There is a theory
that the earth is made up of large loosely connected
rocks that are called plates. These are theorized
that the outer shell of the earth is made up of
thin & rigid plates that move relative to
each other. This theory was formulated in the
year 1960's, and it revolutionized the field of
geology. This theory has helped scientist to explain
and predict many of the earth's natural disasters
and calamities such as earthquakes & volcanic
eruptions this study is also used to study mountain
building and the formation of the oceans and continents.
This theory arose from an earlier theory proposed
by a German scientist "Alfred Weqener"
in 1912. He was a German Metrologist who studied
ancient climates in the 1900's. While studying
the shapes of continents he found that all of
these were one big jigsaw puzzle that fitted all
together in one big piece. Using this observation
and evidence that he found on different continents
he came to the theory of continental drift, which
states that today’s continents were once
one big mass of land.
PLATE TECTONICS
“The plate tectonics theory: The lithosphere
is divided into independent plates. The lithosperic
plates move horizontally by gliding over the plastic
asthenosphere, analogous to sheets of ice floating
on a pond” (Plate Tectonics theory, 2004).
Geologist Scientist of the 1950's and the 1960's
found further evidence in support to Alfred's
theory of the tectonic plates and their movements.
These scientist applied Alfred's theory to various
aspects of the changing earth and they used this
evidence to confirm continental drift. By 1968
all geologic activities into one theory called
the "New Global Tectonics", or more
commonly "Plate Tectonics."
The weakness of Alfred’s theory, and the
reason that it was not readily accepted by geologist
was that it proposed that the continents slide
over the oceans floor. Geologist disagreed, stating
the ocean floor did not have enough strength to
hold the continents and too much frictional resistance
would be encountered.
In the 1950’s and the 1960’s, studies
of the Earth’s magnetic field and how it
varied through time (paleomagnetism) proved that
the continents do indeed drift. To understand
this one must study the Earths magnetic field.
The earth has a magnetic field that causes a compass
needle to always points toward the North Magnetic
Pole. To further understand this theory one could
imagine a large bar magnet in the middle of the
earths crust and the magnetic field is composed
of lines of force. So a compass needle or a magnetic
weight suspended from a string, points along these
lines of force.
These initial studies of how the position of
how the Earths magnetic pole varied were done
in Europe. These studies showed that the magnetic
pole had apparently moved through time. But studies
of a similar kind when were done in North America
showed a different path of the magnetic pole was
found. This indicated that:
1. The earth has had more than one magnetic force.
2. That the different continents have moved relative
to each other over time.
DIVERGENT PLATE BOUNDARIES
“In plate tectonics, a divergent boundary
is a linear feature that exists between two tectonic
plates, where the plates are moving away from
each other” (Divergent boundary, 2004)
At divergent boundaries, two plates move apart
from each other and the space is filled with new
crustal material sourced from molten magma that
forms below the earths crust. The force that drives
these plates apart are not fully understood. There
are basically two theories that prove this one
is the ridge push and the slab pull hypotheses.
The beginning of divergent boundaries is thought
to be associated with the phenomenon known as
hotspots. Exceedingly large convective cells bring
very large quantities of hot atmospheric material
near the surface and the kinetic energy is thought
to be sufficient to break apart the lithosphere.
The hot spot is believed to have created the Mid-Atlantic
Ridge system currently underlies Iceland which
is widening at a rate of a few centimeters per
century.
Plates move in different directions, change direction
over time, and move at different rates. The Pacific
and Cocos plates are the fastest moving and the
Arabian and southern African plates are slowest.
OCEANIC RIDGES AND SEAFLOOR SPREADING
The study of the ocean floor had been given little
attention to. During the World War II the military
had employed geologist to study the topography
of the oceans the main purpose of these studies
was to find out where Allied as well as enemy
submarines were hiding the geologist were studying
the depths of the ocean floor.
These revealed the presence of two important
features of the ocean floor.
1. Oceanic Ridges - long sinuous ridges that
occupy the middle of the Atlantic Ocean and the
eastern part of the Pacific Ocean.
2. Oceanic Trenches - deep trenches along the
margins of continents, particularly surrounding
the Pacific Ocean.
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