Understand the limitations of this method
Plumage
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.
For this information, we must look to another type of feature
entirely: linear chains of seamounts, such as the spectacular one
extending from the Hawaiian
islands northwestward across the Pacific to the Emperor Seamounts.
These chains are thought to form over "hot spots"; places where plumes
of hot material upwell from deep in the mantle. As a plate moves over
the hot spot, new volcanic seamounts form (sometimes reaching the sea
surface as islands), and are then carried away from the hot spot by the
plate's motion. If the plumes do not move with respect to the mantle,
then the direction of absolute plate motion can be found from the trend
of the resulting seamount chain, and the rate of motion can be found
from the progression of volcanic ages.
This demonstrates that the geometry of plate motions, a classic tool of
post-1960 geology, not only remains powerful but can yield new
insights, using only the simplest of tools (paper, pencils, compass and
a straightedge). Geology departments might emulate Plato's Academy and
inscribe over the door "let no one ignorant of geometry enter."
To perform this demonstration in class, you will need:
A printout of the two patterns shown below (or draw your own)
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)
