Deformation

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Deformation
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What is deformation?

• Deformation is the folding, faulting and doming
  of the crust caused by plates colliding,
  diverging or sliding past each other
• The processes of deformation are compression
  (squeezing), tension (stretching) and shearing
  (tearing)

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Folding

• Folding happens where plates converge
• The layers of rock are squeezed to form wave-like
  folds
• Plates buckle or wrinkle
• A fold consists of an anticline (ridge), a
  syncline (valley) and limbs (arms of fold)

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Types of Fold

• Monocline folding caused the horizontal layers
  to tilt in one direction
• Symmetrical Fold both limbs have same slope
• Asymmetrical Fold one limb is steeper than the
  other
• Overfold one limb of an anticline has been
  overturned
• Overturned Fold very intense folding caused one
  limb to completely overturn
• Overthrust Fold very intense folding caused a
  fault to open and one limb is pushed up over the
  other limb

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Caledonian Fold Mountains

• 450 million years ago
• Eurasian North American plates collided
• Approx. 50 million years of folding to form the
  Caledonian Fold Mountains
• They run in a northeast to southwest direction
• They have been severely weathered eroded
• E.g. Dublin-Wicklow Mountains

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Armorican Fold Mountains

• 270 Million years ago
• African Eurasian plates collided
• Layers of limestone, shale and sandstone were
  folded to form the ridge valley landscape of
  Munster
• They run from east to west
• E.g. Comeragh Mountains, Co Waterford
• Ridges of sandstone are separated by valleys of
  limestone shale

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Alpine Fold Mountains

• 30-60 Million years ago
• African Eurasian plates converging
• Himalayas, Andes Rockies all from the same
  period
• Plates are still converging here due to volcanic
  earthquake activity
• Alpine folds are the youngest, highest most
  rugged of all fold mountains
• No examples for Ireland

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Faulting

• Faulting occurs where the folding is so intense
  that it opens up a fracture in the fold
• Continued pressure may open up a fault line
• Movement along a fault may be horizontal or
  vertical

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Normal Faults

• A normal fault is formed where two plates diverge
  and one plates slips downwards along the fault
  plain due to gravity
• A Rift Valley is formed where two normal faults
  are parallel to each other the crust between
  them slips downwards

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Reverse Faults

• Reverse faults are formed when two plates
  converge a fracture may open in the crust one
  plate is forced upwards relative to the other by
  compression
• The Armorican Thrust Fault has developed from
  Dingle Bay to Dungarvan
• The African Eurasian plates collided intense
  pressure caused sandstone to be pushed
  northwards a fault opened and continued folding
  forced the sandstone onto the carboniferous
  limestone (Diagram 4.12 Page 32)

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Tear Faults

• As plates slide past each other in opposite
  directions, tear faults are formed
• As the North American Eurasian plates
  converged, sea floor sediments were compressed to
  form new slabs of crust called terranes
• During the Caledonian Folding period, Donegal was
  put together by terreanes

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Doming

• Doming occurs where upward pressure uplifts some
  crust higher than its surroundings
• Dome shaped mountains such as Slieve Bloom
  (Laois-Offaly) formed during the Armorican
  foldings
• The granite domes of the Mourne Mountains were
  formed 60 millions years ago as hot magma
  intruded under sedimentary rocks weather
  erosion uncovered the granite