Chapter20 Mountain Belts understanding how mountains are created
Mountain belts
are several linear ranges
of mountains. These are
typically on the edges of
the plates, but can be
found in the center of
plate boundaries. The
interior mountain ranges
are remnants of ancient
continent to continent
convergent plate
boundaries (exception
Rocky Mountains).
Mountain belts, even in the center of continents, are associated with earthquakes and the belts on
edges of continents are also associated with volcanoes
Mountain built creation is a combination of three processes: 1. Increase deformation (plate
tectonics), 2. Weathering and erosion and 3.Isostasy. These three different processes interact
differently depending on the location of the mountainous region and the climate to make each
mountain built unique.
Deformation and mountain
building is the shortening of the
continental crust. As plates collide
under the pressure of convergence
the crust is shortened. This
shortening creates folds and faults
with reverse faults being common.
The fold and thrust belts (thrust
faults are shallow angled reverse
faults) found at convergent plate
boundaries are composed of thrust
faults stacked on another with the
rock in between being highly folded.
The book gives a great example of
the impact of this crustal
shortening. The alps are composed of crustal material that (if unfolded and unfaulted) would extend out
500 km wide (310.69 miles) and now has been compressed to a width of 200 km (124.72 miles) a 40%
change in width!
Isostasy is the most difficult concept to understand because it is a balance of forces, gravity
(weight) pulling down on the mountain and the asthenosphere pushing up. Remember the
asthenosphere is part of the upper mantle and is plastic. As a mountain is
formed the weight of it pushes down on the asthenosphere and moves
mantel material away, like sitting on an air
mattress. Your weight pushes the air away from
where you sit. You have equilibrium between
your weight (or the mountains weight,
lithosphere) and the amount of air pushed away
(asthenosphere). This is not a static condition
but will respond to changes in the deformation
due to plate tectonics and weathering and erosion removing material from
the mountain. (page 449 in book). The shortening of the crust from above
creates a tremendous localized increase in weight that then triggers the
isostatic adjustment. Examples of mountains that have been weathered down
and undergone isostatic adjustment are the Appalachians.
http://bcs.whfreeman.com/understandingearth/content/cat_110/ch18/earth4e_1817.html?v=category
&i=18110.01&s=00110&n=18000&o=%7C00510%7C06000%7C14000%7C17000%7C20000%7C23000%7
C22000%7C18000%7C (isostasy video)
Weathering and erosion is the great leveler. Material is removed from the mountains and
transported to other areas of the crust, either continental or ultimately oceanic. The weight removal
then triggers further isostatic adjustment uplifting the root of the mountain higher into the crust. This
continues until the continental crust becomes a uniform thickness. Mountain Building
In the Americas mountain belts run parallel to the coast lines but in Asia they are central in the
continents, such as the Himalayas, the Alps and the Pyrenees. The Appalachian Mountains are rising
from isostatic rebound and are not actively building. The interior plains between the Appalachians and
the Cordillera are the remains of Proterozoic continent building from continent to continent plate
boundaries. The sedimentary rock overlay of these ancient deep seated mountain roots. These are
considered stable and are called the Craton. These rocks are seen in the Grand Canyon, Black Hills and
the Ozark dome as well as some of the Rockies. The figure above is from the Wilson Cycle of plate
tectonics and what the Craton looks like beneath the sedimentary rock.
http://bcs.whfreeman.com/understandingearth/content/cat_110/ch18/earth4e_1817.html?v=category&i=18110.01&s=00110&n=18000&o=%7C00510%7C06000%7C14000%7C17000%7C20000%7C23000%7C22000%7C18000%7C
http://bcs.whfreeman.com/understandingearth/content/cat_110/ch18/earth4e_1817.html?v=category&i=18110.01&s=00110&n=18000&o=%7C00510%7C06000%7C14000%7C17000%7C20000%7C23000%7C22000%7C18000%7C
http://bcs.whfreeman.com/understandingearth/content/cat_110/ch18/earth4e_1817.html?v=category&i=18110.01&s=00110&n=18000&o=%7C00510%7C06000%7C14000%7C17000%7C20000%7C23000%7C22000%7C18000%7C
The sedimentary rock on the Craton is thin, less than 1,000-2,000
meters (0.62-1.24 miles) while the sedimentary rock in the mountain belts is
over 10,000 meters (6.21 miles). This thickness is due to the deformation
with folds and reverse faults. This represents crustal shortening and
deformation.
The Canadian Shield of the North American continent date back to the
more ancient times, the Archean. The advance of the glaciers during the
Pleistocene removed all of the sedimentary rocks. These are complex
metamorphic and plutonic rocks that date back over a billion years. This figure to
the left is the shield.
Continent building takes place with the island arc. These
mountains have little if any
sedimentary rock. Instead
the metamorphic rock
forms from metamorphism
of ocean crust. Once the
volcano starts to build by
rising plutons forming
magma chambers erosion now takes
placed on the volcano creating an
accretionary wedge. This area
undergoes metamorphism forming
blueschist.
There is a complex of
metamorphic and igneous rock
(plutonic rock) found in the heart of
major mountain belts. The
metamorphic rocks are both sedimentary and igneous rock that were deeply buried and now exposed.
This is often record of convergent plate boundary assembly. The images here are convergent plate
boundaries. The mountain range in the Cascade Range and the Andes are this type of plate boundary.
The “wiggled” lines indicate metamorphic rocks. Multiple types of metamorphism represented here.
http://www.google.com/url?sa=i&source=images&cd=&docid=uA1dSaSoS40LYM&tbnid=hNIG7jhYbcPWpM:&ved=0CAgQjRwwAA&url=http%3A%2F%2Fwww.lacusveris.com%2FThe%2520Hi-Line%2520and%2520the%2520Yellowstone%2520Trail%2FThe%2520Bois%2520Brule%2FThe%2520Canadian%2520Shield.shtml&ei=0TspUdOPNqiB2gW6ioHYBw&psig=AFQjCNGds7s5c72uaEPSstz2WN9FyFpYXw&ust=1361743185948322
This last image is continent-continent collision. Here there
are two continents joining. This type of collision created the
Appalachians starting back 550 million years ago. This is the same
boundary that can be seen in the Alps, Pyrenees, and the Himalayas.
These events took place starting in the Mesozoic and are still active
in some of the chains. As you can see the mountain building includes
compression of the crust as well as the rising up of batholiths.
The faulting involved are large thrust fault belts but you can
also find normal faults present. Normal faults indicate extension.
The top of large mountains are
overcome by gravitational
collapse and the igneous rock on
the mountain top is forced to
flow downward by gravity and
joins the molten material rising
(batholiths) upward from the
subducting plate.
Once the mountain range is formed they proceed to weather away. And this is where the
isostacy takes over and the mountains once eroded flat are
uplifted by the flow of the asthenosphere. This is not an
instantaneous event but can be in several decades to
millennium. The Appalachians formed in the Paleozoic time
starting in the Ordovician (488-444 ma) and completed in the
Permian (251 ma). During the Mesozoic Era the Appalachians
weathered away and are now reemerging due to isostacy.
Isostacy will continue the uprising until the continental crust
once more reaches balance.
One possibility for this is a process called
delamination. Here the mountain root heats
(lithosphere) to the point it becomes hot and molten. It
is still colder than the asthenosphere and, therefore,
denser. It breaks apart and sinks into the
asthenosphere and asthenosphere on either side of the
root flows into the void. This phenomena causes uplift and extension with normal faults forming. This is
what is thought to be happening in the basin and range region.
This extension and melting of continental crusts
causes a variety of volcanism, with stratovolcanoes and
basalt flows as well as the material from the mantel rises.
As you can see from the picture above this explains lava
flows found Utah, Nevada, a portion of Oregon and
California, as well as Arizona, New Mexico and Texas.
Mountain formed with island and continent
accretions building our own continent but others. We
know of this by finding areas with geologic material
different than the surrounding land terranes. Many of
these terranes come from the breakdown of mountains created by
orogeny and essentially form in place. Suspect terranes are
terranes that don’t appear to have formed in place. If the terrane is
shown to not have formed on the present continent they are called
accreted terrane and are the result of collision of islands or mini-
continents the size of New Zealand. If they can be shown to have
traveled great distances by fossil assemblage or paleomagnetic
poles they are called exotic terranes. The Carolina terrane forming
the Appalachian Mountains has trilobites associated with England
but not the United States. The image on the left shows the building
of our own western continent.
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.
IgneousRocks Intrusions and Volcanoes
Igneous rocks are associated with molten material. This material can be from the mantel or
melted crust material. Texture, iron content and silica content determine the type of igneous rock. The
texture determines the cooling temperature. Rapid cooling gives small to no crystal formation. This
means that the rock formed on the surface. The main minerals for igneous rocks are quartz, feldspar,
mica pyroxene amphibole and olivine. Mantel rock will have mainly olivine, and pyroxene. Melted
crustal rock has large amounts of quartz in it. Texture is a clue to the environment, internal (intrusive) or
external (extrusive). This was known for 200 years and later confirmed with the developing of polarizing
microscope and the careful grinding of a rock thin section (ground rock so thin light can pass through it).
Texture is created by cooling temperatures. Ions in hot magma have too much energy to bond together
and form a crystal. As cooling proceeds the ions lose energy and can then form crystals. Pressure will
also help in this since it forces ions together regardless of the heat energy. If the cooling proceeds slowly
as in intrusive material you have large crystals, if ejected into the surface you get small crystals or no
crystals.
Intrusive rocks are coarse textured (phanorytic.) These rocks
have slow cooling regimes cooling over thousands and thousands of
years in the crust of our planet. The heat is held in by the overlaying
rock. Elephant rocks in Missouri are a perfect example of such a rock.
This is an igneous intrusive body injected into the crust over a billion of
years ago.
Extrusive rocks have different types of appearances, one composed of
fine grained aphenitic another by glassy (no crystals) rocks. These categories
are dependent on how they erupted from the volcanoes. Lavas have a range
of appearances dependent on their chemical make-up and temperature.
Pyroclastic formation is characterized by violent
eruption with lava thrown into high into the air. If
the material is ejected rapidly and cools rapidly it forms volcanic glass- mineral
free since amorphous.
Pumice
is a form of igneous rock that forms from
volcanic glass with air pockets or vesicles. These vesicles are formed by the
degassing of the molten material (CO2, H2O, etc.). Volcanic ash is composed of
fragmented rocks, lavas and/or volcanic glass. It is thrown high in the air and
smaller fragments will travel around the globe. Bombs have a range of shapes and made up of solidified
lava. They are tossed into the air and fall along the sides of the volcanic cone. The last two are scoria and
pumice. These are gas filed lava. Pumice (to the left) is so light that it floats.
There is one more texture, a mixed texture indicating two different cooling regimes. This is
called porphyritic texture or porphyry. Here a slow cooling regime starts and
large visible crystals form. These are then there is a volcanic eruption before
more large crystals can grow and this gives us the two different crystal types.
Igneous material is also classified by chemical content. We break the material into four types according
to the proportion of silicate minerals. Specific minerals form at specific temperatures. Minerals with a
high proportion of iron and magnesium and calcium compared to silica form this group and are called
mafic from magnesium. The mafic and felsic mineral suites are on page 112 in your book. The feldspar
group is divided into potassium rich (orthoclase), sodium rich plagioclase and calcium rich plagioclase.
The last two the sodium and the calcium are end the
members of a solid solution where the plagioclase
formula is NaAlSi3O8;CaAl2Si2O8. The more calcium in
the plagioclase the more mafic the forming rock and
the higher the melting temperature to the right is an
example of these mixture. The more sodium, the more
felsic the magma melt and the
lighter
the igneous
rock( look at the chart on page 113).
Mafic Rocks a have large amounts of olivine
and pyroxenes giving the rocks their characteristic dark
colors. There may be a small to moderate amount of
calcium plagioclase. The lava form of this is called basalt. There are several areas of sheets of basalt such
the Columbian Plateau along the Columbia River in Washington. India as an even larger area, the Deccan
Traps, where kilometers thick layers of basalt contributed to the Cretaceous extinction event and
another in Siberia with an area as large if not larger also associated with an extinction event (Permian).
There is another mafic rock form. This form is called ultra-mafic. The mineral suite is primarily
made up of olivine with a small amount of pyroxene. This is the material that makes up the upper
mantel. The basalt upwelling at the spreading centers formed the ocean crust.
Felsic Rocks are poor in iron and magnesium. They are also poor in calcium. These rocks tend to
be light in color and one of the most abundant intrusive igneous rocks. They contain approximately 70%
silica and are abundant in quartz and orthoclase feldspar with some
sodium feldspar (albeit minerals). The intrusive form of this igneous
rock is Granite, the extrusive form is Rhyolite. These rocks can appear
as light brown, salt and pepper, pink, or orange or in some cases
almost purple (Missouri rhyolite). The Picture on the left is an image
of this rhyolite. Notice that it is porphyritic.
Between
the end members
of these two rock
types are the rocks
that are called the
intermediate.
These rocks have
less silica then the
Labradorite- tectosilicate
– Ca(50-70%) Na(50-30%) (Al, Si)AlSi2 O8
felsic and more than the mafic. They have some quartz, micas and may have some pyroxene. They may
also have amphiboles. The intermediate is divided into granodiorite and diorite. Granodiortite is
very difficult to differentiate from granite. This is done by looking at difference in the percentages of
quartz, orthoclase and sodium plagioclase. We will then only talk about diorite and its extrusive form
andesite. This material has a mineral suite with pyroxene like mafic and calcium plagioclase but it also
has amphiboles, micas, a mix of the sodium/calcium plagioclase minerals and some quartz. The variation
in the amount of silica gives its melt a variety of properties that can swing from one extreme to another.
The last is the composition factor impacting the melting is the chemical formula. The more mafic the
melt, the higher the melting temperature for mineral formation and mafic melts are characterized by
less silica in the melt and more iron and magnesium. Conversely the more felsic the melt represented by
more silica, the lower the
melting temperature.
` We know from seismic waves that the Earth is solid until we reach the liquid outer core. Where
does the magma come from? While we are
still working on this question we do know
some factors that impact the temperature at
which this solid will melt. One of the big ones
is pressure. Most of these solids are at such a
high temperature that they should have
melted. The pressure prevents this by
preventing atoms from moving apart thereby
keeping it a solid. The only way to overcome
this is an even greater temperature. Water
content also impacts this. Water enters the
system at convergent plate boundaries and by
water circulating naturally close to a magma
body (think Yellowstone geysers).
The last is the composition factor
impacting the melting is the chemical formula. The more mafic the melt, the higher the melting
temperature for mineral formation and mafic melts are characterized by less silica in the melt and more
iron and magnesium. Conversely the more felsic the melt represented by more silica, the lower the
melting temperature.
Looking more closely at temperature it was found that a magma chamber only undergoes partial
melting. This partial melt is determined by the temperature of the chamber and the mineral
composition of the magma. Only certain minerals will melt at a given temperature. It is like the new
“lava cake” desert. The cake turned solid at one temperature but the temperature wasn’t low enough
for the chocolate center to turn solid. As water enters the melt this temperature will lower for many of
the minerals and there will be a more complete melt. We geologist use this information to determine
how different kinds of magma form in different regions of the Earth’s interior. Since magma is formed
from rock in which only the minerals with the lowest temperature melts.
As you go deeper into the Earth the pressure increases. This increase in pressure (as mentioned
earlier) increases the melting temperature. Because of the convection currents mantel material will
move to an area of lesser pressure in the region of the spreading centers. This allows for the
decompression melting of the mantel creating the basalt of our seafloor.
Water impacts the behavior on the melting temperatures. The impact of large amounts of water
on melting temperatures can be seen with the mineral albite which melts at 1000
o
C. When water is
added to the melt drops to 800
o
C. Water is present as a gas and dissolves into the molten albite. There is
a rule to explain this. This states that if you dissolve one material into another lowers the melting
temperature of the solution. It also impacts the melting temperature of mixture of other sedimentary
and other rocks .These rocks are often water rich and will melt.
Magma chambers are formed by the change in density as material is heated. The
magmatic
material will rise through and upward. Being fluid the partial melt moves up through the pores and
boundaries of surrounding rock. As the drops of molten material rises they can coalesce into larger
bodies. It also cam melt the host rock forming magma chambers or cavities in the lithosphere.
Magmatic differentiation can partially account for the different types of igneous rock found on
this planet. In the chamber as it cools high temperature mineral will form. This will remove more iron,
magnesium and calcium/silica tetrahedrons creating the ultramafic and mafic material. This leaves
behind more silicate tetrahedrons per cation so more high temperature minerals that are silica rich and
felsic form. These only form, though, as the melt cools. The process is called fractional crystallization
and gives different minerals in the same chamber. By studying the Bowin’s Reaction Series (bottom of
last page)you can predict a magma temperature by the minerals present.
The diagram to the left demonstrates magmatic differentiation and fractional crystallization. The initial
minerals that come out of the melt are more rich in iron and magnesium. As they crystalize out then
remove these cations leaving behind a progressively more silica rich melt. By the time there is later
crystallization the magma body has cooled and the excess silica gives you the minerals found on the
lower portion of the Bowin’ s reaction series.
While magmatic differentiation and fractional crystallization explains how there are different
minerals in a magmatic intrusion it doesn’t answer the questions of where did all of the granite come
from. Granite is one of the most common igneous continental rocks. The idea rose of a complicated
process that had both partial melting giving basaltic magma at the spreading centers followed by the
formation of intermediate andisitic magma with the mixing of basaltic magma and sedimentary rocks at
ocean-ocean convergent plate boundaries while the melting of igneous, metamorphic and continental
crust at ocean-continental convergent plate boundaries might melt to produce granite if magmas.
Igneous intrusions take
place as the rising magma intrudes
into the country rock. They wedge
open the overlying rock as the
magma lifts up the overlying rock in
extension. This lifting up and
fracturing cam be seen with the
formation of rifts and is presently happening in the Basin and Range of our own west. The
magma can then intrude into these fractures.
As the body rises by breaking off the overlying rocks which may or may not melt into the
chamber. If the rock pieces melt they will change the composition of the melt in that region. If they
don’t melt they remain as xenoliths in the magma that can be seen when the material cools and is
exposed by weathering.
The structures
that form by these process
are called Plutons and can
be from one cubic
kilometer to hundreds of
kilometers in size. The
largest of these plutons is
called a Batholith. These
large structures make up
not a single mountains but entire chains of mountains such as the Sierra Nevada Mountains. Smaller
plutons are called stocks and laccoliths are often the size of a single mountain.
Material that squeezes through the cracks from these bodies can cut across the country rock
making dikes. These magma bodies can also create their own cracks from the pressure they exert as
they rise. These are not a linear structure as they appear in a road cut but are actually a three
dimensional structure or can invade the country rock and spread along it in a horizontal structure called
a sill. You can view a dike here in Missouri at the Silver Mines State Park and on State Highway 72 on the
way to Arcadia.
The last features associated with igneous bodies are hydrothermal
veins. This can also be seen in sedimentary and metamorphic rocks. These can
be as small as millimeters or as large as a km in size. These veins can be in the
form of hydrothermal solutions often with quartz and valuable minerals
dissolved in it. These originate as water that permeates the country rock
(ground water) and forms into hydrothermal If they cool quickly they form
small crystals forming a sheet like tabular structure. These veins are an
important source of metallic ores.
Magmas form at two types of plate boundaries, the mid-oceanic ridges, where there is
divergence. The other plate boundary where magma is common is at the convergent plate boundaries.
There is another major source of magma, the mantle plumes. This are not associated with plate
boundaries and are the result of partial melting and form near the core-mantle boundary.
At the mid-oceanic ridge there is a decompression melt which then seeps up the fissures at the
divergent plate boundaries. The magma forms pillow lava of basalt. These columns of basalt are cut by
dykes cutting into the basalt country rock. Below this is the magma chamber. In the magma
chamber
magmatic differentiation takes place with the olivine and pyroxenes precipitate out to form a peridotite
layer. Adjacent to the magma chamber is a layer of gabbro. The gabbro layer is adjacent to the hotter
magma layer becoming metamorphosed. Above the basalt layer is layers of sediment and sedimentary
rocks. These form the Ophiolite Suites on land. This appears when to plates move so fast that the
ocean
plate is
forced
on and
over the
lighter
continental crust. As the plates move further from the magma more gabbro forms. The areas of
subduction are another area in which magma makes its way to the surface. The composition of the
magmas are dependent on what is being subducted. With this form of magma there is fluid induced
melting. The water in the subducting oceanic crust decreases the melting temperature of the overlaying
mantel material (peridotite rich) and the basaltic crust. There is also a portion of sediment that is left on
this subducting oceanic crust. This material has a very low melting temperature and melts readily.
The composition of these magmas should be basaltic considering they are formed from the
basaltic oceanic crust and the peridotite layer but there is a lot of variation. This variation comes from
the amount of accumulated sediment and sedimentary rock that is incorporated. As the magma rises up
and through the overlaying lithosphere there is also the effect of fractional crystallization giving an
increasingly more silica rich melt. When the oceanic crust is subducted beneath a continental crust felsic
rock melts and contribute to this melt. The different compositions of these melts and the amount of
gases present have a major impact in the eruption style of the volcanoes that are formed in this area.
The last type of “magma factories” is the mantle plume. They originate in the mantle itself and is
thought as a mechanism for cooling the core. It forms a
column of nucleated rock that rises up through the rest of the
mantel in the shape of a diapere. When it reaches the
lithosphere this flattens out and undergoes a decompression
melt forming the magma and the models predict large scale of
eruptions that can last millions of years such as the Deccan Traps
in India and the Siberian Traps in Russia. While the models
predict millions of years eruptions this is often not the case.
This plume is often postulated to be fixed with the
overlaying plate moving over it giving a string of volcanoes from the same magma chamber but of
different ages. While this is often true there are other plumes that are geographically stable such as the
one in Iceland and the Azores off of the coast of Africa.
While plumes can either form flood basalts or strings of volcanoes the volcanic composition can
very. The plume material itself is basalt with high temperature minerals containing a high iron and
magnesium content and low silica content when they appear below a continent other process can take
place. The underlying basalt magma can melt the continental rock; both the granite and the sedimentary
rocks creating a more felsic melt with high silica content.
Volcanic Eruptions
Basaltic lavas are mafic in composition (high iron,
magnesium and calcium) with the lowest of all magma
compositions. The eruption temperatures from these lavas are high,
anywhere from 1000 to 1200
o
C (1832-2192
o
F). This lava has the
fastest downhill speed (62 mph) on a steep slope due to this high
temperature and low silica content.
This gives three different basaltic lava appearances. The
high temperature fast moving is called
pahoehoe a Hawaiian word. This gives
a ropey appearance to the lava field. As
the lava cools a skin forms over the
flow with hot lava continues to flow
beneath. As the lava cools and slows
the “aa” forms. This forms a thick
skin that breaks as it flows giving an angular blocky appearance. The last form that basaltic lava forms is
pillow lava. This lava captures air as the lava flows over itself moving forward. This form develops as the
magma erupts under water. This forms a bulbous form that resembles “pillows”.
Andesitic lavas from the andesitic magmas have a higher silica content than the basaltic lava this
means that the minerals formed at this lava is made up of lower temperature minerals, no olivine. This
decreases the speed and distance of the flows. Their flows are stick forming blocky with few or no air
vesicles. They seldom get beyond the intermediate area of the volcano itself.
Rhyolite lavas are the highest in silica content (over 68%). The minerals are low temperature in
nature and the silicate minerals are high in sodium and potassium. The temperature of this lava is
600-800
o
C (1,112-1,472
o
F). This lava seldom leaves the crater and moves 10 times slower than basaltic
flows.
Volcanic eruptions are not always in the form of lavas. If water comes into contact with hot, gas
charged magma you can have a phreatic or steam explosion. One of the largest in history involved the
island Krakatau. This eruption started from an andesite chamber. The volcanic islands magma chamber
had emptied and collapsed (caldera formation) and sea water poured in triggering a violent phreatic
explosion that sent a major tsunami into much Indonesia as well as sending ash and debris travelled
over water onto the adjacent island of
Sumatra.
Pyroclastic flows and debris form
when water and gases come out of the
magma.
Pressure in
the magma
chamber
will keep
these
volatiles
from escaping. When the pressure drops
during an eruption the gases come out of solution. This can form an explosive eruption. This will shatter
pahoehoe
aa
Pillow Lava
the overlaying rock and also form gas charged fragments in the air.
Pyroclastics or tephra have different sizes and these different sizes have different names: 1. ash,
2. lapelli, 3. agglutinates, 4. bombs and 5. blocks The smallest is the volcanic ash and are less than 2 mm
in size and are usually glass in nature. If you have larger blobs different things are formed. Blocks can are
greater than 64 mm in size and are formed from angular
solid rocks from the plugs in the volcano itself. Bombs are
greater than 64 mm but are formed from molten magma
and can have different shapes. Agglutinates form either
cinders (scoria) or pumice depending on are 2-64 mm in
size and are formed from smaller vesicular blobs. Pumice
is formed from volcanic glass and the air filling the vesicles.
Pumice is characterized by being able to float on water.
The caldera eruption takes place when the magma
chamber partially empties itself and triggers a collapse of
the unsupported material (roof of the chamber). This
then triggers a cataclysmic eruption with the pieces forcing
upward and out with much of the remaining magma in the
camber. The volcano doesn’t present a cone at this stage
but a large valley ringed by the edges of the former magma
chamber. Calder eruptions can take place with any volcano
but are common with volcanoes such as Yellowstone and
stratovolcanoes.
Volcanic
processes- The
anatomy of a volcano can vary depending on the volcano
type. The common features for all volcanoes include a magma
chamber and a transport mechanism. The magma chamber
which lies in the crust portion of the lithosphere. The
chamber is filled by rising magma from the asthenosphere or
by the melting of the overlying rock by the rising magma.
Next is the transport mechanism. This can be in the form of a
central vent and side vents or from a fracture or fissure
through the overlaying rocks and into the magma chamber.
If there is
magmatic
eruption from a vent system you then get these volcanic
features. The most common is the volcanic cone. The overall
shape of this structure is dependent on the eruption type
and the magma type. Craters are a bowel shaped pit at the
Pumice
summit of the volcanic cone. This is over type
volcanic central vent. Another structure is the
volcanic dome. This structure is associated with a
more felsic magma and can act as a plug to the
central vent trapping magma and gas beneath
them. Here the pressure will increase until there is
an explosion.
The last feature is the caldera. Here the magma has
escaped at such a rapid rate that the chamber can
no longer support the overlaying rock. This rock
then collapses into the chamber. This often leads to
an even more violent eruption as the remaining
melted material is expelled from the chamber. The picture on the left is the famous Crater Lake in
Oregon. It is a 6 mile wide caldera that formed after the volcano Mt. Mazamo erupted over 7,000 years
ago. The volcano in the center (Wizard Island) formed much later.
Volcano Types
Fissure volcanos are characterized by large lava fields that latter form plateaus. They are
basaltic in nature and have little to no gas. If gases are present then you will see other volcanic forms
associated with them. Massive flood basalts
from fissure volcanoes have been linked to at
least one extinction event, the Permian and
possible another, the Cretaceous. There have
been many smaller flood basalts from fissures,
one of these is the Columbia Plateau in
Washington and Oregon. Other examples of
fissure volcanism are the massive spreading
centers at the divergent plate boundaries.
Shield volcanoes have a melt with
gases as well as basalt magma. There is a central vent as well as side vents. The temperature and speed
of flow gives the shape of this
volcano. The initial lava is pahoehoe.
This gives the gentle angle of the
shield near the central vent. As the
lava cools it forms aa and has a
steeper angle along the sides of the
volcano giving it the characteristic
shield shape. These volcanoes are
common in areas of divergence as well as oceanic hot
spots such as Hawaii and the Galapagos islands. There are
often fissures and side vents opening up on the sides of the
shields.
Stratovolcanoes form from two different types of
eruptions. You have an alternation of pyroclastics and lava.
This is due to the characteristics of the intermediate melt.
When there is a high gas content combined with a silica
rich melt (more felsic minerals such as sodium plagioclase,
micas and quartz) you have an explosive eruption
and the cone that forms is of rock fragments and
assumes a steep angle. This material can be
covered in a subsequent by lava when the melt is
more basaltic in nature having less silica in it. This
melt would have more high temperature
minerals, plagioclases with more calcium and less
sodium, pyroxenes and little mica. This coats the
rock fragments and maintains the steep angle of
the cone. These volcanoes are seen along
convergent plate boundaries such as the Andes,
the Cascades, Japan and the Aleutians.
The last volcanic cone is the cinder-cone. This is made up by solid fragments builds up into a
cone, this allows for a steep angled cone. These have a small central vent and the magma is gas charged.
These can form on the flanks of shield volcanoes and stratovolcanoes. Once the gases have been
expelled from the magma a side vent often
opens and lava flows out. These volcanoes
erupt only once then their vent seals with cold
magma.
There are multiple hazards with a
volcanic eruption but while we have heard of
many of these hazards such as lava and
massive explosions. VOG or volcanic “smog” is seldom
looked at. Volcanic gases vary in composition. Two of
the most common gases are CO2 and H2O. The amount
of CO2 is 0.25 gigatons a year. Large eruptions can have
profound effects on global warming. Other gases
include H2O. Other toxic gases include: HCL
(hydrochloric acid), HF (hydrofluoric acid), CO (carbon
monoxide) SO2 (sulfur dioxide) and H2S (hydrogen
sulfide). The sulfur compounds will interact
with water to form sulfuric acid. The
picture to the right shows the gases exiting
Kilauea’s central vent. These acidic gases
cause widespread devastation to plant and
animal life on both land and in the sea. This
gas is reportedly equivalent to over a pack
of cigarettes a day. To complicate things if
the sulfur compounds enter the upper
atmosphere the droplets from the acid
they form reflects sunlight and causes
widespread cooling. The eruption of
Tambora caused a year without summer in 1815 and widespread starvation. The eruption of Toba
approximately seventy thousand years ago is credited with a ten year volcanic winter and 1,000 years
of cooling. We are overdue for such an eruption from Yellowstone and Long Valley volcanoes in the
United States.
Ash is a pyroclastic product. It is small enough in size that it and can travel miles away (smaller
particles go worldwide) from the source. Ash clogs the stomata of plants
preventing the exchange of gases and suffocating the plants. Ash is rock and
volcanic glass shards. When animals breathe in ash it enters the lungs and
damaging the alveoli. Exposure can and often does prematurely age these lungs.
This material can also turn into a concrete like material and suffocate people
and animals. Both ash and pumice adds weight to structures roofs. This weight
can collapse buildings. One more problem with the ash is the impact
on engines. These can get clogged by the ash and stop working. Jet
engines are very susceptible to the ash and are the reason that jets are
routed around or cancelled when there are ash clouds. The picture on
the right shows layers of ash from Mt. St. Helens. This is a record of
not just the 1980’s eruption of many past eruptions.
Another hazard is the lahar. This is a mud flow is a mixture
of volcanic debris and water. The water can be from melting
glaciers as in Mt. St. Helens, or rain fall as in Mt. Pinatubo’s
eruption. This mud flow can cover landscapes to hundreds of feet
deep, destroys bridges and homes. With speeds up to
10-60miles/hour a lahar can be linked to thousands of deaths. They
travel along existing water ways onto floodplains.
The deadliest hazard of them all is the pyroclastic flows and surges. Both of these are a mixture
of ash and toxic gases with temperatures as high as 1,000 °C (1,830 °F). Their speed is controlled by the
slope of the volcano (steeper slope more speed) 700 km/h (450 mph). These are normally caused by
the eruption column collapse. There are two layers,
the basal layer will hug close to the ground and
contains larger courser material. The upper layer is
the extremely hot ash plume mixed with the toxic
gases. There is mixing of the cold atmosphere and
the hot gases due to the turbulence causing
expansion and convection.
The pyroclastic surge has less material and
more gas making it act differently than the flow.
Lacking the courser material makes the surge more turbulent and it can rise over ridges and hill crest
while flows are more constrained.
The first thing that I will talk about in predicting volcanic eruptions is what constitutes an active
volcano. If there has been an eruption in the last 10,000 years the volcano or volcanic field it is active.
After saying this there are exception to this rule. These are volcanoes that haven’t had an eruption in
over 10,000 years. They are called active if they have indication of an active magma chamber beneath
them such as thermal features (hot springs, geysers and mud pots), magmatic gases (sulfur gases, and
CO2) and seismic activity. Two examples of ancient volcanoes that haven’t erupted in thousands of
years are Long Valley and Yellowstone.
To predict the eruption you monitor the region for the
things that indicate activity. The first can be seismicity. There is
a type of earthquake called the harmonic tremor that indicates
magma entering a chamber. The seismogram to the right shows
Mt. St. Helens harmonic tremors prior to eruption. Below is a
seismogram supposedly shows Yellowstone with harmonic
tremors during 2008.
You also look for an
increase in the release of magmatic
gases such as CO2 and H2S and SO2.
Both Long Valley and Yellowstone shows such an increase.
Another indication of an impending eruption is ground deformation. This is normally measured
by a tilt meter and indicated magma moving upward towards the vent. Below is a false color map
showing not one bulge in Yellowstone but two.
With magma moving upward closer to the
surface you will also find an increase in surface
temperatures. In 2002 the Norris Geyser Basin (arrow
on the left) ground temperature rose to the
temperature to the temperature of boiling water.
I have been using both Long Valley and
Yellowstone as examples of earthquake prediction for
several reasons. The most obvious is that they both
have all of the eruption indicators but haven’t
erupted. The second reason is what such an eruption
from these volcanoes would mean to mankind.
The image to the right shows the extent
of Yellowstone’s last eruption. The area
stripped of vegetation is the area where
large amounts of ash would be deposited.
This ash deposit impacts the continental
United States as far east as Louisiana and
as far south as far as Mexico. This region
is the bread basket of the world as well as
the United States. So famine would
fallow. It would propel sulfuric acid and
ash into the stratosphere leading to a
global winter for 10 years. The last time there was an eruption close to this magnitude (Toba ~74,000
years ago) there where many extinctions and close extinctions. The current theory is that the population
of the Earth was reduced to 1,000 humans.
