footwall block.
 Reverse Faults and Thrust Faults
• Reverse faults are faults in which the hanging wall block moves
up relative to the footwall block.
• Thrust faults are reverse faults with dips less than 45o
.

24. Folds
Rock Deformation
 Joints
• Joints are fractures along which no appreciable movement has
occurred.
 Strike-Slip Fault
• Strike-slip faults are faults in which the movement is horizontal
and parallel to the trend, or strike, of the fault surface.

25. Four Types of Faults

26. Joints

27. Folded Mountains
Types of Mountains
 Folded Mountains
• Mountains that are formed primarily by folding
are called folded mountains.
 Mountains are classified by the dominant
processes that have formed them.
• Orogenesis is the collection of processes that
result in the forming of mountains.

28. Folded Mountains

29. Fault-Block Mountains
Types of Mountains
 Large-scale normal faults are associated
with structures called fault-block
mountains.
• Fault-block mountains are formed as large blocks of
crust are uplifted and tilted along normal faults.
• Grabens are formed by the downward displacement of
fault-bounded blocks.
• Horsts are elongated, uplifted blocks of crust bounded by
faults.

30. Fault-Block Mountains

31. Domes and Basins
Types of Mountains
 When upwarping produces a circular or
elongated structure, the feature is called a
dome.
• Uplifted mountains are circular or elongated
structures formed by uplifting of the underlying
basement rock.

32. Domed Mountains

33. Quick Review of Convergent
Boundaries
• What happens at these boundaries?
Plates are colliding
• What type of force is involved?
Compression
• What are the results of these boundaries?
Folding, faulting, metamorphism,
volcanoes

34. Convergent-Boundary Mtns.
• Oceanic-Oceanic Convergent boundary:
- one oceanic plate is subducted under
another, cause the sinking plate to melt.
- As magma rises upward an island arc of
volcanic mountains is formed.
Example: Mt. Pinatubo, Philippine Islands

35. Convergent-Boundary Mtns.
• Oceanic-Continental Convergent
boundary:
- Oceanic crust is subducted under
continental crust.
- Subducted oceanic crust melts.
- Magma rises upward forming a
volcanic mountain range on the
continental crust.
Example: Andes Mountains, South
America

36. Convergent-Boundary Mtns.
• Continental-Continental Boundaries:
- Both pieces of crust have relatively
low density and cannot be subducted.
- Plate will instead push together and
get deformed (folded, faulted).
- This will build up thick areas of crust,
thus creating the tallest mountains on
Earth.
Example: Himalayas

37. Nonboundary Mountains
1. Uplifted Mountains – large areas of
gradual upward movement.
(Adirondacks)
2. Fault-block mountains – blocks of crust
lifted or tilted by faults.
3. Volcanoes over hotspots.

38. Mountain Building at Convergent
Boundaries
Mountain Formation
 Most mountain building occurs at
convergent plate boundaries. Colliding
plates provide the compressional forces
that fold, fault, and metamorphose the
thick layers of sediments deposited at the
edges of landmasses.

39. Mountain Building at Convergent
Boundaries
Mountain Formation
 Ocean-Ocean Convergence
• Ocean-ocean convergence mainly produces volcanic
mountains.
 Ocean-Continental Convergence
• The types of mountains formed by ocean-continental
convergence are volcanic mountains and folded
mountains.

40. Ocean-Ocean Convergence

41. Ocean-Continental Convergence

42. Mountain Building at Convergent
Boundaries
Mountain Formation
 Continental-Continental Convergence
• At a convergent boundary between two plates carrying
continental crust, a collision between the continental
fragments will result and form folded mountains.

43. Continental-Continental Convergence

44. Mountain Building at Divergent
Boundaries
Mountain Formation
 The mountains that form along ocean
ridges at divergent plate boundaries are
fault-block type mountains.

45. Mountain Building by
Continental Accretion

46. Non-Boundary Mountains
Mountain Formation
 Not all mountains are formed by plate
boundaries. Some are formed by hot spots
or regional extension or stretching.

47. Continental Accretion
Mountain Formation
 Terranes
• Terranes are any crustal fragments that have a geologic
history distinct from that of the adjoining fragments.
 Accretion is a process that occurs when
crustal fragments collide with and stay
connected to a continental plate.
• Terranes occur along the Pacific Coast.

48. Accretion in Western North America

49. Principles of Isostasy
Mountain Formation
• Isostasy is the concept that Earth’s crust is floating in
gravitational balance upon the material of the mantle.
 Isostatic Adjustment for Mountains
• Because of isostasy, deformed and thickened crust will
undergo regional uplift both during mountain building and
for a long period afterward.
• Isostatic adjustment is the process of establishing a new
level of gravitational equilibrium.

50. Isostasy and Isostatic Adjustment
• Erosion removes weight
• Crust rises
• Eventually uniform
thickness

51. Isostatic Adjustment

52. Isostatic Adjustment in Mountains