11.2A Folds, Faults, and Mountains
Folds and
Faults
Folds
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Over millions of years, stress
forces can bend rock like a ribbon
or soft dough.
Steady pressures of stress over
long periods of time affect
sedimentary layers and can fold
them into dramatic forms.
Folds
Folds :
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During mountain building,
compressional stresses often bend
flat-lying sedimentary rocks into
wavelike ripples called folds.
Folds of sedimentary strata come
in three main types
 Anticlines
 Synclines
 Monoclines
Anticlines and Synclines
Anticlines and Synclines :
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An anticline is usually formed by
the upfolding, or arching of rock
layers.
Often found in association with
anticlines are downfolds, or
troughs, called synclines.
The anticlines are the folds that go
upwards and the synclines are the
folds that go downward.
Dips
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The angle that a fold or fault
makes with the horizontal is called
the dip of the fold or fault.
The more the bend in the fold or
fault, the stronger the dip.
In the figure at right, folds, faults
and dips are visible in B.
In C, the folds are starting to
overturn and D and E the folds
have overturned all the way and
folded over completely.
Monoclines
Monoclines
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Folds are generally closely related
to faults in the Earth’s crust.
Examples of this close association
can be found in monoclines.
Monoclines are large step-like folds
in otherwise horizontal sedimentary
layers.
Monoclines occur as sedimentary
layers get folded over a large
faulting-block of underlying rock.
Monoclines are a prominent feature
of the Colorado Plateau region.
Hanging walls and footwalls
Faults
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Recall that faults are fractures in
the Earth’s crust along which
movement has taken place.
The rock surface immediately
above the fault is called the
hanging wall.
The rock surface below the fault is
called the footwall.
Types of Faults
Faults

The major types of faults are
 Normal faults
 Reverse faults
 Thrust faults
 Strike-slip faults
Types of Faults
Faults
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Normal faults occur due to
tensional stress and reverse and
thrust faults occur due to
compressional stress.
Compressional forces generally
produce folds as well as faults,
resulting in a thickening and
shortening of rocks.
Shearing stresses produce strikeslip faults.
Faults are classified according to
the type of movement that occurs
along the fault.
Normal Faults
Normal Faults
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A normal fault occurs when the
hanging wall block moves down
relative to the footwall block.
Most normal faults have steep dips
of about 60 degrees. These dips
often flatten out with depth.
The movement in normal faults is
mainly in a vertical direction, updown, with some horizontal
movement as well.
Because of the slide down of the
hanging wall block, normal faults
result in the lengthening, or
stretching, of the crust.
Tensional stress pulls the blocks apart
and lets the hanging wall drop downward
Reverse Faults
Reverse Faults:
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A reverse fault is a fault in which the
hanging block moves up (instead of down)
relative to the footwall block.
Reverse faults are high angle compressional
faults with dips greater than 45 degrees.
Thrust Faults
Thrust Faults:
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Thrust faults are reverse faults
with dips of less than 45
degrees.
Because the hanging wall block
moves up and over the footwall
block, reverse and thrust faults
result in a compression,
squeezing and shortening, of the
crust.
Thrust Faults
Thrust Faults:
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Most high-angle reverse faults are
small in scale. They cause only local
displacements in regions that are
already filled with other types of
faulting.
Thrust faults, however, exist at all
scales. Many can be quite large.
In the Swiss Alps, the northern
Rockies, Himalayas, and
Appalachians, thrust faults have
displaced layers as far as 50
kilometers.
The result of this type of
movement is that older rocks end
up on top of younger rocks.
Strike-Slip Faults
Strike-Slip Faults:
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Faults in which the movement is
horizontal and parallel to the line
of the fault is called a strike-slip
fault.
Because of their large scale, and
linear nature ( in a line) many
strike-slip faults produce a trace
that can be seen over a great
distance.
Rather than a single fracture,
large strike-slip faults usually
consist of a zone of roughly
parallel fractures.
Strike-Slip Faults
Strike-Slip Faults:
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The zone of parallel fractures
created by a strike-slip fault
may be up to several kilometers
wide.
The most recent movement is
often along a section only a few
meters wide and may offset
features such as stream
channels.
Crushed and broken rocks
produced during faulting are
more easily eroded, often
producing linear valleys or
troughs that mark strike-slip
faults.
Fence break created by strike-slip fault
11.2B Folds, Faults, and Mountains
Mountains,
Plateaus,
Domes
and
Basins
Types of Mountains
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Folding and faulting produce many
but not all of Earth’s mountains.
In general, mountains are classified
by the processes that formed them
The major types of mountain types
include
 Volcanic mountains
 Folded mountains
 Fault-block mountains
 Dome mountains
Mountain Ranges
Types of Mountains :
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Geologists refer to the collection
of processes involved in mountain
building as orogenesis. The term is
derived from the Greek oros
meaning “mountain” and the –geny
meaning “born”.
Earth’s mountains do not occur at
random. Several mountains of
similar shape, age, size and
structure form a group called a
mountain range.
Mountain Systems
Types of Mountains :
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A group of different mountain
ranges in the same region form a
mountain system.
The Sangre de Cristo and West Elk
mountain ranges form part of the
Rocky Mountain system.
Sangre de Cristo
Mountains Range
Rocky Mountain System
Volcanic Mountains
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Recall from the previous chapters
that volcanic mountains form along
plate boundaries and at hot spots.
In addition, igneous activity forms
rock deep in the crust that can be
uplifted as a result of plate motions
and isostatic adjustment.
Folded Mountains
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Mountains that are formed
primarily by folding are called
folded mountains.
Compressional stress is the major
cause of folded mountains.
Compressional stress helped to
form the Alps in Europe.
Thrust faulting is also important in
the formation of folded mountains,
which are often called fold-andthrust belts.
Folded Mountains
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Folded mountains often contain
numerous stacked thrust faults
that have displaced the folded
rocks layers many kilometers
horizontally.
The Appalachian Mountains, the
northern Rocky Mountains, and the
Alps in Europe are all examples of
folded mountain ranges.
Stacked thrust faults
Fault-Block Mountains
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Fault block mountains; another type
of mountain formation, is the result
of movement along normal faults.
Most normal faults are small and
have displacements of only a meter
or so.
Others extend for tens of
kilometers where they may outline
the boundary of a mountain front.
Examples fault block mountains
Fault-Block Mountains
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Large scale normal faults are
associated with fault-block
mountains
Fault-block mountains form as large
blocks of crust are uplifted and
tilted along normal faults.
Examples fault block mountains
Grabens and Horsts
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Normal faulting occurs
where 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.
Grabens produce an
elongated valley bordered
by relatively uplifted
structures called horsts.
Grabens and Horsts
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The Basin and Range regions of
Nevada, Utah, and California is
made of elongated grabens.
Above the grabens, tilted faultblocks or horsts produce parallel
rows of fault-block mountains.
Sierra Nevada Range
Grabens and Horsts
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In the western US, other
examples of fault block mountains
include the Grand Tetons and the
Sierra Nevada Range in
California.
These steep mountain fronts
were produced over 5 to 10
million years by many episodes of
faulting.
Sierra Nevada Range
Plateaus, domes, basins
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Mountains are not the only
landforms that result from
forces in Earth’s crust.
Up and down movements of the
crust can produce a variety of
landforms, including
plateaus
 domes
 basins.

Plateaus
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A plateau is a landform with a
relatively high elevation and more
or less level surface.
To form a plateau, a broad area
of the crust is uplifted vertically;
raised above the adjoining
landscape.
Plateaus can cover very large
areas of land such as the
Colorado Plateau which stretches
over four states.
Colorado Plateau
Domes
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Broad upwarping in the rock
underlying an area may deform
sedimentary layers.
When upwarping produces a
roughly circular structure, the
feature is called a dome.
Domes often have the shape of an
elongated oval.
You can think of the upwarped
layers that make up a dome as a
large fold.
Basins
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Downwarped structures that have
a roughly circular shape are
called basins.
The central United States
contains a number of basins,
including the large Michigan
Basin.
Michigan
Basin
Basins
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During mountain building, plate
motions can cause the crust to
bend downward and form a basin.
If the basin sinks below sea level,
it may form a shallow sea.
Over time, sediments such as
sand and the skeletons of ocean
creatures are laid down, forming
layers of sedimentary rock.
Michigan
Basin
Basins
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Basins may also form along the
edges of continents where thick
layers of sediment build up. The
weight of the sediment
downwarps the crust to form a
basin.
When forces in the crust uplift
the sedimentary layers, the rock
that fills the basin is exposed at
the surface.
Michigan
Basin
Basins
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Look at the map of the Michigan
Basin to the right; it resembles a
bull’s eye. The oldest rocks are
around the edges of the basin and
the youngest rocks are near the
center.
Michigan
Basin
Basins
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The plate motions that help to
form sedimentary basins can also
destroy them.
For example, when two
continental plates collide, the
ocean basin between them closes
up.
Sedimentary rock in the basin
becomes part of the landmass
formed by the collision.
Michigan
Basin