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Plate Tectonics: Part II Folding and Faulting

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1 Plate Tectonics: Part II Folding and Faulting


3 Horizontal movements of the Earth’s Crust
Takes place from compression and tensional forces Tension – stress tending to pull rock masses apart Compression – Stress tending to squeeze rock masses together Responsible for formation of fold mountains. As a result of compressional forces acting in 2 opposite directions, the crustal rocks bend and form a series of arches & troughs. This is known as folding of crustal rocks that usually occur in a zone where the crust is weak. The lines of fracture are known as faults Both Processes are result in mountain building

4 What is a Fold? A fold is a bend in rock strata resulting from compression of an area of the Earth’s crust . The severity of compression is indicated by the tightness of the folds. Since the compressional force is acting towards the centre, the intervening land rises up in a series of folds

5 Types of Folds: Anticline
An anticline is a convex up fold in rock that resembles an arch like structure with the rock beds (or limbs) dipping way from the center of the structure Note how the rock layers dip away from the center of the fold are roughly symmetrical.

6 Types of Folds: Syncline
A syncline is a fold where the rock layers are warped downward Both anticlines and synclines are the result of compressional stress Note how the rock layers dip toward the center of the fold and are roughly symmetrical

7 Synclinal folds in bedrock, near Saint-Godard-de-Lejeune, Canada

8 More Complex types of Folds
More complex fold types can develop in situations where lateral pressures become greater. The greater pressure results in anticlines and synclines that are inclined and asymmetrical

9 Recumbent Fold A recumbent fold develops if the center of the fold moves from being once vertical to a horizontal position commonly found in the core of mountain ranges and indicate that compression and/or shear forces were stronger in one direction

10 Overthrust Fault Extreme stress and pressure can sometimes cause the rocks to shear along a plane of weakness creating a fault. We call the combination of a fault and a fold in a rock an overthrust fault

11 Fold Mountains The compressional force exerted on the crust is a result of movement of earth’s plates. When the plates converge, the weak rocks and sediments lying between the 2 plates get squeezed and get folded. Parallel folds give rise to long chains of parallel mountain ranges with high peaks. The up-folds form fold mountains while the down folds form longitudinal valleys.


13 The Appalachian Mountains
Mountains of Canada The Appalachian Mountains

14 Mountains of Canada Innuitian Mountains

15 Mountains of Canada The Rocky Mountains
The Valley of Ten Peaks, Banff National Park

16 Understanding Canada’s Mountain Ranges
Read “making connections” textbook pg Answer questions 1-5 on pg. 143

17 Faulting Faults are large scale internal movements causing large scale physical features especially mountains and valleys If the force is moderate only cracks may appear in the rocks. The displacement that occurs results in formation of faults. Such fractures are divided into Joints & Faults. A joint is a fracture where a rock seems to be joined together without any movement it may become a fault when displacement takes place

18 Location of some of the major faults on the Earth.
Where do you notice that a majority of the faults in the world occur?

19 Normal Fault 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

20 Reverse Fault Reverse faults develop when compressional forces exist
Compression causes one block to be pushed up and over the other block.

21 Graben Fault A graben fault is produced when tensional stresses result in the subsidence of a block of rock. On a large scale these features are known as Rift Valleys

22 Rift Valleys (AKA Graben)
Major relief feature of Faulting Activity is Rift Valley. A Rift Valley may be formed in two ways : (1) When the middle portion between two normal faults is dropped downward the sides remain stable. (2) When the middle portion remains stable and the two sides are raised upward These valleys have flat bottoms and steep sides

23 Example: Great Rift Valley, Africa
Why is this important? Because the rapidly eroding highlands have filled the valley with sediments, a favorable environment for the preservation of remains has been created. The bones of several hominids ancestors of modern humans have been found there, including those of "Lucy“

24 Ottawa-Bonnechere Graben
The Ottawa-Bonnechere Graben (also known as the Ottawa Graben), with its branch the Timiskaming Graben, is an ancient rift valley in the Canadian Shield of Northeastern Ontario and Quebec, Canada This rift valley was formed when the Earth's crust moved downward about a kilometre between two major fault zones known as the Mattawa and Petawawa faults These ancient faults are still active and occasionally release stress in the form of earthquakes, such as the 1935 Timiskaming earthquake and the 2000 Kipawa earthquake. The length of the graben is about 700 km (435 mi) Runs from the Montreal area on the east to near Sudbury and Lake Nipissing on the west


26 Barron Canyon in Algonquin Park is along a fault associated with the Ottawa Bonnechere graben.

27 Horst Fault A horst fault is the development of two reverse faults causing a block of rock to be pushed up - In German Horst = 'heap'


29 Fault-Block Mountains
Block mountains are another important feature of Faulting caused by the Tensional features in the Earth’s crust The mechanism of formation of Block mountains is same as the Rift valley The Block mountains are often found on the sides of the Rift valley It is because the land between two parallel faults is either subsides to form a Rift valley or rises to form Block mountains

30 Strike-Slip (transform) Fault
The final major type of fault is the strike-slip or transform fault. These faults are vertical in nature and are produced where the stresses are exerted parallel to each other A well-known example of this type of fault is the San Andreas fault in California.

31 Challenge With the Fault Models can you demonstrate: A normal fault?
A reverse fault? A graben fault? A horst fault?

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