TheEarth'sDeepInterior

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Middle Tennessee State University *

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Geography

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Apr 3, 2024

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Name: ______________________________________________________________________ The Earth’s Deep Interior Part 1 – Seismic evidence for a layered Earth’s interior Before starting this assignment, review the different types of seismic waves and their properties. Refer to the graphs showing changes in seismic wave velocities with increasing depth in Earth’s interior. For each of the major seismic wave discontinuities, answer the following questions. The Mohorovicic Discontinuity (The Moho) What is the approximate depth of the Moho? ______________________________ km Describe how seismic waves change in velocity as they cross the Moho. _____________________________________________________________________________________ What is the cause of the change in seismic wave velocities as they cross the Moho? _____________________________________________________________________________________ The Moho is the boundary between the overlying _____________________________ and the underlying ___________________________. Which of these two layers has the highest density? ______________________

Which of these two layers has the lowest density? ______________________ The Low Velocity Zone What is the approximate depth of the Low Velocity Zone? ______________________ km Describe how seismic wave velocities change as they cross into the Low Velocity Zone. _____________________________________________________________________________________ What is the cause of the change in seismic wave velocities as they cross into the Low Velocity Zone? _____________________________________________________________________________________ The Low Velocity Zone is the boundary between the overlying ________________________________ and the underlying ____________________________ Which of these layers is relatively cold and behaves as a brittle solid when under stress? ______________________________ Which of these layers is relatively hot and behaves as a plastic solid when under stress? ______________________________ The 650-Kilometer Discontinuity What is the approximate depth of the 650-kilometer discontinuity? _____________________________ Describe how seismic wave velocities change as they cross the 650-kilometer discontinuity. _____________________________________________________________________________________ What is the cause of the change in seismic wave velocities as they cross the 650-kilometer discontinuity? _____________________________________________________________________________________ The 650-km discontinuity is the boundary between the overlying ________________________________ and the underlying ____________________________ In which of these layers is internal Earth pressure so high that molten rock is unlikely to exist? __________________________________________ The Lower Mantle – Outer Core Boundary What is the approximate depth of the lower mantle-outer core boundary? _____________________________ Describe how seismic wave velocities change as they cross the lower mantle-outer core boundary? _____________________________________________________________________________________ What is the cause of the change in seismic wave velocities as they cross the lower mantle-outer core boundary? _____________________________________________________________________________________

The lower mantle-outer core boundary is the boundary between the overlying ________________________________ and the underlying ____________________________ Which of these layers is solid rock? ____________________________________ Which of these layers is molten iron and nickel? ______________________________ The Outer Core-Inner Core Boundary What is the approximate depth of the outer core-inner core boundary? _____________________________ Describe how seismic wave velocities change as they cross the outer core-inner core boundary? ____________________________________________________________________________ What is the cause of the change in seismic wave velocities as they cross the outer core-inner core boundary? _____________________________________________________________________________________ The outer core-inner core boundary is the boundary between the overlying ________________________________ and the underlying ____________________________ Which of these layers is solid iron and nickel? ____________________________________ Which of these layers is molten iron and nickel? ______________________________

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Part 2 – The Geothermal Gradient The Geothermal Gradient is the average increase in temperature with increasing depth in the Earth’s interior. Mathematically, the geothermal gradient is defined as: Change in Temperature/Change in Depth  The below graph shows the average geothermal gradient as the red curve.  Orange regions in the above graph show the temperatures and depths at which the mantle is partially molten.  Yellow regions in the above graph show the temperatures and depths at which the mantle is entirely molten. What is the increase in temperature (in o C) between Earth’s surface and 50 km depth? __________________________

What is the increase in temperature (in o C) between 50 km depth and 100 km depth? __________________________ What is the increase in temperature (in o C) between 100 km depth and 150 km depth? __________________________ Is the geothermal gradient (the average rate of temperature increase with increasing depth) higher closer to the Earth’s surface or deeper within the Earth? ____________________________________________ What is a possible explanation for your answer to the above question? Does your answer to the above question make sense based on what you know about the primary source of heat in Earth’s interior? If the geothermal gradient were constant, what would be the state of the Earth’s mantle? What are the relationships between geothermal gradient and ‘metamorphic grade’? Where do low- grade metamorphic, intermediate-grade metamorphic, and high-grade metamorphic rocks form along the geothermal gradient curve?