# C4L2 Enrich Magnetic Reversals

```Name
Date
Class
The Theory of Plate Tectonics
Use the graph to answer the questions that follow on a separate sheet of paper.
Magnetic Reversals Through the Ages
How often does Earth’s magnetic field reverse itself? The graph below shows the record geologists have
put together for the last 65 million years. As you might know, the last of the dinosaurs died about 65
million years ago. So you can think of this graph as the record of earth’s reversals since the dinosaurs
became extinct.
In this graph, each dark band represents a “normal” magnetic field, as it is today. Each light band
represents a reversed magnetic field.
1. Was Earth’s magnetic field “normal” or reversed 65 million years ago? __________________________
2. About how long ago was the last time Earth’s magnetic field reversed? __________________________
3. Can you see any pattern in how often Earth’s magnetic field reverses?
______________________________________________________________________________________
4. How would this history of reversals show itself on the ocean floor?
______________________________________________________________________________________
5. From this graph, when would you predict the next reversal would occur? Give reasons for your answer.
______________________________________________________________________________________
Name
Date
Class
The Theory of Plate Tectonics
If the statement is true, write true. If the statement is false, change the underlined word
or words to make the statement true.
1. _____________ Earth’s plates meet at boundaries.
2. _____________ Breaks in the crust called faults form where plates meet.
3. _____________ Plates slide past each other at convergent boundaries.
4. _____________ A(n) rift valley forms where plates diverge on land.
5. _____________ Ocean currents drive Earth’s plates.
6. _____________ Most transform boundaries where plates move apart occur
along the mid-ocean ridges.
Write the letter of the correct answer on the line at the left.
7. ___ Which theory describes the motion of and
force driving Earth’s plates?
A first law of motion
B third law of motion
C continental drift
D plate tectonics
9. ___ The San Andreas fault is an example
of which kind of boundary?
A transform
B divergent
C oceanic
D convergent
8. ___ What is the motion of Earth’s plates
like?
A fast and in stages
B slow and in stages
C fast and constant
D slow and constant
10. ___ Which feature forms at convergent
boundaries?
A mountain range
B trench
C rift valley
D mid-ocean ridge
The Theory of Plate Tectonics
9. rift valley
10. plate tectonics
1. The top surfaces o f the clay blocks were flat
and smooth. After the clay blocks collided,
the top surfaces of the clay blocks were
pushed up, some had broken, and some
2. The edges of the clay blocks represent the
plate boundaries or edges of Earth’s plates.
3. The Earth’s plates move very slowly, so we
cannot detect the movement.
4. Accept all reasonable responses. Students may
say mountain ranges, valleys, or volcanoes.
11. transform boundary
12. divergent boundary
1. reversed
2. slightly less than 1 million years ago
3. Answers will vary. Some students may correctly
say that there is no discernible pattern.
4. The history of reversals would show itself in
the form of magnetic strips on both sides of
the mid-ocean ridge.
Plate movements produce changes in Earth’s
surface and on the ocean floor that include the
occurrence of earthquakes and the formation
of volcanoes, mountain ranges, rift valleys, and
deep-ocean trenches. The movement of Earth’s
plates has also greatly changed the location of
Earth’s continents, landmasses, and oceans.
suggest that the next reversal may be soon,
since the magnetic field often reverses in less
than a million years.
1. true
2. true
4. true
1. divergent boundary
3. transform
boundaries
2. convergent boundary
5. Convection
6. divergent
boundaries
7. D
8. D
9. A
10. A
3. transform boundary
4. a. The denser plate subducts below the
other at a deep-ocean trench. b. The collision
squeezes the crust into mountain ranges.
c. The denser oceanic plate subducts below
the continental plate
5. The force of convection currents in the mantle
caused the movements of the plates that carry
the continents.
6. convergent boundary
7. faults
8. plates
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