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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 (30 o C/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 75 o F/mile depth in areas not

volcanically active. In regions volcanically active it is

150 o F/mile. The temperature at the bottom of the crust is

1600 o F. 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.

These

forces are

called

stresses.

These stresses are tension, compression and shear

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

upper crust: brittle

deformation

predominates

lower crust ductile

deformation

predominates

low temperature high temperature

low confining stress high confining stress

high strain rate low strain rate

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 dip-

slip 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 strike-

slip 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 S-

wave 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 10 R Richter Magnitude ML:

calculated from the largest amplitude of any of the seismic

waves formula is M=10 R 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/sec 2 ) 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.