Gravity anomalies


Objective: To create a physical situation that changes in time and mimics a geologic situation with different gravity anomalies.


Demonstration: Invite students to a party and place 3 to 4 ice cubes in the graduated cylinder, then add the isopropyl alcohol. The ice cubes will remain at the bottom and represent a material of high density at an interface at depth. There is no surface expression, therefore all anomalies would be positive, i.e. measured, free air, Bouguer and isostatic. During a normal class period the ice will become neutrally buoyant and all anomalies vanish; there is still no surface expression. The buoyant object would still be detectable seismically. Toward the end of the period the ice cubes will float and protrude above the fluid surface. Now the measured values would be negative over the mountains, the free air positive, the Bouguer negative and there would be no isostatic anomaly. This demonstration appears to help the students understand how gravity data can help is learn what is at depth.

Before:

The ice cubes, colored with red food coloring, sink to the bottom of the alcohol in the graduated cylinder. The experiment is shown against a background of a geology textbook page illustrating this concept in the geological realm.

After:

Gradually, the ice cubes rise to the top of the alcohol solution. Here they are seen floating on the top of the solution in the graduated cylinder.