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Deformation
Every body of rock, no matter how strong, has
a point at which it will bend or break.
Deformation is a general term that refers to all
changes in the original shape and/or size of a
rock body. Rocks bend, twist, and fracture.
Most crustal deformation occurs along plate
margins because plate motions create forces
that cause rock to deform.
Most deformation occurs much too slowly for
human observation.
Deformation
Stress is the force per unit area acting
on a solid. Uniform stress is pressure.
When rocks are under stresses greater
than their own strength, they begin to
deform, usually by FOLDING, FLOWING, or
FRACTURING.
The change in shape or volume of a
body of rock as a result of stress is called
strain.
Deformation
When stress is gradually applied, rocks
can be bent into folds without breaking
because they deform elastically. These
changes are recoverable, and the rock will
return to almost its original size and shape
when the force has been removed.
However, once the elastic limit or
strength of a rock is surpassed, it either flows
(ductile deformation) or fractures (brittle
deformation). These kinds of deformation are
permanent.
Factors That Influence Deformation:
Temperature
Confining pressure
Near the surface, where
temperatures and
confining pressures are
LOW, rocks usually
behave like brittle solids
and fracture once their
strength has been
exceeded resulting in
BRITTLE FAILURE or
BRITTLE DEFORMATION
Factors that Influence Deformation
Deep inside the Earth,
temperatures and
confining pressures are
HIGH so rocks undergo
DUCTILE DEFORMATION
that produces a change
in the size and shape of
an object without
breaking it.
Factors That Influence Deformation:
Rock Type
Another factor that influences the type of
deformation is the ROCK TYPE because of
mineral composition and texture.
– Rocks with strong internal molecular bonds
usually fail by brittle fracture. Examples:
GRANITE and BASALT (igneous).
– Sedimentary rocks that are weakly cemented or
metamorphic rocks with zones of weakness
(such as foliation) are more likely to deform by
ductile flow. Examples: GYPSUM, SHALE, ROCK
SALT; sometimes LIMESTONE, MARBLE, and
SCHIST.
Factors That Influence Deformation:
Time
TIME is another factor
that influences deformation.
Small, short-term stresses
that do not cause
deformation can cause rock
to flow when maintained
over long time periods.
Marble benches have
been known to sag under
their own weight after time
spans of nearly a century.
Types of Stress on Rocks
• Tensional
Pulled in opposite directions
• Compressional
Squeezed or shortened
• Shear
Distorted due to forces
acting in opposite directions
Types of Folds that Rocks Form
During mountain building, flat-lying sedimentary
and volcanic rocks are often bent into folds:
Types of Folds That Rocks Form
• Anticline
Layers bend downward in opposite
directions from the crest (letter “A”
shape)
• Syncline
Layers dip upward toward each other
• Monocline
Large, step-like fold in otherwise
horizontal sedimentary strata, usually
over an underlying faulted block.
Types of Folds That Rocks Form
Fault Types
Faults are fractures in the crust along which
movement has taken place.
For faults with vertical offsets, the footwall is
the block beneath the fault plane. The hanging
wall is the block above the fault plane.
Types of Faults: Dip-Slip
1. NORMAL
Hanging wall block moves DOWN relative to the
footwall block (vertical offset).
Crust is stretched and extended. (Tensional forces
and gravity, @ divergent plate boundaries)
Types of Faults : Dip-Slip
2. REVERSE
Hanging wall block moves UP relative to the
footwall block (vertical offset).
Crust is shortened. (Compressional forces @
convergent plate boundaries)
Types of Faults : Dip-Slip
3. THRUST
Reverse faults with dips of less than 45° (shallow).
Hanging wall block moves UP relative to the footwall block.
Crust is shortened (compressional forces @ convergent
plate boundaries).
Types of Faults : Dip-Slip
Dip-slip faults can be OBLIQUE which means
they have horizontal as well as vertical offsets.
Types of Faults: Strike-Slip
4. STRIKE-SLIP
Movement is predominantly horizontal, parallel to
the trend or strike of the fault surface
Crust is displaced without extending or
compressing by shear forces (@transform plate
boundaries)
Source: http://www.livescience.com/37052-types-of-faults.html
Can You Identify the Types of Faults?
A
B
C
D
Seen from above
Mountain Formation
• Geologists refer to the collection of processes
involved in mountain building as orogenesis.
The term is derived from the Greek words
oros meaning “mountain” and -geny meaning
“born”.
Mountain Ranges and Systems
• Earth’s mountains do not occur
at random. Several mountains of
similar shape, age, size, and
structure form a group called a
mountain range.
• A group of different mountain
ranges in the same region form a
mountain system.
• The Brooks Range (AK),
Canadian Rockies, Absarokas
(MT/WY), Grand Tetons (WY),
and Sangre de Cristo (CO)
mountain ranges form part of
the Rocky Mountain system.
Types of Mountains
There are four main types of mountains based
upon their formation process:
1.
2.
3.
4.
Dome or upwarped
Volcanic
Fault-block
Folded
(complex)
Dome or Upwarped Mountains
• Broad upwarping in the
rock underlying an area
may deform
sedimentary layers.
• You can think of the
upwarped layers that
make up a dome as a
large fold.
• Normal faults can
develop as the rock
layers stretch forming
fault scarps.
Dome or Upwarped Mountains
• Examples are the Black Hills of SD and
Adirondack Mountains of NY
Volcanic Mountains
• Form from the buildup of lava
and pyroclastic material
• Continental volcanic arcs and
island volcanic arcs along
convergent plate boundaries
are volcanic mountain ranges
such as the Andes and
Cascade Ranges and the
Aleutian Islands
• Hot spots such as Mauna Loa
in Hawai’i
• Continental rift volcanoes
such as Mt. Kilimanjaro in
Africa
Fault-Block Mountains
• Fault-block mountains form as large blocks of
crust are uplifted and tilted along large-scale
normal faults.
• Some form by tilting of fault
blocks by broad crustal
uplift while others occur
due to tensional stresses.
Often show one
steep flank
(fault scarp)
while the other
flank is more
gradual.
Fault-Block Mountains
• 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.
• A graben produces an elongated valley bordered
by relatively uplifted structures called horsts.
Fault-Block Mountains
• In the western US,
examples of fault block
mountains include the
Grand Tetons (WY) and
the Sierra Nevadas in
California.
Folded (Complex) Mountains
• Mountains that are
formed primarily by
folding of rock strata are
called folded mountains.
• Compressional stress is
the major cause of
folded mountains.
• Examples are the
Himalayas in Asia, the
northern Rockies and
the Appalachians in
North America, and
the Alps in Europe.
Plateau
• A plateau is a large highland area of fairly
level land separated from surrounding land
by steep slopes.
• Some plateaus, like the plateau of Tibet, lie
between mountain ranges. Others are higher
than surrounding land.
• Plateaus are widespread, and together with
enclosed basins they cover about 45 percent
of the Earth's land surface.
http://www.edu.pe.ca/southernkings/plateaurh.htm
Plateau