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Paleolithic See technology.
paleomagnetism Paleomagnetism refers to the ancient
records of the Earth’s magnetic field (paleo- means ancient).
The magnetic field of the Earth is believed to result from the
circulation of liquid metal in the inner Earth. Volcanic rocks
on the Earth’s surface contain iron minerals which, like compass
needles, orient themselves in line with the magnetic field
of the Earth while the lava is cooling. The remnant magnetism
of the rock can be measured. If the Earth’s magnetic
field does not change, and the rock does not move, the iron
minerals in the rock will be oriented parallel to the magnetic
field. However, both of these things change:
• Reversal of magnetic fields. The polarity of the Earth’s
magnetic field has changed frequently during Earth history.
During the past 35 million years, the polarity of the Earth’s
magnetic field has reversed (as indicated by the iron minerals
in rocks) more than a hundred times. When this occurs,
the north and south magnetic poles switch positions. The
periods between magnetic reversals have averaged, during
the Cenozoic era, about a half million years apart; however,
they can be as long as several million years or as short
as a few thousand years apart. The magnetic field has not
reversed for almost a million years. The Earth’s magnetic
field has been diminishing in intensity during the past century,
leading some scientists to predict that another magnetic
reversal is about to occur, within a few more millennia.
The cause of the reversals is unknown.
• Movement of the continents (see continental drift;
plate tectonics). The Earth’s magnetic field has not only
a north-south component but also a vertical component. It
is nearly horizontal at the equator and vertical at the poles.
Many volcanic rocks on continents in polar regions have
horizontal magnetic fields, which indicates that the continents
were near the equator when the rocks were formed
and have drifted to their present locations.
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Within any one location, the pattern of reversals produces
a barcode-like signature of magnetic patterns. The ages
of different formations can be determined by lining up their
pattern of reversals with one another. Paleomagnetism, along
with radiometric dating, allows geologists to provide very
accurate age determinations for volcanic deposits and, therefore,
the fossil-bearing deposits that are between them.
A strong magnetic field protects the Earth from cosmic
radiation. While the reversal is occurring, the strength of the
magnetic field is temporarily reduced. This allows higher levels
of cosmic radiation to reach the Earth’s surface. There
are, however, no patterns of increased extinction in connection
with reversals of the magnetic field during the history
of the Earth. While a reversal of the magnetic field would be
unlikely to cause disaster to the natural world, it would be
likely to disrupt many electronically based economic activities
of modern humans.
Paleozoic era The Paleozoic era (the era of “ancient life”)
is the first era of the Phanerozoic Eon, or period of visible
multicellular life, which followed the Precambrian time
in Earth history (see geological time scale). The Paleozoic
era began with a massive diversification of species (see
Cambrian explosion) and ended with the Earth’s greatest
extinction event (see Permian extinction; mass extinctions).
The Paleozoic era consists of six geological periods
(see Cambrian period; Ordovician period; Silurian
period; Devonian period; Carboniferous period; Permian
period).
During the middle of the Paleozoic, a great deal of continental
landmass was centered in the Southern Hemisphere.
By the end of the Paleozoic, all the continents coalesced into
a single continent, Pangaea. Climatic conditions were cold
and dry in the middle and along the southern edge of Pangaea
during the Permian period.
During the first two periods, almost all life-forms were
aquatic. Probably every animal phylum (including vertebrates