A theory that can be explained by plate tectonics is a theory called continental
drift. According to the theory of continental drift, the world was made up of a single
continent through most of geologic time. That continent eventually separated and drifted
apart, forming into the seven continents we have today. The first comprehensive theory
of continental drift was suggested by the German meteorologist Alfred Wegener in 1912.
The hypothesis asserts that the continents consist of lighter rocks that rest on heavier
crustal material, similar to the manner in which icebergs float on water. Wegener
contended that the relative positions of the continents are not rigidly fixed but are slowly
moving at a rate of about one yard per century. However, one of the strongest objections
to Wegener’s theory was that it did not describe how the plates moved.
According to the generally accepted plate-tectonics theory, scientists believe that
Earth's surface is broken into a number of shifting slabs or plates, which average
about forty kilometers in thickness. These plates move relative to one another above a
hotter, deeper, more stationary zone at average rates as great as a few inches per year.
Most of the world's active volcanoes are located along or near the boundaries between
shifting plates and are called plate-boundary volcanoes. The areas of the Pacific Ocean
Basin, containing the boundaries of several plates, are dotted with many active volcanoes
that form the so-called Ring of Fire. The Ring provides excellent examples of plate-
boundary volcanoes, including Mount St. Helens. Important evidence that supports the
theory of plate tectonics comes from the studies of the magnetic properties and the ages
of igneous rocks on the ocean floor. Some igneous rocks contain minerals that are
magnetic. These minerals provide a record of the direction of the Earth’s magnetic field
at the time when the molten lava that formed the rock, cooled. A discovery that
geologists made was that some of the rocks also recorded reversals in the direction of the
magnetic field. Geologists also record the ages of igneous rocks on each side of what
what is known a mid-ocean ridge. Earth’s mid-ocean ridges stretch around the planet.
Tectonically and volcanically active, they are the places where new crust forms. The
mid-ocean ridge system hosts one of the most complex environments on our planet.. The
mid-ocean ridge is featured at a diverging plate boundary where magma rises between the
separating plates. Another type of plate boundary that geologists have discovered is the
convergent plate boundary. This means that the plates are actually moving towards each
other. There are three types of destructive margins, hose where oceanic lithosphere
meets oceanic lithosphere, continent-continent collisions and that where continental
lithosphere meets oceanic lithosphere. The features in each case are similar. However, a
continent-continent collision results in little or no volcanism. Pacific Plate and the
Eurasian Plate are examples of destructive plates. The history of the Earth has shown
that the size of the Earth itself has not change significantly. And because we know that
new crust is being formed everyday that means that old crust must be getting destroyed
somewhere else. A transform boundary is classified as the area between two plates
sliding horizontally past one another. This idea originated from Canadian
geophysicist J. Tuzo Wilson. The Pacific Plate has been grinding against the North
American Plate for tens of millions of years at a rate of 5 cm/yr. This is the reason that
places like Los Angeles A large majority of transform faults are found on the ocean floor.
The most famous example of a transform fault is the San Andreas Fault in California.
The San Andreas Fault is about 1,300 km long and in some places it is tens of
kilometers wide. This Fault splits two thirds and San Francisco have so many problems
with earthquakes. All of these examples are very supportive of the plate tectonic theory,
but we also have to look at other theories as well, to get both sides of this mystery.
There are obviously other theories about the changes in Earth’s geological
features. One theory states that the expansion of earth is caused by a gain in mass. A
man named Jeffreys argued that the earth had cooled and contracted since its creation.
Another man named Macdonald concluded that the Earth had expanded, others say the
Earth is just randomly pulsating. Clearly there is not a lot of supporting evidence behind
these theories. Most of theorists generally accept the theory of plate tectonics.
The emergence of the theory of plate tectonics started a revolution in the earth sciences.
Since then, scientists have verified and refined this theory, and now have a much better
understanding of how our planet has been shaped by plate-tectonic processes. We now
know that, directly or indirectly, plate tectonics influences nearly all geologic processes,
past and present. Indeed, the notion that the entire Earth's surface is continually shifting
has profoundly changed the way we view our world.
Bibliography
- www. agiweb.org
- Handout 8.1 from class
- Handout the wrong answer to the right question
- Canadian Encyclopedia 1999
- http://pubs.usgs.gov/publications/text/developing