Transcript Deformation of the Crust

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 EurasianMelanesian 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?