Download Ocean Floor Magnetism Mini-Lab

Name: _______________________________
Date:______________
Block: ______
Ocean Floor Magnetism Mini-Lab
Background: Important evidence that supports the theory of plate tectonics comes from studies
of the magnetic properties and the ages of igneous rocks on the ocean floor. Some igneous rocks
contains minerals that are magnetic. These minerals provide a record of the direction of Earth’s
magnetic field at the time when the lava that formed the rock cooled. At certain times in the
past, the present magnetic north pole became the magnetic south pole. When the magnetic pole
is in the north, is it called normal polarity and when the magnetic pole is in the south, it is called
reversed polarity. A number of magnetic reversals have taken place at different times over a
period of millions of years. Currently we are “due” for a magnetic reversal. A mid-ocean ridge
is a long chain of volcanic mountains on the ocean floor with a deep central valley where magma
moves up and cools on the surface of earth. It was discovered that the magnetic reversals are
recorded in the bands of rock on either side of a mid-ocean ridge.
Pre-Lab Question: What is sea-floor spreading?
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Materials:
Shoebox lid
Sheet of white paper
Scissors
2 different colored pencils
Ruler
Procedure:
1. Cut the paper in half length-wise, making two long strips.
2. Insert each strip of paper into the slit of the shoebox, leaving about 3 centimeters of strip
showing on the top of the box. Fold the 3 centimeters of paper back so it is flat against
the box lid.
3. Color the part of the strip that is showing.
4. Pull out each strip another 4 centimeters. Color this new area a different color.
5. Pull out each strip 2 centimeters. Color this new area with this the first color.
6. Repeat steps until the paper reaches the end of the lid and fold it under lid, alternating
between the two colors.
Analysis:
1. What landform does the slit in the shoebox represent?
2. What does the end of the lids represent?
3. What do the stripes represent?
4. Where is the youngest rock located on a mid-ocean ridge? Where is the oldest rock?
5. What would you expect to find if you measured the temperatures of rocks on the ocean
floor at various distances from a mid-ocean ridge? Explain your reasoning.
6. The Earth is about 4.6 billion years old. Based on observations of your sea-floor
spreading model, why do you think that the oldest ocean floor is only about 200 million
years old?
7. On the real ocean floor, alternating stripes of normal and reversed polarity are not all of
equal width. What does this tell you about the lengths of time represented by normal and
reversed polarity?
8. How might you make this model more realistic?