Download Lab- Magnetics and Seafloor Spreading

Lab-
Magnetics and Seafloor Spreading
Name __________________________
Figure 1 shows the three basic types of plate
boundaries. Warm mantle material
upwells at spreading centers, also known as midocean ridges, and then cools. Because the
strength of rock decreases with temperature the
cooling material forms strong plates of new
oceanic lithosphere. The cooling oceanic
lithosphere moves away from the ridges, and
eventually reaches subduction zones or trenches.
We track these motions because the cooling rock
at midocean ridges is magnetized by the earth’s
magnetic field, that reverses from time to time.
The ocean crust thus acts as a tape recorder.
Knowing the history of the earth’s field,
magnetic anomaly patters have been dated and
given numbers.
The magnetic anomaly time scale.
1. Trace the magnetic profile from the central anomaly to
anomaly 5 on the east half of the Eltanin-19 profile. Use
the tracing to identify the corresponding
anomalies on the west flank.
2. Use magnetics and topography to identify anomalies on
the Conrad-12 profile. (Since the two profiles are near
each other, peaks should look similar: This would not be
true if they were from different areas).
3. If the central anomaly (1) has age zero, plot ageversus-distance for the anomalies on EL-19 using the time
scale provided. Determine spreading rates on the east and
west flanks. Do the same for the Conrad-12 profile.
4. Plot the seafloor depth versus the square root of age
for EL-19 and Conrad-12. How well does a straight line fit
these data?
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5. Now that you are proficient with magnetic anomaly data,
try something a bit more difficult. First use the North
Pacific data in the age versus distance plot to find the
spreading rate as a function of time. Do the same for the
Pacific-Antarctic ridge. Are the two the same? Next
identify anomalies in the South Atlantic by comparison, and
find the spreading rate there from an age versus distance
plot. You can see why computer modeling programs are so
useful! Compare your answer to one you get using the Deep
Sea Drilling Project results (age determined from
microfossils) which are also from the South Atlantic.
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6. Cut out the southern continents and reconstruct their
fit 200 million years ago. (The straight lines are
irrelevant boundaries). Try it with and without Madagascar;
notice that the Antarctic peninsula causes some problems
(it is often rotated for better fits). You can see that
paleomagnetic and geologic data would help quite a bit.
What is the mystery block and where does it fit? You can
see some of the problems involved in reconstructions.
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.