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Introduction
Plate tectonics is arguably the most important scientific revolution of the 20th Century. Earth scientists believe that the surface of the earth is covered by 14 major mobile tectonic plates that move around the earth. Some plates move apart, some move together and some simply slide by each other. The critical thing to remember about plate tectonics is that isis an all-encompassing theory that explains almost everything we observe on the surface of the earth and at depth. The animation below describes these plates and their movement.
Historical Perspective
Scientific enquiry is in many ways like a legal trial. Much of the evidence used to convict the accused often is circumstantial evidence that, taken alone, would not convict but when combined with other circumstantial evidence creates a scenario that can only be explained by guilt. Plate tectonics is a good example of the construction of an extremely compelling argument based on a substantial array of independent circumstantial evidence. In the interactive exercise below you may explore the different lines of evidence that support plate tectonics, and learn a bit about the history of plate tectonics.
To test your understanding of the lines of evidence supporting plate tectonics click the link below and work with a set of hypothetical plates to determine how best they fit together.
Crustal Materials
The plates that form the mosaic covering of the earth are composed of two types of materials, continental and oceanic.
Continental crustal material is composed of all of the major rock groups with sedimentary rocks forming a thin veneer over much of the surface. Continental rocks are the oldest crustal material and more enduring than the oceanic crust. Continental landmasses may be rifted apart or collide to form larger landmasses, but they never disappear. Continental crust is less dense than oceanic crust and "floats" above the oceanic crust, and is never subducted and destroyed like oceanic crust. If we melted continental crust and then let it cool and recrystallize, it would most closely resemble a granite.
Oceanic crustal material is in a constant state of renewal and destruction. It is composed of mafic basalt covered with a thin layer of sedimentary rock in most locations. All of the oceanic crust now present is less than 200 million years old, less than 5% of the total history of the earth. The oldest continental crust is 4.6 billion years old. The oceanic crust is much more homogeneous than continental crust. Oceanic crust is created at mid ocean ridges (divergent boundaries) and destroyed at subduction zones (convergent boundaries) with the rates of creation equaling the rates of destruction on a worldwide scale. Click Next to move to the next page.
A Brain Teaser exercise is included below.