Where Mountains Form?

  • A mountain is a large mass of rock that rises a great distance above its base.
  • Are formed due to tectonic activity – due to convergence of plate boundaries
  • Most of the world’s mountains are formed in long belts due to the whole side of plates crashing into other plates
  • A mountain belt is a region where mountains are forming or have formed in the past

Mountain Belts, Continental Margins

  • A cordillera is a mountain belt that runs down the length of a plate
  • A mountain belt is made up of smaller groups of mountains called mountain ranges example: Cascade range is part of the North American Cordillera
  • Some mountains lie on current plate boundaries – such as the Himalayas, while others lie on boundaries that existed millions of years ago – such as the Appalachians.

Continental Margins

  • A Continental Margin is a boundary between continental crust and oceanic crust
  • Active continental margins occur along plate boundaries, while passive continental margins occur at the boundary that marks a continent and the oceanic crust.

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  • Mountain building takes place along active continental margins only.
  • Passive continental margins are areas where sediment accumulation takes place

How do Mountains form?

  • At convergent plate boundaries – the stress due to pushing of plates causes fracturing, folding and stretching

Types of stress:

  • Compression: rock layers squeezed inward, tends to make rock layers thicker and shorter
  • Tension: rock layers being stretched, tends to make rocks thinner and longer
  • Shear stress:  rock layers being pushed in two different, opposite directions.  Tends to distort the shape of the rocks
  • This is an upward fold of strata; found in the Alps.

Folds

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  • During plate collisions, stress can cause rock layers along continental margins to crumple into folds.
  • Folds are described as being anticline and syncline
  • Anticline: is an upfold in the rock layers, a syncline is a downfold in the rock layer.
  • The two sides of a fold are called limbs – limbs represent the intensity of the folding.
  • Limbs may be gently dipping, steeply dipping, straight up or down or even overturned.
  • The compass direction of the fold or of the rock layers exposed at the surface along the fold is called the strike

Faults

  • Faults:  a fault is a break in the lithosphere along which movement has occurred
  • The part of the fault above the fault plane is called the hanging wall and the part below is called the footwall.
  • A normal fault occurs when a hanging wall moves down with respect to the footwall.
  • What would you call a single-limbed fold like this?
  • A normal fault occurs in areas where tension is pulling the crust apart
  • A reverse fault occurs when the hanging wall moves up with respect to the footwall.  Reverse faults are caused by compression
  • A thrust fault is a reverse fault in which the fault plane dips 45 degrees or less from the horizontal
  • Thrust faults are common in many mountain belts.
  • A strike slip fault, the rocks on opposite sides of the fault plane move horizontally past each other example – San Andreas fault
  • Extremely long strike slip faults are found in the Himalayas.
  • Joints: like faults, are breaks in bedrock.  Joints are breaks along which no apparent movement has occurred.
  • Joints provide channels through which fluids enter and move through bedrock.

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