Global topography illustrates the geometry describing the motion of the great plates of Earth's lithosphere in many ways. Most obvious are features showing "relative" plate motions between neighboring plates. At the long midocean ridges, hot material upwells from the deeper mantle and cools to form plates which then diverge. Subduction takes place at deep-sea trenches, where one plate dives under another and reenters the mantle. From the fit of continents (split when new ocean basins formed), we can determine past plate motions. However, while these features tell us much about how the plates have moved with respect to each other, they reveal little of how the plates have moved with respect to the underlying mantle.
To perform this demonstration in class, you will need:
Before the demonstration:
Photocopy (or draw with the pen) the two patterns onto the transparencies. Cut the Plate transparency along the thick line. Tape the Mantle overhead onto the projector stage. Double check that the overhead projector in the room is working.
1. Place the two plate pieces on top of the mantle. Point out the features associated with each part of the plate (ridge, transform fault, etc.).
2. Show that the two plates can move in several different ways and still create the same relative positions by holding Farallon fixed and moving Kula, then repeating the motion with Kula fixed and moving Farallon. (NOTE: Especially dextrous teachers may be able to move both Farallon and Kula relative to the Mantle while spreading.)
3. Have a student hold the pen (representing a plume) fixed at one point over the mantle. Move the two plates, and show the line (representing the seamount chain forming the hot spot track) that is created by the motion. Repeat with several different positions of the pen and senses of relative motion.
Seth Stein (Edited, John DeLaughter, October, 1997)
This demonstration was adapted from Seth Stein's article "Hotspotting in the Pacific", which appeared in Nature, 387, 345-346, 1997
This page designed by John DeLaughter email@example.com Update: Oct 17 1997