Transcript Earth Science Ch 11 Review : Mountains

Earth Science Ch 11 Review : Mountains
Mountain Building: Review
Mountain Building : Deformation
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Over millions of years, these mountains
formed when plate motion and other forces
uplifted the crust of the Earth. At the same
time, weathering and erosion shape the
crust into peaks and other formations. The
process begins when plate motions produce
forces in rock that cause it to bend or
break.
Deformation is any change in the
original shape and/or size of a rock body. In
Earth’s crust, most deformation takes place
along plate boundaries.
Deformation occurs because of stress in a
body of rock. Stress is the force “per unit
area” acting on a mountain. ( pressure per
square inch, foot, or meter for example)
Elastic deformation
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When rocks are under stress that is greater
than their own strength, they begin to deform.
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Usually they deform by one of the following:
Folding, Flowing, Fracturing
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The change in shape or volume of a body of
rock is called strain. When stress is gradually
applied, rocks first respond by responding
elastically.
A change that results from elastic
deformation can be reversed. Like a rubber
band; the rock will return to it’s original size
and shape once the force upon it is removed.
Once the elastic limit or strength for a
rock is surpassed, the rock either flows or
fractures.
Ductile and brittle deformation
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The factors that affect the deformation of
rock include: Temperature, Pressure, Rock
type, Time.
Rocks deform permanently in two ways:
Brittle deformation, Ductile deformation
Rocks near the surface, where temperatures
and pressures are low, usually behave like
brittle solids and fracture once their
strength is exceeded
Brittle materials break
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Ductile deformation is a type of solid-state
flow that produces a change in the size and
shape of an object without fracturing the
object. Ductile materials buckle or bend.
Ductile materials bend
Ductile deformation and brittle fracture
The mineral composition and texture of a rock
also greatly affects how it will deform.
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Rocks like granite and basalt that have a
strong internal molecular bonds usually fail
by brittle fracture.
Sedimentary rocks or metamorphic rocks
are more likely to deform by ductile
deformation; bending or buckling.
Small stresses applied over long time spans
eventually cause the deformation of rock.
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.
Brittle materials break
Ductile materials bend
Types of Stress
Plate motions cause different types of
stress in the rocks of the lithosphere.
The three types of stress that cause
deformation of rocks are
 Tensional stress
 Compressional stress
 Shear stress
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When rocks are squeezed the stress is
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When rocks are pulled in opposite directions,
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Shear stress causes a body of rock to be
compressional stress.
the force is tensional
distorted.
stress.
Isostosy
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Earth’s crust floats on top of the denser more
flexible rocks of the mantle. The concept of a
floating crust in gravitational balance is called
isostosy.
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One way to understand the concept of isostosy is to think
bout a series of wooden blocks of different heights
floating in water. The thicker blocks float higher than the
thinner blocks.
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In a similar way, many mountain belts stand high above the
surface because they have less dense “roots” that extend
deep into the denser mantle. The denser mantle supports
the mountains from below.
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What would happen if another small block of wood was
placed upon one of the floating blocks? The combined
blocks would sink until a new balance of gravity was
reached.
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However, the top of the pair of blocks would be higher
than it was before and the bottom would be lower. This
process of finding a new level of gravitational balance is
called isostatic adjustment.
Over time, the roots of the
mountains rise up in
Isostatic adjustment
Isostatic adjustment
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Applying this concept of isostosy, we should
expect that when weight is added to the crust,
the crust responds by sinking lower. Also , when
weight is removed the crust will rebound and
rise again.
As erosion reduces the summits of mountains,
the crust will rise in response to the reduced
load of weight.
The process of erosion and uplift together will
continue until the mountain block reaches it’s
normal crust thickness; it’s balancing point or
equilibrium.
When this occurs, the mountain will be eroded
to near sea-level and the once deeply buried
interior of the mountain will be exposed at the
surface.
Over time, the roots of the
mountains rise up in
Isostatic adjustment
Folds and Faults
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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.
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
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
angle that a fold or fault makes with the
horizontal is called the dip of the fold or fault.
A vertical cliff straight up would be a 90 degree
dip.
Folds and Faults
 Folds are generally closely related to faults in
the Earth’s crust. Examples of this close
association can be found in monoclines.
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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.
Faults: 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.
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The major types of faults are
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Normal faults
Reverse faults
Thrust faults
Strike-slip faults
Folds and Faults
Faults: Normal faults occur due to tensional stress
and reverse and thrust faults occur due to
compressional stress.
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Compressional forces generally produce folds as
well as faults, resulting in a thickening and
shortening of rocks. Shearing stresses produce
strike-slip faults. Faults are classified according
to the type of movement that occurs along the
fault.
Normal Faults: A normal fault occurs when the
hanging wall block moves down relative to the
footwall block.
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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 and Strike-Slip 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 of the crust.
Most high-angle reverse faults are small in
scale. Thrust faults, however, exist at all
scales. Many can be quite large and account for
some of the largest mountain ranges in the
world such as the Alps in Europe. The result of
this type of movement is that older rocks end
up on top of younger rocks.
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 strikeslip faults produce a trace that can be seen
over a great distance.
Strike-slip fault
4 types Mountain building
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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
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Earth’s mountains do not occur at random.
Several mountains of similar shape, age, size
and structure form a group called a
mountain range.
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A group of different mountain ranges in the
same region form a mountain
system.
Rocky Mountain System
Volcanic and Folded Mountains
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Volcanic Mountains : 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.
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.
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Thrust faulting is also important in the
formation of folded mountains, which are
often called fold-and-thrust belts. Folded
mountains often contain numerous stacked
thrust faults that have displaced the folded
rocks layers many kilometers horizontally.
Stacked thrust faults
Fault Block Mountains
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Fault block mountains; another type
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Large scale normal faults are associated
of mountain formation, is the result of
movement along normal faults.
with fault-block mountains .
Fault-block
mountains form as large blocks of crust
are uplifted and tilted along normal faults.
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Grabens and Horsts: 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.
Fault Block Mountains
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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
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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.
Plateaus, Domes and 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.
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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.
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.
Down-warped structures that have a roughly circular
shape are called basins. The central United States
contains a number of basins, including the large
Michigan Basin.
Colorado Plateau
Mountains and Plates: Ocean-Ocean Convergence
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Mountain building still occurs in many places
worldwide. The jagged mountain peaks of the
Grand Teton Range in Wyoming began to form
about a million years ago and is still rising to this
day. In contrast, older mountain ranges, such as
the eastern Appalachians, are deeply eroded.
With the development of the theory of plate
tectonics, a widely accepted model for mountain
building became available. Most mountain building
occurs at convergent plate boundaries. Colliding
plates provide the compressional forces that fold,
fault, and metamorphose the thick layers of
sediments deposited at the edges of landmasses.
Ocean-Ocean convergence: The convergence of two
oceanic plates mainly produces volcanic mountains.
Recall that this process occurs where oceanic
plates converge in a subduction zone.
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The result of this is the formation of a
island arc on the ocean floor.
volcanic
Convergent Boundary Mountains
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Ocean-Continental Convergence: The convergence
of an oceanic plate and a continental plate
produces volcanic mountains and folded and
faulted mountains. Mountains develop in two belts
that run parallel to the edge of a continent.
Continental volcanic arcs form when an oceanic
plate is subducted beneath a continental plate.
Convergent Boundary Mountains: Another process
forms a belt of coastal mountains made up of
folded and faulted rocks. During subduction,
sediment is eroded from the land and scraped
from the subducting plate. This sediment becomes
stuck against the landward side of the trench.
Along with scraps of oceanic crust, the sediment
forms an accretionary wedge. A long period
of subduction can build an accretionary wedge
that stands above sea level. California’s coastal
ranges formed by this process.
Convergent Boundary Mountains
Continent-Continent Convergence:
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At a convergent boundary, a collision between
two plates carrying continental crust will form
folded mountains. The reason for this is
the continental crust is not dense enough,
compared with the denser crust of the mantle,
to be subducted. An example of such a collision
began about 45 million years ago when India
collided with the Eurasian Plate to form the
Himalayas.
Before this event, India was part of Antarctica.
It slowly moved thousands of kilometers north
of millions of years. The result of this collision
was the formation of the Himalayan Mountains.
Today, these sedimentary rocks and slivers of
oceanic crust are elevated high above sea-level.
The closing up of the ocean between India and
the Eurasian plate is an example of how plate
motions can destroy a sedimentary basin.
Divergent Boundary Mountains:
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Most mountains are formed at convergent
boundaries, but some are formed at divergent
boundaries, usually on the ocean floor. These
mountains form a chain that curves along the
ocean floor at the ocean ridges. This mountain
chain is over 70,000 kilometers long and rises
2000 to 3000 meters above the ocean floor.
The mountains that form along ocean ridges at
convergent plate boundaries are fault-block
mountains made of volcanic rock. The mountains
are elevated because of isostosy. Rock at the
ridge is hotter and less dense, so it rises higher
than older, colder oceanic crust.
Non-Boundary Mountains: Some mountains occur well
within plate boundaries. Volcanic mountains at
hot spots, as well as some upward mountains and
fault- block mountains, can form far from
boundary plates. The Hawaiian islands are a well
known example of volcanic mountains at a hot
spot.
Non-Boundary Mountains:
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Non-boundary mountains formed by
upwarping and faulting include the
southern Rocky Mountains. The southern
Rocky Mountains began to form about 60
million years ago with the subduction of
an oceanic plate more than 1600
kilometers away.
At first, compressional forces deformed
the crust. Than the subducting plate
separated from the lithosphere above.
This allowed hot rock to upwell from the
mantle, pushing up the crust and forming
the southern Rockies.
As the crust bent upwards, tensional
forces stretched and fractured it,
forming the fault-block mountains of the
Basin and Range region.