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Name________________________
Drifting Continents and Magnetic Fields
Adapted from Southeast Missouri State University
Part A- Magnetic Dip
Materials: protractor, ruler and pencil
Background: The earth is surrounded by a magnetic field which is strongest near the
north and south magnetic poles. At the equator, a magnetic compass “points” north in a
direction which is horizontal, or parallel to the surface of the earth. However, at the north
magnetic pole a compass would dip straight down vertically. Most compasses are not
able to move vertically, so a dip needle can be used to illustrate the vertical movement.
The angle formed by the dip needle and the horizon is called the magnetic dip.
When basalt cools, its iron minerals form magnetized to the magnetic field of the
earth. These rocks will have their own weak magnetic field, which can be detected and
recorded in the field where the rocks are in their original position before being collected.
This will provide evidence of the Earth’s magnetic field at the time of the rock formation.
Worksheet 1 part A
1. Label the north and south poles on Worksheet A
2. Notice that horizon line has been drawn at location C. A horizon line is a straight
line touching the circle but not intersecting it (a tangent line). The most precise
way to draw a tangent or horizon line at a point is to first draw a radius to that
point and then draw the tangent at right angles (perpendicular) to the radius.
Draw horizon lines for points A, B, E and D on Worksheet 1 (part A)
3. The dip angle for C is shown as an arrow pointing into the earth at the same angle
as the magnetic field at that point. The dip angle is measured as the angle between
the dipping arrow and the horizon line. Note that the dip angle will never be
higher then 90o. What is the dip at C? __________ Is the dip up or
down?____________
4. Does that dip mean that it is from the northern hemisphere or the southern
hemisphere?________________
5. Draw and measure the dip angles of the other locations
A_________________ B__________________
C_________________D_________________ E__________________
6. Draw the arrows showing the dips on the rock samples
7. Which dip was the least?_________
8. What is the relationship between the dip and the distance from the pole?
9. What is true of the dip of the magnetic field in the southern hemisphere?
Part 2 Rocks from India
What could cause the magnetic dip of the rocks found in India to change during the last
225 million years? There are three possibilities
a. the north magnetic pole has moved
b. India has moved
c. Both the north magnetic pole and India have moved
Evidence from other continents show that the magnetic poles have always remained
relatively close to earth’s geographic poles despite a small amount of polar wandering.
Therefore, explanation b is the only acceptable explanation. Since the magnetic and
geographic poles are close together, you can assume for this activity that they are indeed
the same. Therefor, the angle of the magnetic dip can give you latitude and distance from
a geographic pole if you use this graph (figure 2)