Download Magnetic Reversal LAB 2017

Name _____________________________________________________
Lab Group Members _________________________________________________________________________
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Date ______________________
Lab # ________________
The Theory of Plate Tectonics
and Magnetic Reversals
Magnetic Reversals Locked into the Seafloor
INTRODUCTION:
You can’t see it, but there’s an invisible force field around the
Earth. Okay, not a force field, exactly, but a gigantic magnetic field
surrounding the Earth, and it acts like a force field, protecting the planet –
and all the life – from space radiation.
The Earth is like a great big magnet. The north pole of the magnet is near
the top of the planet, near the geographic north pole, and the south pole is
near the geographic south pole. Magnetic field lines extend from these
poles for tens of thousands of kilometers into space; this is the Earth’s
magneto sphere.
The geographic poles and the magnetic poles are far enough apart that scientists distinguish them differently.
If you could draw a line between the magnetic north and south poles, you would get a magnetic axis that’s
tilted 11.3 degrees away from the Earth’s axis of rotation. And these magnetic poles are known to move
around the surface, wandering as much as 15 km every year.
Scientists think that the Earth’s magnetic field is generated by electrical currents flowing in the liquid outer
core deep inside the Earth. Although it’s liquid metal, it moves around through a process called convection.
And the movements of metal in the core sets up the currents and magnetic field.
How do we know this? Navy scientists, using magnetic instruments (magnetometers) began recognizing
odd magnetic variations across the ocean floor. This finding, though unexpected, was not entirely
surprising
because it was
known
that basalt the
rock that makes
up the ocean
floor contains
iron-rich
magnetite. The
presence of
magnetite gives
the basalt
measurable magnetic properties. Scientists use magnetometers to detect very small changes in the earth's
magnetic field.
In 1962 the US Navy published their surveys of the oceans, and areas on either side of the mid-ocean
ridges showing patterns of magnetic changes that were repeated identically (mirrored) on either side of the
spreading ridge. Two British geologists, Frederick Vine and Drummond Matthews, read the information
and hypothesized that the magnetic striping was produced by repeated reversals of the Earth's magnetic
field, Vine and Matthews suggested that the earth's magnetic field periodically reversed its direction (North
became South, south became north) over many thousands of years.
Careful measurements showed that the patterns of North and South alignment matched exactly on
either side of the spreading ridge as seen on the above. Vine and Matthews deduced that new crust forming
along the divergent plate boundaries recorded the magnetic orientation of Earths poles at the time of
solidification.
Pre-lab:
1. What is a magnetic field and why does Earth have one? ____________________________________
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2. How does the Earth create a record of the orientation of its magnetic fields” __________________
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3. What two scientists developed the theory that Earth’s magnetic field reverses itself every few
thousand years?
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Take it further:
Why do you think that the Navy would have been interested in finding large metal objects in the oceans?
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INSTRUCTIONS: Check off each box in order as you work!
 Please plot the data below on the graph by using placing a N or S in the appropriate
places using the AGE and LONGITUDE data.
 Look at the magnetic reversals, do you see a pattern?
Circle one Yes / No
 Starting at the middle of your diagram and working out to the right and then left
side, connect areas with N polarity using a line of one color and the areas with S
polarity using a line of a different color. Lightly shade the area inside the lines.
 Be sure to include a key below showing which color represents N and S polarity.
 At the bottom of the graph, label where South America coast, the Mid Atlantic Ridge
and the African coast would be.
Graph of the Magnetic Reversals of the Sea-floor over time
_____________________coast
_____________________Ridge
Be sure to check off that you have completed:




Labeling
Coloring each magnetic reversal
Creating a color key
Labeling South America coast, the Mid Atlantic Ridge and
the African coast.
____________coast
KEY
Color of magnetic polarity
 Magnetic field points to
North Pole
 Magnetic Field points to
South Pole
ANALYSIS of DATA
1. State the relationship between age of oceanic crust and distance from divergent plate boundary:
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3. What is magnetic polarity? How does the oceanic crust record the polarity?
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4. What type of rock is oceanic crust generally made of? ________________________________________
5. What type of rock is continental crust generally made of? _____________________________________
6. Which type of crust is thicker? ________________________________________________________________
7. Which type of crust is denser? ___________________________________________________________
8. As you travel from a spreading center towards a coastline, what happens to the thickness of sediment on
the sea-floor? Circle one: Sediments become
Thicker /
Thinner
Explain why.
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9. How does the observed change in sea-floor age support the theory of sea-floor spreading?
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10. How does the observed pattern of magnetic reversals support the theory of sea-floor spreading?
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11. If there is constantly new sea-floor being created along mid-ocean ridges, how come the planet is not
increasing in size?
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Questions 12 and 13 on back