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
 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 midocean 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