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F E M A
50
S
E I S M I C
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L E U T H S
TEACHING CLUES AND CUES
To demonstrate paleo-
magnetic alignment,
make large blowups of
the South American and
PART THREE
M
APPING
P
REHISTORY
A. Introduction
Tell students that in the last activity they modeled the creation of new
ocean floor at a spreading mid-ocean ridge. Project the transparency of
Master 2.2b and review the section of Master 2.2a entitled Three
Kinds of Plate Movement, Four Kinds of Boundaries, and discuss.
Then discuss the driving mechanism for plate tectonics, so students
understand some of the complicated forces and that scientists are still
exploring how these forces work. Tell the class that in this last activity
they will look at one more line of evidence that the continents
African continents, or draw their
outlines on the overhead. Demon-
strate the process of alignment and
distance measurements.
Be sure to pass out two
copies of Master 2.2d to
each group.
have been moving. Review the process of magnetism and how rocks
pick up the polar orientation during their formation. Explain that
sensitive instruments called magnetometers can analyze samples of
rock to discover where the north pole was located during their
formation. In this next activity, students will use magnetic data
collected from several different locations to establish plate motions.
Make sure that the grid
north line remains
parallel to the longitude
lines while you complete
step 2.b.
B. Procedure
1. To each small group, pass out scissors, tape, a ruler, two copies of
Master 2.2d, Continental Pieces, and one each of Master 2.2e, World
Map Grid, and Master 2.2f, World Map Grid, 120 MYA. Point out
that there are two north arrows on each of the continental pieces. The
N stands for north today and the PN stands for paleonorth, or the
approximate location of the north pole 120 million years ago (MYA).
The line that lies across the arrow, labeled G, represents grid north,
and should always line up so it is parallel to longitude (see example on
Master 2.2e, World Map Grid.) The numbers in the tables on the grid
masters tell the distance to the north pole from the tip of the arrow
point at the time that sample was collected.
2. Give these instructions:
a. Cut out the continents on one copy of Master 2.2d along the dotted
continental outlines.
b. To place the continents on Master 2.2e, World Map Grid, begin
with any continent. Place the ruler alongside the present day north
arrow line (N) on the continent to align it with the intersection of the
Prime Meridian (O) and the north edge of the map. Next, align the
grid north line (G) so it is parallel to the nearest longitude. When both
arrows are aligned, measure the distance from the pole to the point
where the two arrows cross. Tape the continent in place.
c. Repeat the process until all the continents are in place. This creates
a map of the world as it appears today.
d. Now repeat the process with a new copy of Master 2.2d,
Continental Pieces, and a copy of Master 2.2f, World Map Grid l20
MYA, this time lining up the PN arrows. This creates a map of the
world as it may have appeared 120 million years ago.
