Faults and Folds, Deformation of the Planet and formation of metamorphic rocks
Metamorphic Rocks
Metamorphism is change
in form in response to changes in
temperature and pressure. This
change takes place in the mineral
content of the rock, but it happens
in a solid form and is due to plate
tectonics. By saying this it is
implied that metamorphism takes
place at plate boundaries but that
is not totally true. One form takes
place in basins were sediment is
deposited and undergoes
increasing pressure at depth.
As different events come
to play different states of
metamorphism comes to play. An
example of this progression of one form changing into another is below. Yet despite these changes there
is a preservation of their former conditions and the events led to metamorphism in the minerals that
have formed. The image above shows not only a change in particle size with increase degree of
metamorphism but a change in the mineral composition of these rocks. These changes are still limited
by the source rock. Chemical rocks with only one mineral present such as limestone and chert are
restricted in the metamorphic rocks that they can become.
In the process of metamorphism temperature plays a crucial role. As plate tectonics moves the
sediment from the surface to depth there is an increase in temperature (30oC/Km depth). The rocks
adjust to these higher temperatures by recrystallizing. Each mineral is stable at a specific temperature.
We use this information as a geothermometer to reveal the temperature at which the rock formed.
Remember when we talk about stability we are not talking about an immediate change but a geologic
time frame. You can observe high temperature minerals on the surface. These minerals will change at a
faster rate than low temperature minerals. An example is olivine. This mineral will break down to
limonite if exposed to humid climates in a few decades.
Pressure also affects the rocks during metamorphism. Here the rocks are exposed to two types of
pressure, confining and directed pressure. Confining pressure is a pressure that is all around you. You
are surrounded by ~1atm pressure as you read this. If you dive in the ocean the pressure increases
around your entire body until you’re crushed like a crushed bear can. Confining pressure increases with
depth of the Earth.
Directional pressure is forces directed in a particular
direction. This is called differential stress and is
concentrated along discrete planes. Heat softens the rock
and the directional pressures causes severe folding. Minerals maybe compressed elongated or rotated.
These directional pressures create different textures. The minerals oriented perpendicular to the forces
form foliation, a set of wavy parallel cleavage planes. This texture is formed by the platy minerals such
as the micas. Metamorphic rocks are classed according to four main criteria, the metamorphic grade,
crystal size, type of foliation and banding.
Fluids have a major role in metamorphism. The source of the water is hydrothermal and introduced
to the depths by water saturated minerals such as clays in convergent plate boundaries and from cracks
in the crust that let surface waters inter the depths of the lithosphere. The fluids bring CO2 and other
substances such as gold copper and silver into contact with the rocks minerals. This allows for
metasomatism or change in the rocks composition by these fluids.
This metasomatism may be the reason for granoblasitic rocks. These are rocks that are
nonfoilated with crystals that are equidimensional in shapes such as spheres or cubes. Examples of this
are marble and quartzite. Here the existing crystals have be altered, increasing in size and intergrowing.
Quartzite, formed from sandstone, is some of the hardest and most weathering resistant rock known.
While we have discussed metamorphic rocks and some of the minerals the question is where is
metamorphism taking place? What is putting the pressure on the rocks and inciting the changes? There
are different types of metamorphism and each type as a specific cause.
The types of metamorphism are: 1. Cataclysmic, 2. Contact, 3. Hydrothermal , 4. Regional and 5. Burial
or dynamic.
Cataclysmic or shock metamorphism takes place during an impact from asteroids, comets and
meteorites. It can also appear during an extremely violent volcanic eru ption.
The metamorphism is due the heat and the shock wave generated by this
event. The country rock is often shattered and “shatter cones” are. The
shattered country rock may also melt making tektites. Individual mineral grains
can shatter creating shocked quartz. The heat can form new high pressure
minerals.
Contact Metamorphism takes place when magma from dykes, sills and
other magma bodies or hydrothermal fluids in veins
come into contact with the country rock. There a
metamorphic aureole formed a banding of different
metamorphic minerals according to the heat. This
type of metamorphism has high temperature with
very low pressures. The size of the aureole is directly
proportional to the size of the igneous body. There is
also another consideration and that is the amount of
water that the magma body releases. Water will
increase the degree of metamorphism and create new
mineral. This is called metasomatism. Metasomatic
aureole forms different minerals then just heat. This is also seen in hydrothermal metamorphism.
Regional Metamorphism takes place at convergent plate boundaries. Here you get both high
temperature low to high pressure and low temperature and high pressure metamorphism. The high
temperature low pressure is similar to contact metamorphism while low temperature and high pressure
metamorphism takes place during plate compression. The
deeper crust is where high pressure and high temperature
pressure takes place. Regional metamorphism creates linear
features of mountain belts.
The last form of metamorphism is dynamic or burial
metamorphism. As sediment is transported into basins the
basins subsides allowing for deeper and deeper burial. The
temperature increases 75oF/mile depth in areas not
volcanically active. In regions volcanically active it is
150oF/mile. The temperature at the bottom of the crust is
1600oF. As the sediment becomes lithified with burial it now
undergoes a low grade metamorphism of low temperature and
low pressure.
During these different processes forces are placed on
the crust.
upper crust: brittle
lower crust ductile
These
deformation
deformation
forces are
predominates
predominates
called
low temperature
high temperature
stresses.
low confining stress
high confining stress
These stresses are tension, compression and shear
high strain rate
low strain rate
and be related to plate boundaries. The rocks
response to stresses by undergoing strain, the change is size (volume) or shape or both. There are three
types of strain: 1. plastic, 2. elastic, 3. brittle.
Ductile or plastic deformation takes place when
the temperature and pressure are higher. This is where
the metamorphism takes place. Different types of rock
are more susceptible to ductile (plastic) deformation;
sedimentary rocks are more prone to ductile
deformation then igneous and metamorphic rocks.
These have a tendency to fracture along fault planes. If
the forces acting on the rocks whether sedimentary or
igneous deforms the rock layers quickly then the rock will fracture. There are many factors that will
determine if there will be ductile deformation of brittle deformation. Temperature is a large
determinant, the higher the temperature the more ductile and less brittle the rock becomes. This is
because the minerals will stretch their bonds creating new minerals while at low temperatures the
bonds hold until fracture takes place. The higher the confining stress the rock is prevented from fracture
and will fold instead. The rate of strain is another factor. The faster the rate the less time the molecules
have to rearrange bonds and the more likely it is to fracture.
When ductal deformation takes place folds appear. Folds
are visible where planer structures such as found the bedding
planes in sedimentary rocks have been warped into a curved
structure. Folds can also be found in metamorphic rocks. Gneisses
often show folds from forces applied to them. Folds don’t have to
be small but can make huge landforms.
Folds can be classed as
anticline or syncline. Showing is easier than describing. If the limbs
of the fold are equal then it is a symmetrical. Bisecting the fold is the
axial plane. If the axial plane is perpendicular to the fold then this is
a symmetrical
fold. If the
axial plane is
at an angle to
the horizon you
get a plunging
fold. Synclines
also have an
axial plane and
again if the
plane is at a dip from the horizon it is a plunging anticline. The
illustration to the right shows how weathered anticline and
weathered synclines
would appear. Fold can
be tilted with one limb
of a fold longer than the other. They can also be overturned
with one limb being
excessively longer
than the other
making an “S” form.
It is obvious from
looking at the picture on the right that folds make large
landforms.
Regional metamorphism takes place during the
formation of the folds. With the folds you have the necessary
heat and pressure to metamorphose minerals in the rocks. The
rocks under compression cause the minerals to line up perpendicular to these compression forces.
Going back to strains on rocks the easiest strain to explain is brittle deformation. If you look
back at the table on page two you will notice that this is a shallow crust response When stresses are put
on a rock formation it breaks. This can be seen with magma pressing into rock layers fracturing it. It can
be seen as one plate boundary is put under tension and fractures at a divergent plate boundary and
when placed under compression at convergent or shear at transform.
Elastic deformation actually takes place before brittle deformation. Stress is placed on the rock
and the rock will bend or deform. Everything has some elastic property. The amount of force necessary
to rupture that material can very according to the confining pressure that holds the substance that is
under stress, the strength of the material itself and the presence of water. Before the material breaks it
will deform. When the stress is removed it will return to its original shape. This ability to bend then
return is elastic deformation. Elastic deformation is seen in rock layers that are undergoing stresses
before earthquakes.
Fractures of the rock layers create faults. There are three types of stresses acting on the rock
layers, tensional, compression and shear. These can be associated with plate boundaries. Tension is
associated with divergence, shear with transform plate boundaries, and compression with convergent
boundaries. These create faults.
The faults have distinct parts. The block of rock incorporated in the rock is the fault block. The surface or
plain that the rock layers move on is called the fault plane. The portion of the fault block that normally
doesn’t move is the foot wall. The hanging wall moves on the fault
plane. How it moves determines the type of fault. The plane separating
the hanging wall from the footwall is called the fault plane. If the
hanging wall moves down with respect to the footwall then you have a
normal fault. If it moves up with respect to the footwall then you have a
reverse or thrust fault (steep angled are called reverse faults, shallow
angled are called thrust
faults). These are called dipslip faults and the angle of
the fault plane is the dip of
the fault plane. If the
hanging wall moves
horizontal compared to the
foot wall then it is a strikeslip fault. The strike is a line
perpendicular to the dip.
As the faults move
one wall against the other
stress is released and heat
generated. The fault plane
will do one of two things. The first is the rock crumbles under the stress and
forms a fault breccia; you can see this on I 55 at the north side of the Arnold
exit. The other is the formation of slickensides. This is a smoothly polished surface of the fault plane
created by the friction and forcing the minerals into fibers.
Divergent boundaries place tension on the
lithosphere as the convection currents and “slab
pull, ridge push” mechanism pull the lithosphere
apart. As this takes place the rock fractures and the
hanging wall slides down extending the crust. Look
on the picture on the right on the page before this
and see the offset beds or layers of rocks. This is
not one normal fault but two. These linked faults are called
conjugate normal faults, and is the mechanism that forms the rift valley. This type of fault is called a dip
slip fault. This means that the fault block is moving down (up in another fault) on the fault plane.
Normally this type of fault gives relative small earthquakes. This is because there is little friction
involved.
Convergent plate boundaries place compression on the lithosphere. This compressive force
squeezes the plates. The thrust fault is often seen at the convergent plate
boundaries. To the left is an image of ocean sediment shales sitting on the
continental crust of the coast of Oregon. During compression at these
boundaries the lower crust forms folds while the upper crust develops
thrust faults. This creates the fold thrust regions. It is the convergent plate
boundaries that are the land mass builders.
The last form of fault associated with a plate
boundary is the strike-slip fault. Here the movement is not on the fault plane but on the
strike. The strike is the compass direction of the fault where it intersects with a horizontal
surface. This is a “horizontal movement”. While there isn’t as much friction as in a
reverse or thrust fault, there can still be enough to
cause large earthquakes and metamorphism. The
fault
is associated with the transform plate boundary. The
fracture zones that offset the divergent plate
boundaries are examples as these. The San Andreas
Fault
system is also an example. The strike can be seen in
the
San Andreas Fault system with offset streams and roads.
While there are many faults associated
with active plate boundaries there are even
more associated with old plate boundaries and
are now found far from an active plate
boundary. Our plate has been created by
multiple continent to continent plate boundary
collisions. An image of our country alone makes
it look like a patchwork quilt of different
smaller continents that merged to make our
continent. Each one leaves behind faults.
Another source of faults is failed rifts. One our
continent was even partially assembled there
was an attempt to rip it a part. One of these
failed rifts is the mid-continent rift that extends
Kansan to Michigan. Another failed rift is our
own seismically active New Madrid Seismic Zone. This fault zone has had the largest earthquake in the
continental United States.
99%+ Earthquakes are associated with
the faults. There are a few other situations
that cause earthquakes that won't be
discussed in this class. The focus is the actual
location of the earthquake. Here is where the
rocks are under stress and are undergoing
brittle deformation and fractures. The
epicenter is merely the surface location
directly above the focus.
We only think of the shaking and
falling of buildings when we think about an
earthquake. What is happening is an energy
release that has built up over years (as few as decades and as many as 10 millennia). This energy is
released in two forms, heat (can be enough to cause metamorphism) and a set of waves that pass
through the planet. There are four different waves and their properties determine their destructive
nature. An idealized wave is used to explain the terminology when talking about the seismic waves. The
wave base is the ground before earthquake and is 0. The
amplitude is a measurement of ground the movement.
The wavelength is the length of the seismic wave. The
next image shows a time measurement P which is the
time it takes for a seismic wave to pass a fixed point.
When talking about the seismic wave it is talked about
in terms of its frequency just like a radio wave. This is calculated by f=1/p and is measured in hertz.
P waves (primary waves) are the first wave generated during an earthquake. This wave is a
compression wave with the movement of the wave in the direction of travel. Since it is a compression
wave it can travel through solid, liquid and gas (you can hear the wave). This wave is the fastest of the
waves traveling ~ 18,000 mph and arrives at seismic stations first. This wave is also called a body wave
because it passes through the body of the planet and is crucial for mapping the interior of our planet.
S- wave is a shear wave also called a secondary wave and is a body wave as well Here the
particle motion is perpendicular to the direction of travel
giving it an undulation form similar to a snake. This wave,
because of the particle movement being perpendicular to
the direction of movement can't pass through liquids.
This wave and the P wave have mapped the interior of
our planet. These waves have solved the mystery of
Farallon, a plate whose final subduction took place 45
million years ago (started 460 million years ago). You can
go to the tomogram map of this plate on page 362.
While the book only talks about three waves
there are actually four. The last two waves are known as
surface waves and are trapped in the crust of the planet.
Since this is where the rocks have the least amount of
density these waves travel the slowest and are the most
destructive. The Love wave is a horizontal shear wave and
destroys foundations. Since it is a shear wave it can't
travel through liquid. The last wave is the Rayleigh wave.
This wave has elements of both the P-wave and the S-wave, moving in a retrograde circle. This is the
most destructive wave of all. Since these waves travel through the crust their energy is lost with
distance from the epicenter.
Earthquake location is determined by the difference in the
travel times of the P and S waves. The closer the seismic station is
to the epicenter, the small the
difference. Think of the
epicenter being the start of a
race between a corvette and a
4 cylinder auto and the seismic
station is the end of the race. If
the seismic station is close to
the epicenter (finish line)
corvette doesn't have enough
time to open up the distance between it and the 4 cylinder. If the
seismic station is far away the difference in arrival time is huge. The P-wave is our corvette and the Swave is the 4 cylinder car. Taking the time difference to the S-P difference curve you find the distance
from that seismic station to the epicenter. This gives you the radius of a circle and the epicenter can be
anywhere on the circumference of that circle. By calculating the distance of 3 seismic stations from the
epicenter you can triangulate the position of the epicenter.
Earthquakes are measured in magnitude and intensity. Magnitude measures the Energy
released by the earthquake. While there are many different magnitudes we will only talk about two, the
Richter Magnitude and the Moment Magnitude. Richter magnitude is
calculated by the ground movement. This generates a value R. The
magnitude (R) of an earthquake is 10R Richter Magnitude ML:
calculated from the largest amplitude of any of the seismic
waves formula is M=10R M = is the amount of ground movement as
measured by a seismograph. A magnitude 4 earthquake is ten times
stronger than a magnitude 3. R= the Richter value
To find the Richter magnitude you can take the largest
amplitude of the seismic wave on the seismogram and take to a
chart like the one on the left and draw a line bisecting the distance
and the amplitude and it passes through the magnitude This
magnitude is no longer used for a host of reasons. The first one is that
it only deals with close earthquakes, which is logical since it was developed in California adjacent to the
San Andreas Fault system. This should tell you that this magnitude is only good for shallow earthquakes
since it this is a strike-slip fault system. This magnitude also has limited accuracy since it is only good for
magnitudes of 3-6.
The more accurate magnitude is the moment magnitude. This takes into account the rock
strength; it is easier to rupture limestone then granite, the length of the fault displacement and the
amount of displacement. This can be calculated by a seismograph by plotting the frequency of the
seismic wave against its amplitude. This works by indicating the rock type. Frequency is dependent on
velocity, seismic waves move through harder rocks faster than softer rocks. Generally the amplitude is
less on these harder rocks. To have large amplitudes and high frequency indicates a rupture of stronger
rocks. Moment Magnitude Mw calculated by the rigidity of the rock multiplied by the average
amount of slip on the fault and the size of the area that slipped.
While magnitudes measure energy release it is not a true indication of damage done. That
measurement is in the Intensity Scale. Intensity or degree of damaged is determined by several factors:
1. Depth of focus, deeper earthquakes seismic waves lose energy as they travel to the surface. 2.
Distance to epicenter, the closer you are to the epicenter the more the damage. 3. Duration of shaking,
the longer the shaking the more likely it is for well-built structures to fail. 4. Ground acceleration, how
easy and therefor how fast is it for the ground to move. Soft sediments offer little resistance to
movement. A simpler way of saying this is what
you are built on determines how much damage
you will receive despite differences in magnitude.
The reason is the response of the surface wave to
the different substrates. Notice that igneous rock
has a high frequency but low amplitude and as you
move to softer substrates the amplitude increases
as the frequency decreases. It is the amplitude that
indicates ground movement and the amount of
damage you can expect.
Two earthquakes illustrate that magnitude does not correspond to intensity. The first
earthquake was the Haitian earthquake with a magnitude of 7. While the press harped on how bad the
building codes were no one reported that this was a shallow earthquake and the epicenter was in a
populated area. They also didn't mention that the structures were built on sediment. Japans earthquake
had a magnitude 9 with ground movement being 100 times greater than Haiti's earthquake. Why was
the greatest amount of damage due to the tsunami and not the shaking? Japan's epicenter was 300 km
from the population centers, in the ocean off of the coast. The focus was deeper and Japan's structures
were built on stiffer rock with less ground acceleration.
There are numerous earthquake hazards that can take place. These hazards span from fault
movements to health hazards. We shall go over just a few. Fault movement or displacement has been
linked to creating tsunamis as it displaces the water. Fault displacement ruins roads, can split homes,
and as in the New Madrid Earthquakes of 1811-1812, create lakes. Normally the fault movement itself is
not the main cause of damage. Landslides are triggered as angled
grains are disturbed and topple. This can ruin roads, train tracks
and crush buildings. Liquefaction can be a major killer. There are
multiple forms of liquefaction that is gone over in Earthquake
and Society. During liquefaction the sub-straight itself takes on a
fluid like quality. This can lead to mud flows, fissures and a "quick
sand" effect as seen in the picture on the left with buildings sinking into the substraight.
Tsunamis that are linked to earthquakes take place for a number of reasons. The first is the fault
displacement itself. As dip-slip fault slips it creates either uplift or downdrop. Both actions disturbs the
water, a thrust fault pushes it out of away, and a normal fault creates a void and water rushes in then
rushes back out. Another mechanism is the landslide, both submarine and surface. One of the largest
tsunami wave height, over 300 feet in height took place because a small earthquake triggered a
landslide. The problem with a tsunami is the large area it can affect and the fact that it isn't one wave
but multiple waves. In the open
ocean this wave has a long
wavelength and a small
amplitude but tremendous
velocity (500 mph). As the wave
approaches the shoreline it
intersects with the floor of the
ocean and friction slows down
the wave. Correspondingly the amplitude or wave height increases. This slowing doesn't mean you can
out run it. Now it may be traveling at 45 mph but the wave height may be as high as 30 to 60 feet.
A secondary hazard is fire. When there is a large enough earthquake gas lines are ruptured or
charcoal burners are toppled and fire results. The main problem with fire is the inability to fight it. Often
water lines are also broken and there is no way of fighting a fire.
Region that have frequent
earthquakes have their ground
analyzed for ground acceleration. This is
an excellent indication of potential
damage. A car accelerating at 1g
(32feet/sec2) would travel over 300 feet
in 4 seconds. This would be the force
exerted on structures and on life on
living in this area. 1g acceleration is
strong enough to toss you into the air. If
you look at the map on the next page
you would notice that the one area in the continental United States is the New Madrid Seismic Zone.
The reason for this is that region is on a failed rift with rock down over a kilometer and material that
people built on and live is alluvium, silt and mud. These materials give maximum amplitude to the
seismic waves.
When looking at earthquakes the importance of predicting them is obvious. This means lives
saved, environmental hazards avoided. To do
this a number of methods are used. Earlier in
this chapter foreshocks were talked about.
These are small earthquakes that take place days, weeks before main shock. While 44% of all major
earthquakes are preceded by a foreshock only 5-10% of smaller earthquakes are foreshocks. This is the
reason that we are not using these smaller earthquakes to predict larger ones.
The geologist tries to determine the reoccurrence interval of earthquakes along a fault system.
This is an indication of the amount of time is required to accumulate enough strain to trigger an
earthquake. This can be done by examining old seismograms in areas that have frequent earthquakes,
or written records from earthquake survivors. Another technique used is age dating earthquake
features. While this gives an estimate there is a problem. For example the estimate for a repeat of the
New Madrid Earthquakes is 200-400 years from 1811.