Deformation of Crust Fall 2013

How the Crust is Deformed Deformation- the bending, tilting, and breaking of the earth’s crust Major cause- plate tectonics Plate movement is not the only thing that shapes the earth’s crust

Isostatic Adjustment Some changes in the earth’s crust occur because of changes in the weight of some part of the crust When parts of the crust become thicker and heavier, they sink more deeply into the mantle When parts of the crust become thinner and lighter, they rise higher on the mantle

Isostatic Adjustment The up & down movements of the crust occur because of two opposing forces The crust presses down on the mantle Mantle presses up on the crust Isostacy- balance of these two forces Up & down movements of the crust to reach isostacy are called isostatic adjustments As isostatic adjustments occur, areas of the crust are bent up and down, pressure created by this causes the rocks in that area of the crust to deform Constantly occurs in areas of the crust with mountains Can also be found in areas where there are large bodies of water or where glaciers once were

Stress Isostatic adjustments and plate movement cause stress in rocks that make up the earth’s surface Stress- amount of force per unit area that is placed on a given material Crustal stress occurs when lithospheric plates collide, separate, or rub together Causes strain in crust rocks Strain is a change in the shape or volume of rocks that results from the stress of being squeezed, twisted, or pulled apart

3 Types Of Stress Compression- occurs when crustal rocks are squeezed together Often reduces the volume of rocks Tends to push the rocks up higher or deeper down into the crust

3 types of Stress Tension- force that pulls rocks apart Rocks tend to become thinner

3 types of Stress Shearing- pushes rock in opposite horizontal directions Rocks bend, twist, or break apart as they slide past each other

The Results of Stress High pressures and temperatures caused by stress in the crust generally deform rocks. If the force exceeds a certain limit, the shape of the rock changes permanently

The Results of Stress Faulting Cooler temperatures and lower pressure near the earth’s surface often cause rock to respond to stress by breaking 2 categories of breaks in rocks Fracture- when there is no movement in the rock along either side of the break Faults- when there is movement in the rock along either side of the break

The Results of Stress Faulting cont… Fault plane- surface of the fault along which any motion occurs Hanging wall- rock above the fault plane Foot wall- rock below the fault plane

Types of Faults Normal fault Fault in which the hanging wall moves down relative to the footwall Form along divergent boundaries Usually occur in a series of fault lines, forming step-like land forms Ex: Great Rift Valley

Types of Faults Reverse fault Forms when compression causes the hanging wall to move up relative to the footwall Thrust fault- special type of reverse fault in which the fault plane is at a low angle or nearly horizontal The rocks in the hanging wall are pushed up and over the rocks in the footwall because of the low angle of the fault plane.

Types of Faults Strike-slip fault Rock on either side of the fault plane slides horizontally Often occur at transform boundaries Ex: San Andreas Fault Faulting

Mountain Formation Mountain range- a group of adjacent mountains with the same general shape and structure Ex: Mount Everest is in the Himalaya Range Ex: Mount St. Helens is in the Cascade Range Mountain system- a group of adjacent mountain ranges Ex: Great Smoky, Blue Ridge, Cumberland, & Green mountain ranges all make up the Appalachian mountain system

Mountain Formation Mountain belt- group of large mountain systems 2 major belts on earth 1) circum-Pacific belt- forms a ring around the Pacific Ocean 2) Eurasian-Melanesian- runs from the Pacific islands through Asia and southern Europe and into northwestern Africa

Plate Tectonics and Mountains Both the circum- Pacific and Eurasian-Melanesian mountain belts are located along convergent plate boundaries Leads scientists to believe that most mountains were formed when lithospheric plates collided

Plate Tectonics & Mountains Collisions Between Continental & Oceanic Crust Oceanic crust is subducted beneath the continental crust Subduction of the oceanic crust causes partial melting of the overlying mantle material, producing magma that may eventually erupt & form volcanic mountains

Plate Tectonics and Mountains Collisions Between Oceanic Crust and Oceanic Crust Volcanic mountains sometimes form where two plates with oceanic crust at their edges collide One plate subducts beneath the other, water from the subducting plate induces partial melting of mantle material to form magma Magma rises and breaks through the ocean crust Forms an arc of volcanic mountains on the ocean floor Ex: Mariana islands

Plate Tectonics & Mountains Collisions Between Continents Two continents collide Ex: Himalayas

Types of Mountains Scientists classify mountains according to the way in which the crust was deformed and shaped by mountain-building forces. Types of Mountains: Folded Mountains & Plateaus Fault-block Mountains & Grabens Volcanic Mountains Dome Mountains

Folded Mountains and Plateaus Highest mountain ranges in the world Commonly found where continents have collided Tectonic movements have squeezed rock layers together like an accordion Plateaus Large flat topped rocks high above sea level Formed when thick, horizontal layers of rock are slowly uplifted Most are found next to mountain ranges Ex: Tibetan Plateau is next to the Rockies

Fault-Block Mountains and Grabens Formed where parts of the earth’s crust have been extended and broken into large blocks & faulting tilted the blocks and caused some blocks to drop down relative to other blocks Ex: Sierra Nevada range Grabens Develop when steep faults break the crust into blocks and a block slips downward Ex: Death Valley

Volcanic Mountains Mountains that form when molten rock erupts onto the earth’s surface May develop on land or on the ocean floor Ex: Cascade Range Some of the largest volcanic mountains are found along divergent plate boundaries, which form the mid-ocean ridges Ex: Hawaiian Islands- tips of high volcanic mountains that formed over a hot spot on the sea floor

Dome Mountains Unusual type of mountain Formed when molten rock rises through the crust and pushes up the rock layers above it Ex: Black Hills in South Dakota

http://youtu.be/zPfILoG7ojo http://youtu.be/uoyrqhUbiko Formation of the Rocky Mountains