Deformation of the Crust By: Mrs. Severe Earth Science Section 1 – How the Crust is Deformed Objectives: 1. Predict isostatic adjustments that will result from changes in the thickness of the earth’s crust. 2. Identify sources of stress in crustal rock. Deformation • Bending, tilting, and breaking of the earth's crust • Plate tectonics is the major cause of crustal deformation, but is not the only force that shapes the earth’s crust. Other Forces • Changes in the weight of some parts of the crust – Thicker and heavier = sink more deeply into the mantle – Thinner and lighter = rise higher on the mantle Isostatic Adjustment • Up-and-down movements of the crust occur because of two opposing forces. • Crust presses down on the mantle • Mantle presses up on the crust • When the two forces balance, the crust moves neither up nor down Isostatic Adjustment Continued… • When weight is added to the crust, it sinks until a balance of the forces is reached again • Balancing of the two forces is called: • ISOSTASY Isostatic Adjustment Continued… • Up-and-down movements of the crust to reach isostasy is called • ISOSTATIC ADJUSTMENTS • As the adjustments occur, areas of the crust are bent up and down • Pressure from this causes rocks in that area of the crust to deform Isostatic Adjustment Still Continued… • Isostatic adjustments occur constantly: – Mountain ranges • *As the crust becomes lighter, the region may rise – Rivers flow into large bodies of water carrying large amounts of mud, sand, and gravel • *Added weight cause the floor to sink Isostatic Adjustment STILL Continued… • Isostatic adjustments occur constantly: – Glaciers once covered the land • *Weight of the ice caused the crust underneath it to sink • Glacial ice retreats • Land slowly begins rising again in response to its reduced weight Isostatic Adjustment FINAL! Stress (Not the kind you cause your teacher to have) • Isostatic adjustment and plate movement cause stress in rocks that make up the earth’s crust • Amount of force per unit area that is placed on any given material Strain • Crustal stress occurs when lithospheric plates collide, separate, or rub together • A change in the shape or volume of rocks that results from the stress of being squeezed, twisted, or pulled apart Types of Stress • Compression - occurs when crustal rocks are squeezed together – often reduces the volume of the rocks • Tension - the force that pulls rocks apart – rocks tend to become thinner • Shearing - sliding rocks past each other in opposite horizontal directions Types of Stress Review 1. Explain isostatic adjustment… 2. Define Stress and Strain… 3. Draw a diagram of each of the following: – Compression – Tension – Shearing Section 2 – The Results of Stress Objectives: • Compare folding and faulting as responses to stress • Describe four types of faults The Results of Stress Introduction • High pressure and temperatures caused by stress deform rocks • Stress applied slowly = rock may return to its original shape as the force is removed • If the force exceeds an acceptable amount, the shape of the rock changes permanently • Rock may also break because of extreme stress Folding • Rock responding to stress by becoming permanently deformed without breaking • Usually occurs deep in the crust where overlying rocks create great pressure so behavior is more plastic 3 Types of Folds • Anticline • Syncline • Monoclin Anticline • Up-curved fold in which the oldest layer is in the center of the fold • Generally forms a ridge • Can you think of examples… Syncline • Down-curved fold in which the youngest layer is in the center • Generally forms a valley • Can you think of examples… Monocline • Fold in which both limbs remain horizontal • Gently dipping one way or the other • Can you think of examples… Faulting • Breaks in rocks when the rocks on either side of the break move is faulting • Breaks in rocks when the rocks on either side of the break do not move is fracture • Near the crust’s surface rocks are more brittle and tend to break, not bend Fault Vocabulary • Fault plane - surface of a fault along which any motion occurs • Hanging wall - in a non-vertical fault, the rock above the fault plane • Footwall - rock below the fault plane 4 Types of Faulting • Normal Fault • Reverse Fault • Thrust Fault • Strike-slip Fault Normal Faulting • Hanging wall moves down relative to the footwall • Form along divergent boundaries • Usually occurs in a series of parallel fault lines Reverse and Thrust Fault • Reverse - Forms when compression causes the hanging wall to move up relative to the footwall • Thrust - Fault plane is at a low angle or nearly horizontal. • Common in steep mountains such as the Rockies and Alps Strike-slip Fault • Rock on either side of the fault plane slides horizontally • Often occur in transform boundaries • Example: San Andreas Fault Review 1. What results when rock responds to stress by permanently deforming without breaking? 2. Explain why faulting is more likely to occur near the surface than deep within the earth… 3. Draw and describe four types of faults Section 3 – Mountain Formation Objectives: • Identify the types of plate collisions that build mountains. • Identify four types of mountains and discuss the forces that shaped them. Mountain Formation Introduction • A mountain range is a group of adjacent mountains with the same general shape and structure • A group of adjacent mountain ranges make up a mountain system • Largest mountain systems are part of still larger systems called mountain belts – Circum-Pacific Belt – Eurasian-Melanesian Belt Plate Tectonics and Mountains • Circum-Pacific and Eurasian- Melanesian mountain belts are located along convergent plate boundaries • Scientists think this is evidence that most mountains were formed when lithospheric plates collided How Plate Collisions Form Mountains • Collisions between Continental and Oceanic Crust = Subduction zones form coastal volcanoes • Collisions between Oceanic Crust and Oceanic Crust = Volcanic island arcs form on ocean floor • Collisions between Continents = Crust crumples and rises (Example are the Himalayas) Types of Mountains • Mountains are complicated structures with rock formations that yield evidence of the forces that created them. • Classified by: deformation and shape • 4 Types Folded Mountains and Plateaus • Continental Crust is pushed together and up • Highest mountain ranges in the world • Plateaus are large uplifted flats are also formed near folded mountains Fault-Block Mountains • Range fronts rise along normal faults as a result of crustal stretching • Examples: Lost River Range, most of Nevada Volcanic Mountains • Mountains that form when molten rock erupts onto the earth’s surface • Hot Spots – formed on the ocean floor Dome Mountains • Formed when molten rock rises through the crust and pushes up the rock layers above it • When pushed up, rock layers are worn away exposing hardened rock • Where the rock wears away and leaves separate high peaks is dome mountains Review 1. Describe the types of lithospheric plate collisions that build mountains. 2. Name the four types of mountains and explain how each is formed. 3. How do volcanic mountains grow?