Page 48 - The Restless Earth Fossils

so Many Fossils, so little time 47

generated by our entire planet. This also meant that Lord Kelvin’s
calculations were way off the mark because radioactivity would
have kept the Earth’s interior hot and toasty for a very long time.
Not only that, the emission of subatomic particles by decaying
radioactive elements occurs in such a regular fashion that it can
be used as a kind of slow-ticking clock—just the kind needed to
keep track of old Mother Earth’s age. Also, temperature, pressure,
and chemical reactions do not affect these “atomic clocks.”
The bad news about radioactive elements is that you have to
look in igneous rocks, like those found in volcanoes, to find
them rather than in the sedimentary rocks that hold fossils.
However, layers of igneous rocks often bracket layers of fossil-
bearing rocks, so you can determine minimum and maximum
ages for various fossils. By dating the oldest rocks found on Earth
and by dating meteorites that were formed at about the same
time as the Earth, scientists ultimately pegged her age at 4.6 bil-
lion years.

the Modern geological tiMe scale

The modern geological time line shows Earth’s entire history as
recorded in rocks. This timeline has been pieced together since
William Smith’s day by overlapping sections from all over the
world. Eons are the longest blocks of time, followed by eras,
periods, and epochs. The Phanerozoic Eon covers the span
of time in which you can see fossils of one sort or another with
the naked eye. It begins 542 million years ago with an error of 1
million years in either direction (this is indicated by the + and
– symbols next to each number; the error comes from uncer-
tainties in the dating techniques used). Evidence of microscopic
life in rocks extends back 4 billion years to the beginning of the
Archean Eon. For the most part, paleontologists study fossils
from the three eras of the Phanerozoic: The Paleozoic Era (age
of old life), Mesozoic Era (age of middle life), and Cenozoic Era
(age of recent life).

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