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 crustWhen parts of the crust become thicker and
heavier, they sink more deeply into the mantleWhen 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 forcesThe 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 mountainsCan 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 rocksStrain 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 togetherOften reduces the volume of rocksTends 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
directionsRocks 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 structureEx: 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 Folded Mountains
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 Fault-Block Mountains
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 mountainFormed 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/uoyrqhUbikoFormation of the Rocky Mountains