FREE ESSAY ON PLATE TECTONICS

PLATE TECTONICS

Plate Tectonics
Since the beginning of human kind there has been a cloud of wonder regarding the origin
of our planet. Scientist interested in this field through out the years have developed
many different theories to how our planet came about. Before the Twentieth Century,
scientist and geologist alike revolved in the idea that Mountain building was due to the
massive contractions of the earth caused by the gradual cooling of molten rocks. In 1900,
American scientist Joseph Le Conte, published an article in the Appleton's Popular
Scientific Monthly. He described that the problem in understanding mountain building was
establishing the cause of sideways pressure. "The most obvious and as yet the most
probable view is that it is the result of the secular contraction of the earth which has
gone on throughout the whole history and is still going on." (2) 
It was not until 1910, that an American Geologist named F.B.Taylor, proposed the idea of
a continental drift. His scientific peers dismissed Taylor's idea, because there was just
not enough proof. However, Taylor's idea was then resurfaced by a German scientist named
Alfred Wegener. He proposed that the continents surrounding the Atlantic Ocean, Africa
and South America fit together like a jigsaw puzzle. Wegener was a balloonist,
astronomer, explorer, meteorologist, while in his spare time he researched paleontology,
geophysics and geology. His broad range of studies enabled him to incorporate his theory
of Plate Tectonics. Wegener, later in 1915 while in sick leave from the German military
published his idea that interpreted how his continental drift theory worked. He proposed
that a huge landamass called Pangea, meaning (all land") existed 200 million years ago.
He furthered explained that this super continent began to drift apart very slowly
throughout millions of years into what it looks like now. Wegener went on several
expeditions through out his life to the continents of South America and Africa. He came
back with evidence to support his claim of plate tectonics. Wegener and his expedition
brought back rock structures, fossils, and evidence of ancient climates. 
When it came to rock structures, Wegener looked for the similarities of the rocks found
along coastlines and the mountain belts found on the continents. They looked at the
mountain belts of the Appalachian Mountains and the Caledonian Mountains in Scandinavia
and the British Isles. These two continuos belts contained similar rock structures and
the ages of these rocks matched the on both ends of the African and South American
continents. Wegener also found evidence of massive climatic changes. He researched and
found out that ancient glacial deposits were found indicating that ice sheets covered a
large part of the Southern Hemisphere, India and Australia. Wegener's explanation of the
glaciation was that the "landmasses where all fitted together as a super continent with
South Africa centered over the South Pole." (1.) Pag. 474) He furthered explained that
the Northern Hemisphere was near the equator supporting the idea of coal deposits being
located there. Wegener was more intrigued of the fact that fossilized remains were being
found along South America and Africa. Remains of the aquatic prehistoric dinosaur called
Mesosaurus were only found in eastern South America and Africa. Plant remains were also
being found. For example the fossil fern Glossopteris were found sparingly throughout
Africa, Australia, India, and South America. Furthermore, this plant was found in polar
climates, supporting his theory of connected landmasses. Organisms like the Australian
Marsupials and the marsupial opossums found in the Americas, which have a definite fossil
link also supports Wegener's theory. 
Alfred Wegener's theories were not immediately accepted by the scientific community. One
of his faults was that he could not prove how the continents moved across the globe.
Wegener's suggestion was that "tidal influence from the moon"(1. Pag. 475) was to be
credited. However the science community proved him wrong. Eventhough he was in the right
direction, he could not completely and precisely determine how the continental drift was
accomplished. Therefore, his theories were dropped as rubbish. Essentially, Wegener was
ahead of his time. It was not until years later that other scientist cleaned up some of
Wegener's mistakes and resurfaced the Plate Tectonics theory.
Thanks to the study of magnetism a whole new window of scientific evaluations emerged to
support the theory of Plate Tectonics. S.K. Runcorn proposed an interesting idea. It was
found that "the magnetic alignments in the iron-rich minerals in lava flow of different
ages was found to vary widely."(1. Pag. 478) This was strong evidence that the magnetic
poles of the earth had migrated through time, meaning that lava flows had moved and so
did the continents. This event is called polar wandering. 
For a long period of time there was still a lot of questions that needed to be answered
about the Plate Tectonics theory. Thanks to technological efforts and the mapping of the
ocean floor, it became known that there was an oceanic ridge system. Ocean Ridges are
slices in ocean floor that extended for 70,000 kilometers. It was not until 1960, when a
professor from Princeton University, Harry Hess, suggested Sea Floor spreading. His idea
stated that new sea floor was being spread through mid-ocean ridges. As scientist clashed
ideas they suggested that continental drift and ocean floor spreading were incorporated
in the theory of plate tectonics. They implied that the outer layer of earth called the
lithosphere overlaid another layer of earth called the aesthenospshere. It was later
learned that the lithosphere was broken into seven large and small plates. Research
concluded that these plates are in motion and have been changing shape through billions
of years. However, scientists were clear to verify that these plates only moved one to
two centimeters a year. Over millions of years these centimeter accumulated from the big
landmass of Pangea to what the continents look like now.
In order to understand the movement of these plates one has to look at the how and where
these plates move. Divergent boundaries are known as plates that move away from their
ridge axis. This is due to the hot molten rock submerging from the mantel into the ocean
floor, creating new oceanic crust, also known as seafloor spreading. Some of these
divergent zones can be found in the Mid-Atlantic Ridge, Red Sea, and the Gulf of Mexico,
where it separates the Baja Peninsula from Mexico. Convergent Boundaries take place where
the oceanic plates descend into the asthenospshere. This region of activity is called
subduction zone. As the plate bends it creates a deep-ocean trench. These zones can
converge in three ways. Oceanic to continental, Oceanic to Oceanic, and Continental to
continental. Some examples of Convergent Zones take place in the Nazca Plate where it
crashes into South American plate to form the Andes Mountains. The Pacific Plate which
crashes into the North American plate, and the Australian-Indian Plate which also slams
into the Eurasian Plate to form the Himalayas. Another type is the Transform Fault
Boundaries. This type occurs when plates grind past each other. An example of a Transform
fault would be the San Andreas Fault in California. However this can also take place in
the ocean. An example of this would be where the Antarctic plate is grinding past the
African plate.
There are three main reasons why the theory of plate tectonics is supported. A link
between plate boundaries and earthquakes show that the earthquakes occur with in
subduction zones rather than in the mantel therefore providing a way to track a plate
decent. Secondly, the ages and thickness of sediments from ocean floor give scientist an
out look of how rocks submerged giving them a rough estimate of how long plate tectonics
has been taking place. Finally, Island chains that were created by hot spots can provide
evidence on the directions that the plates are moving. 
There was still the question on what was the driving force for the plate's movements.
Several models where proposed to the scientific community. "The convection current
hypothesis stated that convection cells in the mantel carried plates like a conveyor
belt." (1) The slab-pull hypothesis stated that "when cold, dense oceanic material is
subducted it pulls the trailing lithosphere along. The Slab-push may occur when gravity
sets the elevated slabs astride the ridge crest in motion."(1) Pag.505) The final model
encouraged that hot plumes or rock in the mantel created movement with in the plates. 
Plate Tectonics have opened a new perspective of geologic time. It was not know until
recently how old Earth was, which is now known to be 4.6 billion years old. This means
that in Geologic time humans have only been here for a fraction of earth's existence. The
theory of Plate Tectonics has brought all fields of science including chemistry, biology,
geology and paleontology together. This broad base of knowledge can now be used in future
research to unlock the mysteries of earth.
Bibliography:
1. Tarbuck, Edward J., and Frederick K. Lutgens.
Earth: An Introduction to Physical Geology. New
Jersey: Prentice Hall, 1999.
2. Tectonic Designs, www.platetectonics.com, accessed 10/15/00.
Bibliography:
1. Tarbuck, Edward J., and Frederick K. Lutgens.
Earth: An Introduction to Physical Geology. New
Jersey: Prentice Hall, 1999.
2. Tectonic Designs, www.platetectonics.com, accessed 10/15/00.