Print Moving Plates Reading Comprehension
||edHelper's suggested reading level:
||grades 9 to 10
||Flesch-Kincaid grade level:
||convecting, deformable, nonsubducting, silica-rich, sea-floor, subducts, radioactivity, subducting, rift, magnesium, fracture, slab, lithosphere, oceanic, divergent, collide
||Pacific Ocean, Pacific Ring
By Trista L. Pollard
1 Once Wegener and other scientists began studying the movement of our continents, more hypotheses and theories were developed. During the 1960's scientists developed the theory of plate tectonics. This theory explains the reason continents move and change our planet's crust.
2 The lithosphere, formed by the Earth's crust and the upper part of the mantle, provides the thin outer shell for our planet. It is sectioned into tectonic plates. These plates move along the asthenosphere which is a deformable layer of the mantle. Scientists believe the asthenosphere is "plastic" rocks located just below the lithosphere. This solid rock is under an extreme amount of pressure and flows very slowly. In other words, the asthenosphere provides the lubrication the tectonic plates need to move.
3 Scientists have also divided our crust into the oceanic crust and continental crust. The dense oceanic crust is formed by rocks that contain iron and magnesium. The continental crust, however, is low in density and contains silica-rich rock. Our planet's tectonic plates can include both types of crust or each type alone. According to the theory, the continents and oceans are moved by these tectonic plates.
4 There are fifteen major tectonic plates that have been identified by scientists. Some plates are easy to identify because they are bordered by mountain ranges or deep ocean trenches. Others are hard to identify because they may be located within continents away from major surface features. Scientists have used earthquakes and volcanoes to identify plate boundaries. Earthquakes occur when there are sudden shifts along plate boundaries as the plates are moving. Scientists have found that frequent earthquakes in one area occur where two or more plates may meet. When moving plates generate magma that may erupt on the surface, volcanoes may form. Lying in the Pacific Ocean is the Pacific Ring of Fire, an area of active volcanoes. This ring of volcanoes encircles the Pacific Ocean and is one of our major earthquake zones. Based on this evidence, scientists have determined that the Pacific Ocean is surrounded by plate boundaries.
5 There are three types of plate boundaries. They may be located around the edges of continents, in the middle of the ocean floor, or within continents. Each of the boundaries has specific characteristics. Divergent boundaries form where two plates move away from each other. The result is that magma from the asthenosphere rises to the surface, and new oceanic lithosphere is created. This new rock, which is warm and light, moves above the sea-floor to form the mid-ocean ridge. A narrow rift valley is formed in the center of the ridge as the plates move apart. Our planet's ocean floor generally contains divergent plates. Continental rifts' valleys have been discovered, allowing scientists to study lithosphere separation on land.
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