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Knowing the fossil record lets a geoscientist place a particular fossiliferous
rock layer into the scale of geologic time. But the time scale given by
fossils is only a relative scale, because it does not give the age of
the rock in years, only its age relative to other layers. Long after the
relative time scale was worked out from fossils, geologists developed
methods for finding the absolute ages of rocks, in years before the present.
These methods involve radioactivity. Here's how one of the important ones
works.
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© National
Park Service
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Geolgogic
time scale for the Miocene-age rock.
Miocene-age rocks in the Niobrara River wetlands and contain
an excellent fossil records. Twenty million years ago animals
such as the Dinohyus (giant pig-like animal), Stenomylus (small
gazelle-camel), and Menoceras (short rhinoceros) roamed the
plains. There were also carnivorous beardogs wandering around,
and the land beaver Paleocastor dug spiral burrows that remain
as today’s trace fossils (Daemonelix) into the ancient riverbanks.
There are remnants of the ancient grasses and hoofprints of
prehistoric animals in Miocene sediments preserved in the
park, as well as layers of fossilized bones.
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Some minerals contain atoms of the radioactive chemical element uranium.
Now and then, an atom of uranium self-destructs to form an atom of lead.
Scientists know the rate of self-destruction. They grind up a rock to
collect tiny grains of minerals that started out containing some uranium
but no lead. Then they use a very sensitive instrument, called a mass
spectrometer, to measure how much of the uranium has been changed to lead.
Using some simple mathematics, they can figure out how long ago the mineral
first formed. It is possible to date rocks as old as four billion years
this way.
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Copyright © Micromass UK LTD
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Photograph of a mass spectrometer. Mass spectrometers are
used to quantitatively date rocks based on methods of distinguishing
the different isotopes of an element.
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Absolute dating of rocks has provided many "tie points" for
the relative time scale developed from fossils. The result is an absolute
time scale. When you collect a fossil from a rock, you can place it in
the relative time scale. Then you also know about how old it is in years
(or usually millions, or tens of millions, or hundreds of millions of
years). Even though modern technology makes it possible to date some rocks,
the relative time scale is still very important. This is because it takes
a lot of time and money to obtain an absolute date, and not all rocks
can be dated using radioactivity.
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