Moving Plates By Trista L. Pollard

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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.
 
⁶     Convergent boundaries are formed when two plates collide. At this boundary three types of collisions may occur. The dense oceanic lithosphere subducts or sinks under the continental lithosphere as they collide. These areas are called subduction zones. You will also find deep ocean trenches in these zones. Heated fluid is released into the overlying mantle as the oceanic plate subducts. The mantle is melted by the fluids, and magma is formed. This magma rises and forms volcanic mountains. The second type of collision causes the uplifting of mountains. Two continental plates collide, causing the colliding edges to crumble and thicken. Subduction doesn't occur because both plates are either not dense enough or have the same density. Neither plate would sink. The last type of collision occurs with two plates of oceanic lithosphere. As the plates collide, one plate subducts under the other. In addition to forming deep ocean trenches, island arcs may be formed. During the subduction, fluids are released, mantle rock is melted, and magma is produced. The rising magma forms a chain of volcanic islands.
 
⁷     Transform boundaries occur where two plates slide past each other horizontally. The edges of these plates scrape against each other in sudden spurts of motion. Earthquakes are usually felt along these boundaries. Transform boundaries do not produce magma. These boundaries are also found along mid-ocean ridges at fracture zones. Fracture zones are short segments of connected transform boundaries. So, why do we have all of this plate movement?
 
⁸     That is the million dollar question that scientists are still looking to answer. In general, they have strong theories about the forces behind plate movement. However, it is difficult to understand the true process without traveling to the center of the Earth. Most scientists believe that tectonic plates move through convection. Convection or the movement of heated material based on their densities occurs when there are differences in temperature. Simply put, cooler denser material sinks, while hotter less dense material rises. This process creates a cycle called a convection cell.
 
⁹     Scientists studying plate movements believe that our planet is a convection system. Mantle convection occurs when energy produced by the Earth's core and radioactivity heat the material in the mantle. This mantle material becomes less dense and rises through the cooler denser material that surrounds it. As this heated material rises, the cooler material moves away and sinks to replace the heated mantle. It's the flowing of the cooler material that moves or drags on the bottom of tectonic plates. Well, at least scientists believe mantle convection is the reason the plates move. In addition to the convection currents, scientists have also identified two other forces that affect plate motion.
 
¹⁰     Ridge push occurs at mid-ocean ridges. When cooling rock or lithosphere sinks, the asthenosphere underneath it moves away. As the asthenosphere is moving away, it exerts force on the overlying plate, causing the plate to move away from the mid-ocean ridge. The plate is being pushed away from the ridge by the asthenosphere. At convergent boundaries scientists believe there is a slab pull force behind plate movement. Plates at the mid-ocean ridge pull away from each other and magma rises to the surface from the asthenosphere. The cooling magma forms new lithosphere that becomes denser as it moves away from the ridge. This dense lithosphere subducts into the asthenosphere as it is moving. The slab pull occurs because the lithosphere is pulling the rest of the plate behind it as it is sinking. Scientists have found that subducting plates in general move faster than nonsubducting plates. Based on this evidence, scientists believe the driving force of the tectonic plates is provided by the pulling action of the subducting lithosphere.
 
¹¹     Earth's tectonic activity, earthquakes, and volcanic eruptions have given scientists valuable information about plate movement. The mechanisms of Earth's convecting system- drag on the bottom of plates, ridge push, and slab pull- have been working together to move our tectonic plates constantly for millions of years.

Copyright © 2008 edHelper

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Moving Plates

1.   Plates that are subducting move slower than plates that are not subducting.   False   True	2.   Name and describe the three types of collisions that occur at convergent boundaries.
3.   The only boundaries that do not produce magma are ______.   Transform boundaries   Divergent boundaries   Subduction boundaries   Convergent boundaries	4.   Explain slab pull.
5.   Scientists use ______ to identify plate boundaries.   Fracture zones   Volcanoes   Earthquakes   All of the above   B and C	6.   How are transform boundaries different from divergent boundaries?
7.   Tectonic plates include the continental crust.   False   True	8.   Explain mantle convection.

 

Moving Plates - Answer Key

1    False
2  The first type of collision causes subduction between the dense oceanic lithosphere and the less dense continental lithosphere as they collide. These are called subduction zones. As the continental lithosphere sinks under the oceanic lithosphere, magma is formed and rises to the surface. Volcanic mountains and deep ocean trenches are usually produced. The second type of collision causes mountain uplifting. Two continental plates collide, causing the colliding edges to crumble and thicken. Subduction doesn't occur because both plates were either not dense enough or have the same density. The third type occurs when two oceanic plates collide, causing one plate to subduct under the other. Deep ocean trenches and island arcs may be formed.
3    Transform boundaries
4  Slab pull occurs when plates at the mid-ocean ridge pull away from each other and magma rises to the surface from the asthenosphere. The cooling magma forms new denser lithosphere, which is moving away from the ridge. This dense lithosphere subducts into the asthenosphere as it is moving and pulls the rest of the plate behind it as it is sinking.
5    B and C
6  Transform boundaries occur where two plates slide past each other horizontally. The edges of these plates scrape against each other in sudden spurts of motion. Earthquakes usually occur along these boundaries. 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 is how the mid-ocean ridge is formed.
7    False
8  Mantle convection occurs when energy produced inside the Earth heats the material in the mantle. This mantle material becomes less dense and rises through the cooler denser material that surrounds it. As this heated material rises, the cooler material moves away and sinks to replace the heated mantle.