Plate Tectonics

June 23, 2004

Plate Tectonics

American Military University

SC 106, Introduction to Oceanography

by James Landrith

Comprehending the theory of plate tectonics requires first understanding the concept of “layering.” Geologists believe the Earth is made up of the following layers: the lithosphere (100-200 km thick), the asthenosphere (350-650 km thick), the mantle and the core. These layers are believed to have varying levels of density and temperature zones. Scientists have formed the theory regarding the layering of the Earth based on astronomical data collection, earthquake shock studies and volcanic gas measurements.

The least dense layer of the Earth, the lithosphere, “floats” on top of the asthenosphere, which in turn floats on the mantle and core due to the principle of buoyancy. In short, the lithosphere literally sits on top of the asthenosphere and floats, moves and shifts like a boat on the ocean.Further contributing to the floating effect is the Earth’s internal heat, which keeps the asthenosphere flexible. This flexibility allows the lithosphere to move and adjust.

Further complicating the theory of plate tectonics are the seismic waves released during earthquakes. Scientific studies of seismic waves have traced their refractions as they travel through the Earth. Plotting the various refractions and wavelength changes of seismic waves has allowed researchers to map the various layers of the planet.

In addition, volcanic activity and earthquakes attest to the theory of constant motion in the inner layers of the Earth. Further, the existence of continental drift also bolsters the theory of plate tectonics. Meteorologist Alfred Wegener formed the theory of continental drift based on the puzzle piece shapes of the continents. Eventually other scientists backed up Wegener’s claim through studies of deep sea earthquakes as a means of drifting the continents apart. These studies led to the concept of seafloor spreading, which explained the changing surface of the ocean floor. According to this theory, as the seafloor moves and changes and forms new crust, portions of the Earth’s crust are destroyed elsewhere by falling into the mantle during a process known as subduction.

Eventually, the theories of seafloor spreading and continental drift were merged to form a unified theory of plate tectonics. This theory divides the planet up into multiple plates that move and float on the asthenosphere. As these plates move and collide in the spreading system, plates overlap and form mountains. Further, parts of plates descend into the mantle and outer core, while the outer core vents mass into the asthenosphere, which eventually finds its way into the lithosphere and the plate system. All of this movement is fueled by the Earth’s internal heating system.

However, some questions remain unanswered. For instance, how long will plates continue to move and reform the surface of the planet? Further, at what distance to the plates extend into the Earth? Despite the unanswered questions and for lack of a better theory, I would have to fall on the side of supporting the theory of plate tectonics.


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