Geology Reference
In-Depth Information
Another important aspect of the magnetic fi eld is that
the magnetic poles, where the lines of force leave and en-
ter Earth, do not coincide with the geographic (rotational)
poles. Currently, an 11.5˚ angle exists between the two
(Figure 2.8). Studies of the Earth's magnetic fi eld show that
the locations of the magnetic poles vary slightly over time,
but that they still correspond closely, on average, with the
locations of the geographic poles.
Experts on magnetism do not fully understand all as-
pects of Earth's magnetic fi eld, but most agree that electrical
currents resulting from convection in the liquid outer core
generate it. Furthermore, it must be generated continuously
or it would decay and Earth would have no magnetic fi eld
in as little as 20,000 years. The model most widely accepted
now is that thermal and compositional convection within
the liquid outer core, coupled with Earth's rotation, produce
complex electrical currents or a
self-exciting dynamo
that, in
turn, generates the magnetic fi eld.
N
Cretaceous
Triassic
Permian
Devonian
Silurian
Cambrian
Path of
North American
paleomagnetic
pole
Path of
European
paleomagnetic
pole
POLAR WANDERING
Interest in continental drift revived during the 1950s as a re-
sult of evidence from paleomagnetic studies, a relatively new
discipline at the time.
Paleomagnetism
is the remanent mag-
netism in ancient rocks recording the direction and intensity
of Earth's magnetic poles at the time of the rock's formation.
When magma cools, the magnetic iron-bearing miner-
als align themselves with Earth's magnetic fi eld, recording both
its direction and strength. The temperature at which iron-bearing
minerals gain their magnetization is called the
Curie point
. As
long as the rock is not subsequently heated above the Curie point,
it will preserve that remanent magnetism. Thus, an ancient lava
fl ow provides a record of the orientation and strength of Earth's
magnetic fi eld at the time the lava fl ow cooled.
As paleomagnetic research progressed during the 1950s,
some unexpected results emerged. When geologists measured
the paleomagnetism of geologically recent rocks, they found
that it was generally consistent with Earth's current magnetic
fi eld. The paleomagnetism of ancient rocks, though, showed
different orientations. For example, paleomagnetic studies of
Silurian lava fl ows in North America indicated that the north
magnetic pole was located in the western Pacific Ocean at
that time, whereas the paleomagnetic evidence from Perm-
ian lava fl ows pointed to yet another location in Asia. When
plotted on a map, the paleomagnetic readings of numerous
lava fl ows from all ages in North America trace the appar-
ent movement of the magnetic pole (called
polar wandering
)
through time (
S
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Figure 2.9
Polar Wandering The apparent paths of polar
wandering for North America and Europe. The apparent location
of the north magnetic pole is shown for different periods on each
continent's polar wandering path. If the continents have not moved
through time, and because Earth has only one magnetic north pole,
the paleomagnetic readings for the same time in the past, taken on
different continents, should all point to the same location. However,
the north magnetic pole has different locations for the same time in
the past when measured on different continents, indicating multiple
north magnetic poles. The logical explanation for this dilemma is
that the magnetic north pole has remained at the same approximate
geographic location during the past, and the continents have moved.
from all continents indicated that each continent seemingly
had its own series of magnetic poles. Does this really mean
there were different north magnetic poles for each continent?
That would be highly unlikely and diffi cult to reconcile with
the theory accounting for Earth's magnetic fi eld.
The best explanation for such data is that the mag-
netic poles have remained near their present locations at the
geographic north and south poles and the continents have
moved. When the continental margins are fi t together so that
the paleomagnetic data point to only one magnetic pole, we
fi nd, just as Wegener did, that the rock sequences and glacial
deposits match, and that the fossil evidence is consistent with
the reconstructed paleogeography.
SEAFLOOR SPREADING
Geologists refer to Earth's present magnetic fi eld as being
normal—that is, with the north and south magnetic poles
located approximately at the north and south geographic
poles. At various times in the geologic past, however, Earth's
magnetic fi eld has completely reversed, that is, the magnetic
north and south poles reverse positions, so that the magnetic
north pole becomes the magnetic south pole, and the mag-
netic south pole becomes the magnetic north pole. During
Figure 2.9).
This paleomagnetic evidence from a single continent
could be interpreted in three ways: The continent remained
fi xed and the north magnetic pole moved; the north mag-
netic pole stood still and the continent moved; or both the
continent and the north magnetic pole moved.
Upon analysis, magnetic minerals from European Silu-
rian and Permian lava flows pointed to a different mag-
netic pole location from those of the same age from North
America (Figure 2.9). Furthermore, analysis of lava flows
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