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, Australia-Antarctica, 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
distance-measuring 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 'super-continent.'
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
trench-like 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.
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