Q
Bunsen burner, cigarette lighter, or other direct heat source
Q
One 3 x 5 file card and sheets of blank paper
Q
Twenty 9 can (3-4 in.) lengths of audiocassette tape (Use an old tape.)
Q
A roll of clear tape
Q
Scissors and a ruler
Q
Pens or pencils
Q
Maps of South America and Africa (
optional
, but desirable)
Q
Map of the Pacific Ocean floor (
optional
, but desirable)
P ROC E DUR E
Teacher Preparation
Practice the magnetizing demonstration before class. Prepare the nail
by stroking it with a magnet until it will pick up paper clips or staples
on one end. Place the nail in a clamp and heat it in the middle until the
clips drop off.
Read Master 2.2a, Tectonics Background, and decide how much time
you will need to spend presenting this information to your students. If
they are already familiar with its outlines, you may just want to
connect the illustrations with the stick-slip movement students saw in
the previous activity.
TEACHING CLUES AND CUES
You may want to take
students through a quick
review of magnetic
properties and the rule
of
PART ONE
P
OLES AT
P
LAY
A. Introduction
Pass out copies of Master 2.2a, Tectonics Background. Project Master
2.2b, Earth Cross Section, and review the first two major sections of
Master 2.2a with the students. Point out to students that on a world
map continental land masses seem to have a jigsaw fit. Early
mapmakers noted this long ago when drawing the first maps of the
new world.
attraction and repulsion of poles.
TEACHING CLUES AND CUES
Be sure students under-
stand that it is the mag-
netic poles that have
changed, not the
B. Procedure
1. Ask the students how magnets work. Some may already know that
alignment of atomic forces creates the magnetic “pull,” or force field.
Tell students that when a magnet is heated it loses its magnetic force,
and when it cools down it again picks up the faint trace of the Earth’s
magnetic field. Demonstrate this with a nail by stroking it with a
magnet and then heating it, as explained above. Ask students where
rocks are heated (in volcanoes and deep within the Earth). Tell them
that in the next two activities they are going to simulate the process of
using paleomagnetism to study plate tectonics.
2. Tell students that in 1963, two English geophysicists, Vine and
Matthews, were using extremely sensitive magnetometers to make
measurements of the seafloor across the Mid-Atlantic Ridge. Scientists
already knew that over the 4.6 billion years of Earth’s history, its
magnetic poles have changed directions more than once, so that the
south pole and the north pole have actually switched
physical ends of the Earth.
A G U
/
F E M A
47
S
E I S M I C
S
L E U T H S
