Download Unit 3 Lesson 3 Mountain Formation

Grade 9 Geography of Canada - Unit 3 Lesson 3
Mountain Formation
Mountains are created by the constant but very slow movement of the Earth's plates.
Math Connection
How many kilometre would a plate travel in 100 million years if the plate moves at a rate
of 2.5 centimetres per year? (Recall: 1 kilometre = 100,000 cm)
100,000,000 X 2.5 cm = 250,000,000 cm
250,000,000 / 100,000 = 2,500 kilometres
Mountain Formation
During the movement of the Earth’s plates, mountains are formed by five processes:
Volcanic activity
Folding
Faulting
Dome building
Erosion.
Definitions
Folding
It is the bending of rock layers
The pushing together of Earth's plates in a roller coaster like series of high points
and low points.
Folding bends layers of rocks without breaking them.
The Appalachian Mountains and Rocky Mountains are
mountain ranges formed by folding.
Faulting
It is the movement along a crack or cracks in the
Earth’s crust.
Layers of the Earth's crust are moved vertically upward
at fault lines (cracks between plates) by pressures
caused by plates colliding. Mountains formed in this
way are called fault-block mountains.
Dome Building
Low mountains formed when the Earth’s crust is
heaved upward without folding or faulting. The heaving
creates a rounded dome called a Dome Mountain.
Erosion
The wearing away of land or soil by the action of wind, water, or ice.
Mountain Formation
How does folding and faulting occur?
The answer is stress and strain.
Stress describes the forces that act on Earth’s materials including the crust. There are
three types of stress that can be applied to rocks.
Compressional Stress applies a squeezing force to the rocks.
Tensional Stress is a stretching force.
Shear Stress is applied to a material when forces act in opposite directions on
opposite sides of the material. To illustrate shear stress, place a deck of cards
between your hands, then slowly move your hands in opposite directions).
Strain describes the change in shape of a material in response to a stress. There are
three ways in which rocks will deform (strain) when stresses are applied.
If a rock undergoes Elastic Deformation, it will change shape in response to a stress,
then snap back into its original shape if the stress is removed (i.e., the strain is
recoverable).
Brittle Deformation occurs when stresses overcome the strength of the material and
it simply breaks.
Plastic deformation will occur where pressures are relatively high and the strain rate
relatively low. During Plastic Deformation, the material flows and changes in shape
are permanent.
Typical folding is Compression Stress coupled with Elastic Deformation.
Normal faults occur when tensional forces act in opposite directions and cause one slab
of the rock to be displaced up and the other slab down. Thus faulting is Tensional
Stress and, since a break occurs, Brittle Deformation