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MOUNTAIN BUILDING
Michele Holler
FACTORS AFFECTING
DEFORMATION
• Deformation refers to all the
changes in the original shape
and /or size of a rock body
• Stress is the force per unit
area acting on a solid
• The changes in shape or
volume is called strain
• There are four factors that
influence the strength of a rock
and how it will deform. They
are temperature, confining
pressure, rock type and time
TEMPERATURE AND PRESSURE
• Rock deforms permanently in
two ways: brittle deformation
and ductile deformation
• At the surface, where confining
pressure (think of hot spots)
and temperatures are low,
rocks act like brittle solids and
fracture. This is called brittle
deformation or brittle fracture
• At depth, where temperatures
and pressure is high, rocks
show ductile behavior. Ductile
deformation produces a
change in the rock size or
shape without fracture
ROCK TYPE
• Mineral composition and
texture greatly affect how a
rock will deform
• Rocks, like granite and basalt,
that have strong internal bonds
will fail by brittle fracture
• Sedimentary and foliated
metamorphic rocks will deform
by ductile flow. Examples are
gypsum, rock salt, and shale
• Rocks of intermediate strength,
like marble and limestone, may
also behave in a ductile
manner
TIME
• How long a stress is
applied to a rock also
plays a role in
deformation
• Forces that are
unable to deform a
rock when first
applied may cause a
rock to flow if
maintained over a
long period of time
THE THREE TYPES OF STRESS
• Tensional stress-when
rocks are pulled in the
opposite directions.
Causes the material to be
stretched
• Compressional stresswhen rocks are squeezed
or shortened
• Shear stress-causes a
body to be distorted with
no changes in volume
THREE TYPES OF FOLDS
• Folds are a series of
wave-like ripples
• ANTICLINES-formed by
upfolding or arching
• SYNCLINES-are
downfolds or troughs and
are a limb of an anticline
• MONOCLINES-large
step-like folds in
otherwise horizontal
sedimentary strata. Are
prominent in the Colorado
plateau
FAULTS
•
•
1.
2.
3.
4.
Are fractures in the
crust along which
movements occurs
There are four major
types of faults:
Normal faults
Reverse faults
Thrust faults
Strike-slip faults
NORMAL FAULTS
• Hanging wall block
moves down relative to
the footwall
• Most have steep dips of
about sixty degrees
• Movement is mainly in a
vertical direction with
some horizontal
movement
• Results in lengthening or
extension of the crust
from tensional stress
REVERSE FAULTS
• Hanging wall block
moves up relative to the
footwall
• Are high angle faults with
dips greater than 45
degrees
• Most are small and only
cause local displacement
• Result from
compressional stress
THRUST FAULTS
• Are reverse faults
with dips less than 45
degrees
• Hanging wall moves
up and over the
footwall
• Have large-scale
movement
• Caused by
compressional forces
STRIKE-SLIP FAULTS
• Movement is horizontal and
parallel to the trend or strike
• Produce a trace that is visible
over great distances
• Consists of a zone of parallel
fractures
• Produce large earthquakes
• Crushed and broken rocks are
easily eroded
• Example is the San Andreas
JOINTS
• FRACTURES WITH
NO MOVEMENT
• MOST OCCUR IN
ROUGHLY
PARALLEL GROUP
• RESULT IN LARGE
SCALE REGIONAL
STRESSES
FOLDED MOUNTAINS
• Formed primarily by
folding
• Caused by
compressional
stresses
• Thrust faulting
produces fold and
thrust belts that have
displaced the folded
rock layers
horizontally
FAULT-BLOCK MOUNTAINS
• Caused by large-scale
normal faults and
tensional stress
• Examples include the
Teton Range in Wyoming
and the Sierra Nevada in
California
• Grabens which are like
trenches and horsts
which are the uplifted
structures are formed
with this type of mountain
graben
horst
DOMES AND BASINS
• Domes are structures
produced by up
warping like the Black
Hills of South Dakota
• Basins are down
warped structures
with a circular shape.
Found in Michigan
and Illinois
MOUNTAIN BUILDING AT
CONVERGENT BOUNDARIES
• Most mountain
building occurs here
• Colliding plates
provide the
compressional forces
that fold, fault and
metamorphose
sediments
OCEAN-OCEAN CONVERGENCE
• When two oceanic
plates collide
• Produces volcanic
mountains
• Examples are the
Aleutian Islands
OCEAN-CONTINENTAL
CONVERGENCE
• Forms a continental
volcanic arc
• Andes are an example
• An accretionary wedge is
formed which is an
accumulation of different
sedimentary and
metamorphic rocks
• Forms volcanic and
folded mountains
CONTINENT-CONTINENT
CONVERGENCE
• Form folded
mountains
• When the Eurasian
and India plate
collided they formed
the Himalayas
DIVERGENT BOUNDARIES
• OCCUR MAINLY ON
THE OCEAN FLOOR
• FORM FAULT
BLOCK MOUNTAINS
• EXAMPLE IS THE
MID-ALTLANTIC
RIDGE
NON-BOUNDARY MOUNTAINS
• Some upwarped
mountains, fault-block
mountains and
volcanic mountains
are not formed at
plate boundaries
• Examples are the
Hawaiian islands
CONTINENTAL ACCRETION
• Accretion is when the
fragments get stuck to or
embedded into the
continent
• Terranes are crustal
fragments that has a
geologic history distinct
from the joining terranes
• Accretion of mountain
arcs may result in
mountains
ISOSTASY
• Floating crust in
gravitational balance
• Mountains stand high
because they have
buoyant crustal roots
that extend dense into
the deeper mantle
• Isostatic adjustment is
when a new level of
gravitational
equilibrium is reached
ISOSTATIC ADJUSTMENT
• BECAUSE OF
ISOSTASY, DEFORMED
AND THICKENED
CRUST WILL UNDERGO
REGIONAL UPLIFT
DURING MOUNTAIN
BUILDING AND A LONG
PERIOD AFTERWARDS
• EROSION CAUSES THE
CRUST TO RISE