Use the following terms to complete the paragraph about processes and forces tha

Question

Use the following terms to complete the paragraph about processes and forces that drive plate motion. You may use a term more than once. (15pts)

ridge push                   sink                  oceanic                       mantle drag                 gravity

friction                         rise                  convection                  slab pull

The forces that drive plate motion include fluid flow by _____ and _______. The subduction of old, dense _____ lithosphere (often referred to as cold lithosphere) is agreed to be an important mechanism in driving plate motion. As the plate subducts, the fluid material in the underlying asthenosphere is displaced upward initiating an upward moving portion of _____. The mass of the subducting plate is pulled downward by _____ leading to more rapid plate motion than plates that do not subduct. This process is called _____.

Another mechanism driving plate motion occurs along the ocean ridges. Here, the massive rocks comprising the elevated flanks of the ridges are pulled downward by _____ in a process called ______. This process contributes less energy to plate motion and as a result, plates driven strictly by the upward moving component of_____ at regions of sea floor spreading or continental rifting move slowly.

Although geoscientists do not yet fully understand the entire process of fluid flow by _____ within the mantle, they do agree that cold, dense old lithosphere will _____ (i.e. downward moving portion) while warm, fluid rock will ____ (i.e. the upward moving portion). Horizontal movement of fluid rock in the asthenosphere called, _____, may influence the rate of plate motion.   If the fluid rock of the asthenosphere is moving more slowly or in a different direction than the overlying plate, _____ is increased and the overlying plate slows. If the horizontal rate of fluid rock in the asthenosphere is moving in the same direction but faster than the overlying plate, then ________ can increase the speed of plate motion.

Explanation / Answer

ANSWER: The forces that drive plate motion include fluid flow by ridge push and slab pull. The subduction of old, dense oceanic lithosphere (often referred to as cold lithosphere) is agreed to be an important mechanism in driving plate motion. As the plate subducts, the fluid material in the underlying asthenosphere is displaced upward initiating an upward moving portion of convection. The mass of the subducting plate is pulled downward by mantle drag leading to more rapid plate motion than plates that do not subduct. This process is called slab pull.

Another mechanism driving plate motion occurs along the ocean ridges. Here, the massive rocks comprising the elevated flanks of the ridges are pulled downward by gravity in a process called  ridge push. This process contributes less energy to plate motion and as a result, plates driven strictly by the upward moving component of convection at regions of sea floor spreading or continental rifting move slowly.

Although geoscientists do not yet fully understand the entire process of fluid flow by convection within the mantle, they do agree that cold, dense old lithosphere will sink (i.e. downward moving portion) while warm, fluid rock will rise (i.e. the upward moving portion). Horizontal movement of fluid rock in the asthenosphere called, convection, may influence the rate of plate motion. If the fluid rock of the asthenosphere is moving more slowly or in a different direction than the overlying plate, friction is increased and the overlying plate slows. If the horizontal rate of fluid rock in the asthenosphere is moving in the same direction but faster than the overlying plate, then friction can increase the speed of plate motion.

The answers are given in bold and underlined manner.

[Explanation of some terms:

Plate movements occur due to convection processes in the mantle. The heat source for this convection is the radioactive decay of isotopes in the mantle. The plate motion is also caused by the subduction of older, colder, denser oceanic lithosphere. There are two forces that help in plate motion: mantle drag or slab pull and gravity or ridge push. Due to mantle drag, heavy lithosphere goes into the trench, this process is known as slab pull. In this process when the plate sinks into the mantle, it also pulls the rest of the plate behind it. Whereas the another force is ridge push, which is caused by gravitational action. This occur in spreading ridges. The horizontal movement of fluid in the mantle is known as convection process. During this process, cold, old heavy lithosphere sink and warm fluid rock will rise. Friction between the convection currents in the asthenosphere and the more rigid overlying lithosphere is one of the reason of plate motion. It can increase or decrease the motions of plates.]

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