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Purpose: Demonstration of the Core Shadow Zone
Supplies: Large lens, clear (or translucent) hemisphere, cardboard circle
Background and Demonstration:
We know that the Earth has a low-velocity core because seismic P waves are focused into the core, leaving a shadow zone on the surface from angular distances of 104 degrees to 140 degrees. We can draw raypaths on a 2-d cut-away section of the Earth, using Snell's Law to define the bending of rays toward vertical as they encounter the lower-velocity core. However, it is difficult for students to appreciate that our observations are limited to the surface, and that the shadow zone is a doughnut-shaped region on the surface of the Earth.
If you have a clear or translucent half-sphere available (ours is a broken clear-plastic globe used in Structural Geology class), you can illustrate the refracting properties of the core and how it generates the shadow zone on the surface. Hold the hemisphere in front of a light source (like an overhead projector or slide projector) so that the light shines into the inside of the hemisphere. [If the hemisphere is clear, sprinkle some chalk dust on the inside so that the light can be seen shining on the hemisphere.] Now hold a large lens (like a magnifying glass) between the light source and the hemisphere (at approximately the center of the sphere). The lens focuses the light so that a relatively dark band appears on the surface of the hemisphere. Inside of this there should be a bright circle of light where the lens has focused the light. If need be, move the lens forward or backward so that the appropriate effect is achieved.
We know that the Outer Core of the Earth is liquid because, in addition to the P-wave shadow zone demonstrated above, there is a shadow zone for S waves which is total for angular distances greater than 104 degrees. Once you have demonstrated the focusing of P waves, you can illustrate the blockage of S waves by the core by inserting a cardboard circle between the source of light and the hemisphere. The cardboard circle casts a shadow with no bright circle of light at its center (obviously). Thus, since the core focuses P waves, the P wave velocity in the Outer Core is lower than that of the overlying Mantle. But, since S waves are blocked, the shear-wave velocity of the Outer Core must be zero, indicating zero rigidity, or a fluid.
Jeffrey S. Barker (SUNY Binghamton) Demonstrations of Geophysical Principles Applicable to the Properties and Processes of the Earth's Interior, NE Section GSA Meeting, Binghamton, NY, March 28-30, 1994.
Questions or comments: jbarker@binghamton.edu
Last modified: March 18, 1996 (content), June 6, 2021 (reformatted and moved to Google sites)
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