places. When a volcano erupts, the volcanic rocks record the direction
of the poles at that time. The rocks are magnetized in somewhat the
same way as the nail was by the magnet in your demonstration. What
Vine and Matthews discovered was a pattern of polar reversal stripes
on both sides of the ridge. The purpose of this activity is to simulate
those patterns.
3. For each group of two or three students, pass out 20 audiotape
strips, one 3 by 5 card, glue or tape, and a magnet.
4. Give these instructions:
a. Tape 6 or 7 strips of audiotape to the card by one end, so they are
parallel to each other and about 1 cm apart. One end will be loose, as
shown on the illustration.
b. Stroke one of the strips with one pole of the magnet. Then stroke
the next strip with the other pole of the magnet.
When the magnet is passed over the loose, ends of the tape, the strips
will move. Their direction will depend on which end of the magnet
they were stroked with.
TEACHING CLUES AND CUES
This activity works best if
the card is held upside
down with the tape
hanging down.
c. Create patterns of magnetism by mixing up the order in which you
stroke the strips of tape, then give the cards to other groups to
interpret.
C. Conclusion
After they have had a chance to create patterns and share them with
other groups, ask students if they were able to interpret the other
groups’ patterns. How did the tape strips record the direction of the
magnetism? (Magnetic particles are embedded in the tape.)
This activity was adapted from a workshop on earth science activities
presented at the Exploratorium in San Francisco, CA.
Be sure all strokes on
the individual strips are
in the some direction. Do
not touch again when
testing polarity.
PART TWO
R
ECORDS IN THE
R
OCK
A. Introduction
Review the structure of the Earth’s interior with the class, drawing on
Master 2.2a. Remind students that in the last activity they saw how
patterns of magnetic polarity can be recorded. What Vine and
Matthews saw, as they explored the Mid-Atlantic Ridge with their
instruments, was a similar pattern recorded in volcanic rocks. Looking
at these patterns on one side of the ridge, they noticed that the opposite
side showed a mirror image of reversals. Their next step was to collect
rock samples from the sea floor and determine the age of the rocks, to
find out when the volcanoes that formed the rocks erupted. They
discovered not only that the patterns of ages were mirrored on the two
sides, but also that the rocks on both sides were progressively older as
they moved away from the ridge. The next activity will help students
understand the process Vine and Matthews followed.
A G U
/
F E M A
48
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E I S M I C
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