The land below us is always in motion. Plate tectonics studies
these restless effects to give us a better understanding of theEarth 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 theEarth 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, theEarth was just a ball of molten
material. Since then, parts of theEarth 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 theEarth 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 theEarth 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 theEarth 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.ENDNOTES1. 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.BIBLIOGRAPHYBird, 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.
. 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. 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