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The land below us is always in motion. Plate tectonics studies these restless effects to
give us a better understanding of the Earth and its past. New molten rocks are poured out
in the form of magma from the mid-ocean ridges. The rock is recycled and re-entered
back into the earth in deep ocean trenches through convection current. The convection
current in the mantle drives plates around either against or away from each other. These
collisions give rise to earthquakes, volcanoes, mountains, and continental drift. The
crashing and spreading of the plates forms the landscape of the Earth as we see it today.
The positions of the land masses today is a result of continental drift. During the Earth's
existance, the magnetic fields have never been stable. Solidified magma containing
magnetic imprints reveal periods of time when the Earth's magnetic fields have actually
been reversed.
Approximately 4.55 billion years ago, the Earth was just a ball of molten
material. Since then, parts of the Earth have cooled forming the solid crust-mantle. This
process has been occurring for roughly about 3.8 billion years. The mantle is about 2900
km. thick, which lies above a layer of molten magma that still exists today. The immense
heat from the magma (approximately 2700(C) causes convection in the mantle (Figure 1).
Convection is caused by non-uniform temperature in a fluid and density differences. This
continuous convection is the cause of plate movement. Each complete cycle, called a
convection cell, drives the plate in the direction of the cell. How does a 'solid' mantle
move? The mantle may be solid but, as with most solids, it will deform if long term
stress is applied; '...like Silly Putty which seeps into the rug when left unattended, mantle
material flows when subjected to small long-term stresses.'1
Presently, there are more than fourteen plates in the Earth's crust (Figure 22).
Upwelling hot magma flows out from mid-ocean ridges and then cools down when
exposed to the cooled environment outside; the layer of cooled magma forms the
lithosphere. When magma flows out from the ridges, the crust is fractured and a new
ocean floor is built spreading perpendicularly away from the ridge. Because of this
constant upwelling, the ocean is relatively shallow in these areas. Sea floor spreading
and continental drift are the products of this continual upwelling. The cooled magma
will, in time, sink back down into the Earth in the deep ocean trenches. The mantle
sinking down produces subduction zones or Benioff zones. The deepest part of the ocean
resides in these areas. There are three types of boundaries where plates meet: divergent
boundaries -- the upwelling of magma; convergent boundaries where the plates collide
producing mountains, volcanoes, and earthquakes; and transform boundaries -- lateral
movement. Transform plates are caused by fracture zones. When a rift opens from the
upwelling of magma it causes a crack in the crust. As new magma rises to the surface,
the crack increases caused by the pressure, resulting in a horizontal faulting. The
fractured plate pieces travel in the same direction as the original plate was traveling -away from the ocean ridge.
During the early 1900's, a theory of a 'super-continent' was developed by Alfred
Wegener. He was ridiculed for his ideas that continental drift produced the present
positions of the continents from a single 'super-continent' called Pangea. This theory is
widely accepted today, however. There was abundant evidence for Wegener to believe in
the existance of Pangea. The shape of the continents could be pieced together like a giant
jigsaw puzzle suggesting that the continents were once 'glued' together. The fossils found
on the continents were not distinct to that particular land, but were also found in lands
that were separated by thousands of kilometers of water. Fossils indicated that identical
species existed in different continents. Geological structures also demonstrated that the
continents were, in fact, one giant land mass; old mountain ranges from one continent
matched with those from another (i.e., South America and Africa).
Ocean spreading has always been moving the continents towards or away from
each other. About 200 million years ago during the Jurassic period, Pangea began to
separate (Figure 33). Pangea's continental crust was subjected to many faults and rifts.
Hot magma would flow out, splitting the land apart and creating a rift valley. When this
valley became deep enough, water flowed in. In time, the rift expanded so much that a
sea began to form between thus creating two continents. About 135 million years ago,
because of sea floor spreading, Pangea separated into two large land masses: Laurasia
(containing North America, Europe, and Asia) to the north, and Gondwana (containing
South America, Africa, Australia, Antarctica, and India) to the south. About 180 million
years ago, Gondwana started to break up into South America-Africa, AustraliaAntarctica, and India. About 130 million years ago, the Atlantic started separating South
America and Africa while India sailed towards Asia, crashing into it about 30 million
years ago. Australia and Antarctica split about 45 million years ago and North America
separated from Europe 5-10 million years later.
To this day, the continents are continually moving and will still be moving until
the liquid inner core cools and solidifies. With the use of a highly-accurate distancemeasuring device known as a geodimeter, the speed at which the continents are moving
and the speed of ocean spreading could be measured. A geodimeter uses a helium-neon
laser that acts like radar to measure distances. The average speed of sea floor spreading
is about 2 cm. per year. Africa, today, is traveling towards Europe and Asia, causing the
Mediterranean to close in; in due time, this sea will vanish. India, which is cemented to
Asia, is an example of continental collision. India's drift speed is about 17 cm. per year;
this collision is shown physically by the Himalayan mountains. In the far future, North
America will, most likely, be placed more the west, possibly colliding with Asia; and
Australia will drift north, colliding with South Asia. Another possibility may be that, in a
few hundred million years, all the continents may join together, creating another 'supercontinent.'
One of the most destructive forces the plates generate are earthquakes. There are
earthquakes occurring every day of different intensity and magnitude, from 500,000 per
year at a Richter scale of 1, to one every few years at a Richter scale of about 8. Faults
are produced when rock strata are stressed beyond their limits, forming cracks in the
crust. These cracks are fault zones where crustal movement is taking place. There are
three types of faults shown in Figure 4: normal, reverse, and strike slip. Normal faults,
also called tension faults, move up and down, caused by two plates pulling away at
divergent boundaries. These vertical movements cause one side of the land to slide
downwards along a plane that is slanted. This kind of 'downward-fault' produces trenchlike valleys called grabens similar to the Rhine Valley on the border of France and West
Germany. Reverse faults, or compression faults, are caused by the collision of two plates
at convergent boundaries. Most faults are produced by this compressional force. Like
normal faults, these faults also cause vertical movements where one side is pushed
upwards vertically on an inclined plane. These faults produce high vertical 'upward-fault'
structures called horst.
Strike slip or transform faults are lateral movements of faults at the transformed
boundaries. Strike-slip faults do not produce any cliffs but they can produce rift valleys.
Tectonic forces deform the rocks on both sides of the fault. At this point, rocks are
bending and storing potential energy. Finally, when the force exceeds the frictional force
between the two rocks, the plates suddenly slip at the most vulnerable place. The initial
slip causes more slippage along the fault which in turn causes energy to be released. The
released energy produces vibrations called seismic waves which originate at the
epicenter. The San Andreas Fault is a well-known example of this released energy from
a transform fault. At this location, an almost straight valley is produced by the parallel
fractures. The Pacific plate, in Canada, is sliding northwards and thus, in the future,
California may end up where Vancouver is, today.
One of the most prominent signs that molten material resides below the crust and
mantle is the display of volcanoes. Magma seeks out weak spots on the crust where it
could seep out. Volcanoes are mostly present at fault lines especially at the ocean ridges
where new magma is constantly being poured out. This accounts for about 81% of all
magma that escapes to the surface. The other 19% rises at certain points rather than
along fissures. On of the most famous examples of volcanic activity is The Ring of Fire,
located around the Pacific Plate. There, a continuous 'ring' of volcanoes exists. 'Island
arcs' are formed there by many volcanoes developing islands in the form of a curve. The
longest island arc is the Aleutian Islands stretching more than 3000 miles from Alaska to
Asia. One explanation for this arc is that the Pacific plate is rotating very slowly. The
westward-moving plate moves away from the source of volcanic activity making the
volcanoes arise in an arc due to the rotation of the plate.
One of the beauties plate collisions could offer are mountains. There are three
types of tectonic mountains: volcanoes, block fault, and folding. One way mountains are
formed are by volcanoes such as the aforementioned island arcs. In time, after numerous
eruptions, more and more sediments are layered and compressed, forming mountainous
islands. Block fault mountains occur when two plates collide, causing one to climb up.
This is known also as a horst mentioned before. Mountains such as the Sierra Nevada
Range is a large tilted fault block. Folding mountains occur when two converging plates
bring two land masses together. When a continent is pushing its way towards another,
the oceanic crust sinks into the subduction zone. As it moves down the zone, the
sediment that makes up the crust is scraped off by the other continent. With the
continental crusts pushing together, the sedimentary rocks are compressed into complex
folds where the folds themselves fold as well. This process forms the high alpine
mountains such as the Himalayas which were caused by India crashing into Asia.
If the mantle is always being convected back down into the depths of the Earth,
then why doesn't the continents disappear in the deep ocean trenches as well? The crust
contains two different crusts: the granite continental crust and the basaltic oceanic crust.
Only the basaltic crust is thrust back into the Earth while the granite crust floats on top of
it. This is due to the difference in densities. The granite crust is less dense (2.7 g/cm3)
and thicker than the basaltic crust (2.8 g/cm3) making it seem as if the land is actually
floating, instead of one big solid mass that extends down to the Earth. Using Broecker
analogy:
'...swimming pool with 4 x 4 hardwood beams and part with 8 x 8 softwood beams. The
softwood beams would float higher for two reasons: they are thicker and they are less
dense.'4
As new crust is formed from upwelling magma, the ocean floor spreads away
from the source. Because molten magma contains metallic substances such as iron, the
cooled rock will possess a magnetic field parallel to the direction of the Earth's field. The
magnetic imprint occurs when certain substances cool after intense heating within a
magnetic field. The rock cools to the temperature when the magnetic field of the rock
becomes permanent; this is called the Curie temperature. During the history of the Earth,
this 'normal' magnetic field (North pole to true North) has not been constant. Over the
past 110 million years, the Earth's magnetic field has reversed about 80 times with North
becoming South and vice versa. Figure 55 shows the chronological reversals of Earth's
magnetic field over the past 4.5 million years. The last major reversal was approximately
700,000 years ago called the Brunhes Epoch. These magnetic reversals are symmetrical
to either side of the ridge. The reversals are also random with no determined period of
time.
Radioactive dating along with magnetic reversals provides a means to record the
speed at which the ocean floor is spreading. The youngest crust is where the magma
flows out from the ridges and the oldest being where the crust flows back in the trenches.
Figure 66 shows the age of the oceanic crust. Deep sea drilling and the art of radioactive
dating could tell us when the magnetic field was reversed. Ships equipped with hollow
drills would obtain samples of the ocean floor from various places around the ridge. The
procedure most widely used to date the ocean floor is the Potassium-Argon dating
method. It relies on any present radioactive material, Potassium-40. Potassium-40
decays slowly (1250 x 106 years) but not as slow as Uranium, which decays too slowly
for this purpose and Carbon, which decays too fast. Potassium-40 decays into Argon-40
and Calcium-40. By measuring the amount of decay, the age of the ocean floor can be
determined. Knowing the time and distance, the velocity of the ocean floor spreading can
then be determined. It takes about 50 to 150 million years for the crust to travel from its
origin to where it will circulate back below. The crust is relatively new because it is
always being renewed.
Using the magnetic orientation of rocks, more evidence could be deduced that
backs of the theory of Pangea: '...it is possible, using simple trigonometry, to determine
the latitude at which the rock was formed and the past orientation of the continent upon
which it lay.'7 This practice is called paleomagnetism. The readings can give the
position of the magnetic North pole in any time period. If the readings from a single
continent is plotted, a smooth curve called the polar wander curve, could be attained. The
plot shows the curve leading away from the present pole. This is only possible if either
the magnetic pole moved or the continent moved. When readings were calculated for
other continents, the curves did not converge at a point. This means that there was only
one magnetic North pole at any one time and indicated that the continents moved in
respect to each other.
Magnetic reversals are still a mystery, but many suggested hypothesis exists.
One reason was that collisions with meteorites or comets may have caused the reversals.
In fact, there was recent evidence that the Earth in fact, collided with a huge meteor. This
hypothesis corresponded to the periods of mass extinctions; '...of the eight species that
vanished from the cores during the 2.5 million years for which the record was most
complete, six disappeared close to the time of a reversal, as recorded in the magnetic
particles of the same core.'8 Tektites, glassy fragments from meteorites containing large
amounts of iron and magnesium were scattered over large sections of the Earth which
corresponded to the last major reversal. The meteorites provided some proof to this
hypothesis. This theory is just one of the many scientists have come up with. Others
believed that the anomalies were formed by the compression of rocks -- the same kind of
compression that existed during mountain building.
The drifting of plates could cause devastation or wonder. Convection cells that
propel the plates produces Earth's surface dynamics. Murderous earthquakes and violent
storms of volcanoes are a result from these ever-dynamic floating plates. Earth's crust
juts out as high as the sky along with the deep valleys that are being produced from the
crashing and spreading of these plates. Upwelling of hot magma separates the land and
continents similar to the separation of Pangea, but in time the continents will meet yet
again to form another 'super-continent.' The ever new sea floor containing magnetic
'footprints' shows us of a time of magnetic field reversals. These reversals could explain
continental drift and its velocity. There has been extensive study in tectonic plates, but
there are still unsolved mysteries for one to discover.
ENDNOTES
1.
Wallace S. Broecker, How to Build a Habitable Planet. (Palisades, New York:
Eldigio Press, 1985), p. 147.
2.
Robert W. Christopherson, Geosystems. 2nd. ed. (New York: MacMillan
College Publishing Company, 1994), p. 341.
3.
Ibid., p. 336-337.
4.
Wallace S. Broecker, How to Build a Habitable Planet. (Palisades, New York:
Eldigio Press, 1985), p. 155-156.
5.
Walter Sullivan, Continents in Motion. 2nd. ed. (New York: McGraw Hill Book
Company, 1991), p. 97.
6.
Wallace S. Broecker, How to Build a Habitable Planet. (Palisades, New York:
Eldigio Press, 1985), p. 159.
7.
Peter J. Smith, The Earth. (New York: MacMillan Publishing Company, 1986),
p. 13.
8.
Waltus Sullivan, Continents in Motion. 2nd. ed. (New York: McGraw Hill Book
Company, 1991), p. 96.
BIBLIOGRAPHY
Bird, John M. and Isacks, Bryan, ed., Plate Tectonics. Washington American
Geophysical Union, 1972.
Broecker, Wallace S. How to Build a Habitable Planet. Palisades, New York: Eldigio
Press, 1985.
Christopherson, Robert W. Geosystems. 2nd. ed. New York: MacMillan College
Publishing Company, 1994.
Erickson, Jon Volcanoes and Earthquakes. Blue Ridge Summit: Tab Books Inc., 1988.
Smith, Peter J. The Earth. New York: MacMillan Publishing Company, 1986.
Sullivan, Walter Continents in Motion. 2nd. ed. New York: McGraw Hill Book
Company, 1992.
Keywords:
land below always motion plate tectonics studies these restless effects give better
understanding earth past molten rocks poured form magma from ocean ridges rock
recycled entered back into earth deep ocean trenches through convection current
convection current mantle drives plates around either against away from each other these
collisions give rise earthquakes volcanoes mountains continental drift crashing spreading
plates forms landscape earth today positions land masses today result continental drift
during existance magnetic fields have never been stable solidified magma containing
magnetic imprints reveal periods time when magnetic fields have actually been reversed
approximately billion years just ball molten material since then parts have cooled forming
solid crust mantle this process been occurring roughly about billion years mantle about
thick which lies above layer molten magma that still exists today immense heat from
approximately causes convection figure caused uniform temperature fluid density
differences this continuous cause plate movement each complete cycle called cell drives
plate direction cell does solid move solid with most solids will deform long term stress
applied like silly putty which seeps into when left unattended material flows when
subjected small long term stresses presently there more than fourteen plates crust figure
upwelling flows ocean ridges then cools down exposed cooled environment outside layer
cooled forms lithosphere flows ridges crust fractured floor built spreading
perpendicularly away ridge because this constant upwelling relatively shallow these areas
floor spreading continental drift products continual upwelling will time sink back down
into deep trenches sinking down produces subduction zones benioff zones deepest part
resides areas there three types boundaries where meet divergent boundaries convergent
boundaries where collide producing mountains volcanoes earthquakes transform lateral
movement transform caused fracture zones rift opens causes crack rises surface crack
increases caused pressure resulting horizontal faulting fractured pieces travel same
direction original traveling away ridge during early theory super continent developed
alfred wegener ridiculed ideas that produced present positions continents single super
continent called pangea theory widely accepted however there abundant evidence
wegener believe existance pangea shape continents could pieced together like giant
jigsaw puzzle suggesting that continents were once glued together fossils found were
distinct particular land were also found lands separated thousands kilometers water
fossils indicated identical species existed different geological structures also
demonstrated fact giant mass mountain ranges continent matched with those another
south america africa always moving towards each other about million years during
jurassic period pangea began separate figure subjected many faults rifts would flow
splitting apart creating rift valley valley became deep enough water flowed time rift
expanded much began form between thus creating million because floor separated large
masses laurasia containing north america europe asia north gondwana containing south
america africa australia antarctica india south million gondwana started break africa
australia antarctica india atlantic started separating while india sailed towards asia
crashing australia antarctica split north separated europe later continually moving will
still moving until liquid inner core cools solidifies with highly accurate distance
measuring device known geodimeter speed which speed could measured geodimeter uses
helium neon laser acts like radar measure distances average speed year traveling towards
europe asia causing mediterranean close vanish cemented example collision year
collision shown physically himalayan mountains future most likely placed more west
possibly colliding colliding another possibility hundred join together creating another
super most destructive forces generate earthquakes occurring every different intensity
magnitude year richter scale every richter scale faults produced rock strata stressed
beyond their limits forming cracks cracks fault where crustal movement taking place
three types faults shown normal reverse strike slip normal also called tension move
pulling divergent vertical movements cause side slide downwards along plane slanted
kind downward fault produces trench valleys grabens similar rhine valley border france
west germany reverse compression collision convergent produced compressional force
normal cause vertical movements side pushed upwards vertically inclined plane produce
high vertical upward fault structures horst strike slip transform lateral movements
transformed strike slip produce cliffs they produce valleys tectonic forces deform rocks
both sides point rocks bending storing potential energy finally force exceeds frictional
force between suddenly vulnerable place initial causes more slippage along turn energy
released released energy produces vibrations seismic waves originate epicenter andreas
well known example released location almost straight parallel fractures pacific canada
sliding northwards thus future california vancouver prominent signs material resides
below display volcanoes seeks weak spots could seep mostly present lines especially
constantly being poured accounts escapes surface other rises certain points rather than
along fissures famous examples volcanic activity ring fire located around pacific
continuous ring exists island arcs formed many developing islands form curve longest
island aleutian islands stretching than miles alaska explanation pacific rotating very
slowly westward moves source volcanic activity making arise rotation beauties collisions
offer three types tectonic block folding formed such aforementioned island arcs after
numerous eruptions sediments layered compressed forming mountainous islands block
occur collide causing climb known horst mentioned before such sierra nevada range large
tilted block folding occur converging bring masses pushing oceanic sinks subduction
zone moves zone sediment makes scraped crusts pushing sedimentary compressed
complex folds folds themselves fold well process forms high alpine such himalayas
crashing always being convected back depths then doesn disappear trenches well contains
different crusts granite basaltic oceanic only basaltic thrust while granite floats difference
densities granite less dense thicker basaltic making seem actually floating instead mass
extends using broecker analogy swimming pool hardwood beams part softwood beams
softwood beams would float higher reasons they thicker they less dense formed spreads
source because contains metallic substances iron rock possess field parallel direction field
imprint occurs certain substances cool after intense heating within field cools temperature
becomes permanent curie temperature history pole true constant over past reversed times
becoming vice versa shows chronological reversals over past last major reversal
approximately brunhes epoch reversals symmetrical either side ridge reversals random
determined period radioactive dating provides means record youngest oldest being shows
oceanic drilling radioactive dating tell reversed ships equipped hollow drills would obtain
samples various places around procedure widely used date potassium argon dating
method relies present radioactive potassium potassium decays slowly slow uranium
decays slowly purpose carbon decays fast argon calcium measuring amount decay
determined knowing distance velocity determined takes travel origin circulate below
relatively renewed using orientation evidence deduced backs theory possible using simple
trigonometry determine latitude orientation upon practice paleomagnetism readings give
position pole period readings single plotted smooth curve polar wander curve attained
plot shows leading pole only possible either moved moved readings calculated curves
converge point means only indicated moved respect still mystery many suggested
hypothesis exists reason collisions meteorites comets fact recent evidence fact collided
huge meteor hypothesis corresponded periods mass extinctions eight species vanished
cores record complete disappeared close reversal recorded particles same core tektites
glassy fragments meteorites large amounts iron magnesium scattered over sections
corresponded last major reversal meteorites provided some proof hypothesis just
scientists come others believed anomalies compression same kind compression existed
mountain building drifting devastation wonder cells propel surface dynamics murderous
violent storms result ever dynamic floating juts high valleys separates similar separation
meet again ever footprints explain velocity extensive study tectonic unsolved mysteries
discover endnotes wallace broecker build habitable planet palisades york eldigio press
robert christopherson geosystems york macmillan college publishing company ibid
wallace broecker build habitable planet palisades york eldigio press walter sullivan
motion mcgraw hill book company wallace build habitable planet palisades eldigio press
peter smith macmillan publishing company waltus sullivan motion mcgraw hill book
bibliography bird john isacks bryan tectonics washington american geophysical union
christopherson robert geosystems macmillan college publishing erickson blue summit
books smith peter sullivan walter mcgraw hill book
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