Travel Reference
In-Depth Information
The most common of these magnetic minerals, magnetite, is a rod-shaped
crystal with a positive and a negative pole, just like any other magnet. Some
of these mineral grains are microscopic in size, and if they exist in a medium
where they can move freely (such as in watet or even in unsolidified lava),
they act as tiny compass needles. Their positive pole will point toward the
negative pole of the earth's great magnet. If present in sufficient quantity,
they endow their enclosing mother rock with a magnetic signal.
Magnetic particles yield useful information about ancient magnetic
fields in volcanic and in some sedimentary rocks. Both of these rock types
preserve an actual record of the earth's magnetic field direction in much the
same way. When hot magma cools and solidifies, the magnetite crystals
found within the cooling lava become aligned to the present field direction.
A similat thing happens when sediments lithify from a wet slurry to solid
rock and, in the process, lock in place the tiny magnetic minerals, all
aligned in one direction hy the earth's magnetic field at the time of the
rock's fotmation.
These two types of rock now contain weak—but measurable—magnetic
signals. // the exact orientation of the rock is known, then a piece of the rock,
carefully removed and taken to the lab, can yield not only its magnetic in-
tensities but also the actual direction of the earth's magnetic field when the
rock was formed. Thus one can learn whether the rock in question was lithi-
fied during a period when the north pole of the earth coincided with the pos-
itive pole of the earth's magnet, or vice versa.
The pioneering geophysicists, when measuring closely spaced samples
from piles of lava, found that there was a random pattern of reversals. Some-
times thick piles of lava recorded numerous reversals; and sometimes an
equal thickness—and, perhaps, an equal time period—recorded no reversals
at all. The reversal themselves had no identity. Like a computer, they simply
record binary data. But just as a simple plus-and-minus computet code can
teveal, retain, or record great quantifies of infotmation when enough data
are accumulated, so too can our rich record of magnetic reversals tell us
much about time.
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