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In-Depth Information
At about the same time, oceanographers made a similar discovery for
underwatet volcanoes. In the eatly 1960s, plate tectonic theory proposed
that new oceanic crust (which is composed of lava) is created by long, linear
volcanoes arranged along submarine mountain chains called spreading cen-
ters. New ocean crust is then carried away from this spteading center, carried
piggyback on a thicker layer of the earth's crust. When oceanographers
towed, over the spreading centers, instruments capable of detecting the ori-
entation of the eatth's magnetic field, they observed regions of normal and
regions of reversed polarity symmetrically arrayed around the spreading cen-
ters. The magnetic signals looked like great striped patterns, which were ul-
timately mapped across all of the earth's ocean bottoms. The only thing in-
vestigators needed to obtain a chronology of the reversal history of the
earth's magnetic field was an age measurement for each stripe. These mea-
surements were soon obtained by a ship specially designed to sample lava
and sediment cores drilled from the bottom of the sea.
By the late 1960s, these cores had provided enough information
about age and polarity for a "polarity time scale" to be constructed. The
greatest advantage of using magnetic reversals as time indicators is that
they are worldwide, or "isochronous," time surfaces. By themselves they are
virtually useless; there have been so many reversals during earth history
that no individual reversal is identifiable. However, combined with othet
dating techniques, such as biostratigraphy (telling time with fossils) and
radiometric dating, the pattern of magnetic field reversals becomes a very
powerful tool. Each time a field reversal takes place, it leaves its indelible
signature in the earth's history and provides a worldwide time marker of
enormous utility. The record of reversal through time is now well known.
That accumulated record is called the geomagnetic polarity time Scale, or
GPTS.
Detecting the record of these geomagnetic reversals is theoretically
simple, but like many
theoretically
simple things, the actual detection process
is often less so. The evidence comes from the directions of untold numbets
of tiny, magnetized mineral particles locked within either sediment or lava.
