Geology Reference
In-Depth Information
assume that the continents must once have been connected.
Recent discoveries of dinosaur fossils in the Gondwana
continents further solidifies the argument that these land-
masses were in close proximity during the Early Mesozoic Era.
Notwithstanding all of the empirical evidence presented by
Wegener and later by du Toit and others, most geologists simply
refused to entertain the idea that continents might have moved
in the past. The geologists were not necessarily being obstinate
about accepting new ideas; rather, they found the evidence for
continental drift inadequate and unconvincing. In part, this was
because no one could provide a suitable mechanism to explain
how continents could move over Earth's surface.
Interest in continental drift waned until new evidence
from oceanographic research and studies of Earth's magnetic
fi eld showed that the present-day ocean basins were not as
old as the continents, but were geologically young features
that resulted from the breakup of Pangaea.
Image not available due to copyright restrictions
Earth can be thought of as a giant dipole magnet in
which the magnetic poles essentially coincide with the geo-
graphic poles (
What is magnetism and what is a magnetic fi eld?
Magnetism
is
a physical phenomenon resulting from the spin of electrons in
some solids—particularly those of iron—and moving electric-
ity. A
magnetic fi eld
is an area in which magnetic substances
such as iron are affected by lines of magnetic force emanating
from a magnet (
Figure 2.8). This arrangement means that
the strength of the magnetic fi eld is not constant, but varies.
Notice in Figure 2.8 that the lines of magnetic force around
Earth parallel its surface only near the equator. As the lines
of force approach the poles, they are oriented at increasingly
larger angles with respect to the surface, and the strength of
the magnetic fi eld increases; it is strongest at the poles and
weakest at the equator.
◗
Figure 2.7). The magnetic field shown in
Figure 2.7 is
dipolar,
meaning that it possesses two unlike mag-
netic poles referred to as the north and south poles.
◗
◗
Figure 2.8
Earth's Magnetic Field
Geographic
north pole
Geographic north pole
Magnetic north pole
Lines of
magnetic force
Magnetic
north pole
Magnetic
equator
Geographic
equator
Magnetic
equator
Geographic
equator
Dip needle
Earth's magnetic fi eld has lines of force like those of a bar
magnet.
The strength of the magnetic fi eld changes from the magnetic
equator to the magnetic poles. This change in strength causes a dip
needle (a magnetic needle that is balanced on the tip of a support
so that it can freely move vertically) to be parallel to Earth's surface
only at the magnetic equator, where the strength of the magnetic
north and south poles are equally balanced. Its inclination or dip
with respect to Earth's surface increases as it moves toward the
magnetic poles, until it is at 90 degrees or perpendicular to Earth's
surface at the magnetic poles.
a
b
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