Laboratory Manual for Introductory Geology 4e

strength, resulting in an observed magnetic field stronger than today’s field (a positive
anomaly). If the paleomagnetic polarity is reversed, the rock’s paleomagnetism
subtracts from the modern field strength, and the result is a measurement weaker
than today’s field (a negative anomaly) (FIG. 2.10).
FIGURE 2.10 Components of the Earth’s magnetic field strength that produce marine
magnetic anomaly stripes.
Earth’s current
magnetic field
(strength and
direction)
Earth
Paleomagnetism Anomalously Earth’s current Paleomagnetism
+
(strength and
high magnetic field (reversed
= +
direction) magnetic field
polarity)
strength
=
Seafloor
Positive
anomaly
Earth
Seafloor
Anomalously
low
magnetic field
strength
Negative
anomaly
+
=
+ =
This pattern of marine magnetic anomalies has now been found at every midocean
ridge, proving that the magnetic reversals are truly worldwide events. Geologists
determined that as lava erupting at mid-ocean ridges cools, it records the
magnetic field polarity in effect at that time. If the Earth’s polarity reverses, new lava
will adopt the new polarity, and the older lava will yield a negative magnetic anomaly,
as shown in Figure 2.9. Exercise 2.4 shows the reasoning by which geologists connected
marine magnetic anomaly stripes in the oceans with plate tectonics theory.
EXERCISE 2.4
Interpreting Magnetic Anomaly Stripes at Mid-Ocean Ridges
Name:
Course:
Section:
Date:
In Figure 2.9a, compare the orientation of the magnetic anomaly stripes near the Mid-Atlantic Ridge with the orientation
of the ridge crest (highlighted by the red line). In Figure 2.9b, do the same for the Juan de Fuca Ridge (shown in yellow)
and its associated magnetic anomaly stripes.
(a) Are the individual anomalies oriented randomly? Are they parallel to the ridge crests? Oblique (at an angle) to the
ridge crests?
(b) Explain how the process of seafloor spreading can produce these orientations and relationships.
(c) Some magnetic anomaly stripes are wider than others. Knowing what you do about seafloor spreading and magnetic
reversals, suggest an explanation.
2.4 MODERN EVIDENCE FOR PLATE TECTONICS
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