# Deformation of the Crust

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Deformation of the Crust
By: Mrs. Severe Earth Science

Section 1 – How the Crust is Deformed
Objectives: Predict isostatic adjustments that will result from changes in the thickness of the earth’s crust. 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

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

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 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 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

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 Explain isostatic adjustment… Define Stress and Strain…
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 What results when rock responds to stress by permanently deforming without breaking? Explain why faulting is more likely to occur near the surface than deep within the earth… 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 Describe the types of lithospheric plate collisions that build mountains. Name the four types of mountains and explain how each is formed. How do volcanic mountains grow?