Laboratory Manual for Introductory Geology 4e

EXERCISE 2.3
Putting the Early Evidence Together (continued)
Name:
Course:
Section:
Date:
(c) Focus on the patterns in the middle of the Atlantic Ocean. Only a few volcanoes are shown along the Mid-Atlantic
Ridge, but earthquakes seem to occur all along it. Volcanic activity does, in fact, occur all along the ridge. Why are
only a few volcanoes shown?
(d) Geologists have discovered rocks and fossils native only to Africa in South America, and they have also found rocks
and fossils native to South America in Africa. Considering your answer to question (c), explain what the geologists
proposed had happened.
2.4 Modern Evidence for Plate Tectonics
The geographic fit of continents and paleoclimate evidence convinced some geologists
that plate tectonics was a reasonable hypothesis, but more information was
needed to convince the rest. That evidence came from an improved understanding
of the Earth’s magnetic field, the ability to date ocean-floor rocks, careful examination
of earthquake waves, and direct measurements of plate motion using global
positioning satellites and other exciting new technologies. The full body of evidence
has converted nearly all doubters to ardent supporters.
2.4.1 Evidence for Seafloor Spreading:
Oceanic Magnetic Anomalies
The Earth has a magnetic field that can be thought of as having
“north” and “south” poles like a bar magnet (FIG. 2.7). Navigational
compasses are aligned by magnetic lines of force that emanate
from one magnetic pole and re-enter the Earth at the other
(as shown by the arrows in Fig. 2.7). This magnetic field has been
known for centuries, but two discoveries in the mid-20th century
provided new insights into how the field works and, soon afterward,
the evidence that confirmed plate tectonics theory.
First, geologists learned that when grains of magnetite or
hematite—minerals containing magnetic materials—crystallize,
their magnetic fields align parallel to the lines of force of the
Earth’s magnetic field. Some rocks that contain magnetite or
hematite therefore preserve a weak record of the Earth’s ancient
magnetic field—a record called paleomagnetism. Then, geologists
learned that the Earth’s magnetic field reverses polarity
from time to time, so that what is now the north magnetic
pole becomes the south magnetic pole, and vice versa. During
periods of normal polarity, the field is the same as it is today,
but during periods of reversed polarity, a compass needle that
FIGURE 2.7 The Earth’s magnetic field is defined
by magnetic lines of force, shown in this diagram by
the arrows.
North magnetic pole
South
geographic pole
North geographic pole
Mantle
South
magnetic pole
Lines of
magnetic force
2.4 MODERN EVIDENCE FOR PLATE TECTONICS
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