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Passing Plates II - Who's Fault?



Passing Plates II - Who's Fault?
Reading Level
     edHelper's suggested reading level:   grades 5 to 7
     Flesch-Kincaid grade level:   7.82

Vocabulary
     challenging words:    asthenosphere, divergent, lithosphere, lithospheric, mid-ocean, oceanic, outermost, reabsorbed, reoccur, seafloor, strike-slip, subducting, subduction, subducts, tectonic, upswelling
     content words:    Passing Plates, United States, Aleutian Islands, San Andreas Fault, Alfred Wegener, Arthur Holmes


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Passing Plates II - Who's Fault?
By Trista L. Pollard
  

1     In Passing Plates I, we took a trip back through history to see where the plate tectonic theory was born. Let's journey deeper into the earth to learn about these plates. Most of the plate "action" occurs within the lithosphere, which includes the crust and upper mantle. The crust is the solid outermost layer of the earth, and the mantle is the layer under the crust where the plate movement occurs. All of this makes the lithosphere one restless place. The places where the plates meet in the lithosphere are called plate boundaries. Earthquakes usually occur in plate boundaries. There are three types of plate boundaries: divergent, convergent, and transform.
 
2     Divergent boundaries or "spreading zones" are found where two plates are moving away from each other. Most divergent boundaries are found in oceans where seafloor spreading occurs. The ocean floor has mid-ocean ridges and underwater mountain chains. In the centers of these mid-ocean ridges lie underwater volcanoes and other "hot spots." These hot spots are areas where magma rises from the asthenosphere. This rising mantle is referred to as an upswelling of magma. As the magma rises, plates in the mid-ocean ridges are forced apart. New earth material is added to the edges which means new oceanic lithosphere is born.
 
3     Convergent boundaries are found where lithospheric plates move toward each other. Eventually as these plates move toward each other, one plate subducts or overrides the other. This boundary is usually referred to as a subduction zone. As the subducting plate continues to move, the other plate is pushed downward toward the mantle where it will start to melt. The melting causes the plate to be reabsorbed into the earth. It is also the cause of some of the world's most destructive earthquakes. The northwest coasts of the United States, western Canada, and southern Alaska and Aleutian Islands have a subduction zone plate boundary.

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