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Chapter 11
Mountain Building
Rock Deformation
It is theorized that all continents were
once mountainous masses and grow by
adding mountains to their edges.
If this is so, then how do mountains grow
in the middle of continents?
Factors Affecting Deformation
Every rock has a point at which it will
bend and/or break.
Deformation is the term that refers to
all changes in the original shape and/or
size of a rock body.
Most deformation occurs at plate
margins.
Stress is the force per unit area acting on
a solid. Under great stress, rocks tend
to deform usually by:
Folding
Faulting
Flowing
Fracturing
The change in shape or volume of a body of
rock as a result of stress is called strain.
Rocks can be bent into folds if stress is applied
gradually and not going beyond the breaking
point.
We already know that rocks under stress have
elastic properties and will go back to their
original shape and size when the force is
removed. (elastic rebound)
Once the elastic limit or strength of a rock
is surpassed, it either flows or fractures.
The factors that influence the strength of
a rock and how it will deform include:
Temperature
Confining pressure
Rock type
Time
Temperature and Pressure
Rocks deform permanently two ways:
Brittle deformation
Ductile deformation
Rocks near the surface, where
temperatures and confining pressures
are low, usually behave like brittle
solids and fracture once their strength
is exceeded.
This type of deformation is called brittle
failure or brittle deformation.
Examples: china, bones, pencils, glass
At depth, where temperatures and confining
pressures are high, rocks show ductile
behavior.
Ductile deformation is a type of solid-state flow
that produces a change is size and shape of
an object without fracturing the object.
Examples: modeling clay, bee’s wax, caramel
candy, and most metals
Question ????????????
Placing a penny on a railroad track and
having a train run over it, causing the
penny to flatten out is an example of?
ductile deformation
Rock Type
The mineral composition and texture of a
rock also affect how it will deform.
Granite and basalt contain minerals with
strong internal molecular bonds will fracture.
Sedimentary rocks that are weakly cemented
or metamorphic rocks that are foliated are
more likely to deform by ductile flow.
Rocks that exhibit ductile flow may include:
Soft
Halite (rock salt)
Gypsum
Shale
Intermediate strength
Limestone
Schist
Marble
Time
Natural stress applied over a long period
of time will fold rocks.
Forces that are unable to deform rock
when first applied may cause rock to
flow if the force is maintained over a
long period of time.
Types of Stresses
Three types of stresses rocks undergo
include:
Tensional forces – rocks being pulled in
opposite directions
Compressional force – rocks are
squeezed and shortened
Shear stress – when a body of rock is
distorted
Stresses
Folds
During mountain building, flat lying rocks
(both sedimentary and igneous) are
bent in wave-like ripples called folds.
There are three types of folds:
Anticline – an upward fold (arch)
Syncline – a downward fold (trough)
Monocline – closely associated with faults,
monoclines are large step–like folds in
otherwise horizontal layers of sedimentary
strata.
Prominent features of the Colorado Plateau
which coves Colorado, New Mexico, Utah,
and Arizona.
Anticline and Syncline Process
Circular Anticline
Syncline
Joints – fractures in rock strata where no
movement has taken place
So faults are……………
Three types of faults
Normal – caused by tensional stress
Reverse (or thrust) – caused by
compressional stress
Strike-slip – commonly caused by shear
stress – San Andreas Fault
Normal
Reverse or Thrust
Strike-Slip
What type of fault? Strike-slip
What type of fault? Normal
What type of fault?
Reverse or
Thrust
Thrust fault
Types of Mountains
The collective processes that produce a
mountain belt are called orogenesis.
The dominant processes that have
formed them classify mountains.
Folded Mountains
Mountains formed primarily by folding are
called folded mountains.
Thrust faults are also important in the
formation of folded mountains.
These mountains are called fold and thrust
belts.
Examples: Appalachian Mountains, the
northern Rocky Mountains, and the Alps of
Europe
Fault-Block Mountains
Large-scale normal faults are associated with
structures called fault-block mountains.
These mountains form when large blocks of
crust are uplifted and tilted along normal
faults.
Examples: Teton Range of Wyoming and the
Sierra Nevada Range of California.
Normal faulting occurs when tensional stresses cause
the crust to be stretched or extended.
As the crust is stretched, a block called a graben,
which is bounded by normal faults, drops down.
Graben is German for ditch or trench.
Grabens produce an elongated valley bordered by
relatively uplifted structures called horsts.
Examples: Basin and Range Province of Nevada,
Utah, and California.
Rift or Grabben
Graben or Rift
example is: Death Valley 250
ft. below sea level
Dead Sea is
1200 ft below
sea level
Domes and Basins
These mountains are produced by
broad upwarping in the basement rock
deforming the overlying sedimentary
strata.
A dome is when upwarping produces a
circular or elongated structure or dome.
How are these domes uncovered?
Erosion strips away the highest portion of
the sedimentary beds exposing older
igneous and metamorphic rocks in the
center.
The oldest rocks form the core of these
mountains.
Example: Black Hills
Downwarped structures having a circular shape are
called basins.
These structures have gently sloping bed similar to
saucers.
Basins are thought to be formed by the accumulation
of sediment, whose weight caused the crust to
subside.
Example: Basins of Michigan & Illinois
In the case of a basin the oldest rocks are found in
the center with the youngest rocks on the flanks.
Mountain Formation
Just how old are mountains?
Appalachians are 100s of million of years
old
Himalayas are about 45 million years
old… Just a youngster!
Mountain Building at Convergent Boundaries
Most mountain building occurs at
convergent plate boundaries.
Colliding plates provide the
compressional forces that fold, fault,
and metamorphose the thick layers of
sediment deposited on the edges of
landmasses.
Ocean-Ocean Convergence
Converging oceanic plates result in
subduction that creates magma leading
to the growth of a volcanic island arc
once it grows above sea level.
Examples: Aleutian Islands in Alaska,
Japan,
Ocean-ocean convergence mainly
produces volcanic mountains.
Ocean-Continental Convergence
Example: West coast of South America – the
Andes Mountains
This produces a continental volcanic arc.
During subduction, sediment scraped from
the subducting plate is stuck against the
landward side of the trench.
This accumulation of sediment and
metamorphic rock is called an accretionary
wedge.
Ocean-continent convergence produces
mountains in two roughly parallel belts.
The continental volcanic arc develops on the
continental block and the accretion wedge is
on the seaward belt.
The types of mountains formed by oceancontinent convergence are volcanic
mountains and folded mountains.
Continent-Continent Convergence
A convergent boundary between two plates
carrying continental crust, a collision between
the continental fragments will result and form
folded mountains.
Example: Himalayan Mountains and the
Tibetan Plateau, European continent collided
with Asia producing the Ural Mountains in
Russia.
Mountain Building at Divergent
Boundaries
These mountains are usually formed on
the ocean floor
Example: Mid-ocean ridges that extend
65,000 kilometers.
The mountains that form along ocean
ridges at divergent plate boundaries are
fault-block type mountains.
Non-Boundary Mountains
Example: Hawaiian Islands
Continental Accretion
Accretion is when small crustal
fragments collide and merge with
continental margins.
Terrane – is any coastal fragment that has a
geological history distinct from that of the
adjoining terranes.
Some may be no larger than volcanic islands,
while other are immense, such as the entire
Indian subcontinent.
Some may have been microcontinents similar
to present day Madagascar.
Many others were island arcs like Japan and
the Philippines.
All these materials add to the continent
width and thickness displacing other
fragments further inland.
Mountains of Accretion
Example: Mountains in western North
America and western Canada contain
rock, fossils, and structures different
from the surrounding area.
These materials have been accreted
(added to) the western margin of North
America.
Principle of Isostasy
This is a gradual up and down motion
of materials that make up the interior of
continents away from continental
margins.
Earth’s crust floats on top of denser
more flexible rocks in the mantle.
The concept of a floating crust in
gravitational balance is isostasy.
Mountain belts float on top of more
dense crustal roots that extend into the
mantle.
The denser mantle supports the
mountains from below.
If more material were added to the top of the
mountains the combined block would sink
until a new isostatic balance was reached.
However, the top of the combined block
would be higher than before and the bottom
would be lower.
This process of establishing a new level of
gravitational equilibrium is called isostatic
adjustment.
Isostatic Adjustment of Mountains
Continental ice sheets provided evidence of
crustal subsidence followed by rebound.
The weight of 3 kilometers of ice depressed
Earth’s crust by hundreds of meters.
In 8000 years since the last ice sheets
melted, uplift of as much as 330 meters has
occurred in Canada’s Hudson Bay region.
Most mountain building causes the crust to
shorten and thicken.
Because of isostasy, deformed and thickened
crust will undergo regional uplift both during
mountain building and for a long period
afterward.
As the crust rises (rebounds) the processes of
erosion increases and the deformed rock
layers are carved into mountains.
Erosion removes material from the
summit reducing the load causing the
crust to rise. This will continue until the
mountain block reaches its “normal”
thickness.