Transcript MOUNTAINS - cravenccgeology

Mountains and
Crustal Deformation
Rock Deformation
We begin our look at mountain building by examining the process of
rock deformation and the structures that result. Every mass of rock, no
matter how strong, has a point at which it will fracture or flow.
Deformation is a general term that refers to all changes in the original
shape and/or size of a rock body. Most crustal deformation occurs
along plate margins.
When rocks are subjected to forces (stresses) greater than their own
strength, they begin to deform, usually by folding, flowing, or
fracturing. Although each rock type deforms differently, there are
specific variables that may influence the strength of a rock and how it
will deform. These variables include temperature, confining pressure,
rock type, and time.
Temperature and Pressure
Rocks near the surface, where temperatures and confining pressures are
low, tend to behave like a brittle solid and fracture once their strength
is exceeded. This type of deformation is called brittle deformation.
- Everyday examples of brittle deformation include glass objects,
wooden pencils, china plates, and even the bones in our body!
By contrast, at depth, where temperatures and confining pressures are
high, rocks exhibit ductile behavior. Ductile deformation is a type of
solid-state flow that produces a change in the size and shape of an
object without fracturing.
- Everyday examples of ductile deformation include modeling clay,
beeswax, caramel candy, and most metals (like a coin on a railroad
track – it flattens but does not break by the train exerting pressure).
Ductile deformation: These layers of sedimentary rock in England,
originally deposited horizontally, have been folded as a result of the
collision between the African and European crustal plates.
Rock Type and Time
In addition to the physical environment, the mineral composition and
texture of a rock will greatly influence how it will deform.
For example, crystalline rocks (such a granite, basalt, and quartzite)
that are composed of minerals with a strong molecular bond tend to fail
by brittle deformation. By contrast, sedimentary rocks that are weakly
cemented, or metamorphic rocks that contain zones of weakness, such
as foliation, are more susceptible to ductile deformation.
Another key factor in rock deformation is time. Forces that are unable
to deform rock when initially applied may cause rock to flow if the
force is maintained over an extended period of time. Deformation due
to material type and time can be seen in every day life as well – a
marble bench might sag under its own weight after 100 years; a
wooden bookshelf may bend after being loaded with books in a year.
Folds
During mountain building, flat-lying sedimentary and volcanic rocks
are often bent into a series of wavelike formations called folds. Most
folds are the result of compressional forces that result in the shortening
and thickening of the crust.
The two most common types of folds are
anticlines and synclines.
Domes
Broad upwarps in basement rock
may deform the overlying cover
of sedimentary strata and
generate large folds. When this
upwarping produces a circular or
elongated structure, the feature is
called a dome.
The Black Hills of South Dakota
is a large domal structure with
resistant igneous and
metamorphic rocks in the core.
Erosion stripped away the
overlying sedimentary layers and
exposed the older rocks.
Basins
When downwarping of
basement rock occurs,
basins are created.
The basis of Michigan and
Illinois have very gently
sloping beds similar to
saucers. These basins are
thought to be the result of
large accumulations of
sediment, whose weight
caused the crust to subside.
Younger rocks are usually
found in the center, with older
ones around the outside.
Faults
Faults are fractures in the crust along which appreciable displacement
has taken place. Some faults might be small (a few feet), but others,
like the San Andreas Fault in California, have displacements of
hundreds of feet which can easily be seen through aerial photography.
Earth scientists use the angle of the fault with respect to the
surface (known as the dip) and the direction of slip along the
fault to classify faults. There are two main types of faults:
Dip-slip faults and strike-slip faults. Dip-slip faults are
further classified as either normal or reverse faults, depending
on the direction of the hanging wall and footwall.
Dip-Slip Faults
It has become common practice to call the rock surface that is
immediately above the fault the hanging wall and to call the rock
surface below, the footwall.
This nomenclature arose
from prospectors and miners
who excavated shafts and
tunnels along fault zones.
In these tunnels, the miners
would walk on the rocks
below the fault zone (the
footwall) and hang their
lanterns on the rocks above
(the hanging wall).
Dip-Slip Faults
There are two major types of dip-slip faults: Normal faults and reverse
faults. Normal faults occur when the hanging wall block moves down
relative to the footwall block. Normal faults occur as a result of crust
forces pulling apart.
Reverse faults are dip-slip faults where the hanging wall block moves
up relative to the footwall block. Whereas normal faults occur in
tensional environments, reverse faults occur when crustal blocks are
moving toward each other. A special type of reverse fault, called a
thrust fault, happens when the fault has a dip of less than 45 degrees.
Strike-Slip Faults
Faults in which the dominant displacement is horizontal and parallel to
the strike of the fault surface are called strike-slip faults.
Many major strike-slip faults cut through the lithosphere and
accommodate motion between two large crustal plates. This special
kind of strike-slip fault is called a transform fault. The best-known
type of transform fault is the San Andreas Fault in California.
San Andreas Fault,
a transform-type of
strike-slip fault.
Joints
Unlike faults, joints are fractures along which no appreciable
displacement has occurred. Although some joints have a random
orientation, most occur in roughly parallel groups.
Columnar
joints form
when igneous
rocks cool and
develop
shrinkage
fractures that
produce
elongated,
pillarlike
columns.
Mountain Building
The name for the processes that collectively produce a mountain belt is
orogenesis. The rocks comprising mountains provide striking visual
evidence of the enormous compressional forces that have deformed
large sections of Earth’s crust.
Although folding is often the most obvious sign of these forces, thrust
faulting, metamorphism, and igneous activity are always present in
varying degrees.
Most mountain building occurs at convergent plate boundaries, which
can involve both oceanic and continental crust. Mountain building
occurs at subduction zones (island arcs), collisional ranges (Himalayas),
and through accreted terranes, which is when pieces and fragments of
crust break off and suture themselves to other pieces of crust.
Mountain building
along a subduction zone
Accretion occurs as
the inactive volcanic
arc collides with
continental crust.
This map shows terranes
through to have been
added to western North
America during the past
200 million years.
Continental rifting can also produce uplift and the formation of
mountains. The mountains that form as normal faults pull apart are
called fault-block mountains. A good example of this is the Grand
Tetons of Wyoming or the Sierra Nevada mountains in California.