2. The Theory of Plate Tectonics
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The photographs above show two basic observations that can now be interpreted through our understanding of "plate tectonics."
First, on the left, is a picture of the Andes Mountains in South America, which in this particular locale are composed of sediments that formed on the seafloor -- miles below the sea surface -- millions of years ago. Now these sediments rest on the top of the mountains, miles above the sea surface -- how can this be? How can something as heavy as the surface of the Earth rise to such a high elevation?
On the right is a picture of the fountains of molten rock, or lava, on the big island of Hawaii. Once again, how can molten (liquid) rock at temperatures more than 1000oC find its way to the surface of the Earth and why should this happen in Hawaii?
In the following expedition, we will explore a concept that explains these observations with a unifying theory -- the theory of plate tectonics - which caused a revolution in scientific thinking only a few decades ago.
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The debate was largely between a group of scientists who believed that the Earth's surface was mobile, albeit by movements of only a few inches per year, versus those who believed in a fixed surface.
An early model of a mobile earth surface, called continental drift, proposed by Alfred Wegener, suggested that the continents formed a super-continent called Pangaea some 225 million years ago. Wegener was actually a meteorologist, not a geologist, but traveled all over the world, especially in the southern hemisphere.
As shown at the left, the super-continent broke up into a series of drifting continents scattered about the Earth to their current configuration.
Early proponents of this theory were met with much skepticism and disapproval by the "conventional wisdom" (dogma?) of earth science community until the 1960's, when data on the nature of the seafloor yielded new insights, which ultimately supported the work of these pioneers.
Even with acceptance of plate tectonic theory, the scientific community is only beginning to comprehend the workings of the Earth.
This field is so young -- and there is so much yet to learn, but as we will see in this expedition, oceanographers played a leading role in turning the earth science community upside down by mapping critical properties of the seafloor shown in the diagram below.
Some of our current ideas will undoubtedly require modification and revision as new data provided by new technologies become available (that is the nature of science, which can always be improved).
Let's back up and first define plate tectonics --
by tectonics, we mean, the processes, stresses (forces) and motions of the Earth's crust and underlying mantle layer.
by plate we mean the rigid, hard outer shell of the Earth's surface which is broken into a network of pieces -- each piece is called a plate.
plate tectonics - the processes, stresses and forces that result in the movement of the plates on the surface of the Earth
Created By:
Don Reed
Dept. of Geology
San Jose State University
©Copyright 2008
Last Updated on
Sept. 22, 2008
When did the continents last
form a single, supercontinent called Pangea?
a) 225 million years ago
b) 135 million years ago
c) 65 million years ago
d) 30 million years ago