Geology Reference
In-Depth Information
PRECAMBRIAN
◗
Figure 19.3
Earth as It May Have Appeared Soon After It Formed
AGE
(Ma)
Eon
Era
Period
AGE
(Ma)
542
Ediacaran
600
700
800
900
1000
1100
1200
630
Cryogenian
850
Tonian
1000
Stenian
1200
a
No rocks are known from this earliest time in Earth's history,
but geologists can make some reasonable inferences about
the nature of the newly formed planet.
Ectasian
1300
1400
1500
1600
1700
1800
1900
2000
2100
2200
2300
2400
2500
2600
2700
2800
2900
3000
3100
3200
3300
3400
1400
Calymmian
1600
Stratherian
1800
Orosinian
2050
Rhyacian
2300
Siderian
2500
2800
b
At about 4.0 billion years old, the Acasta Gneiss from the
Northwest Territories in Canada is the oldest known rock on Earth,
other than meteorites.
3200
4.4 billion years ago. Like the other terrestrial planets and
the Moon, Earth was bombarded by meteorites, the rocky
debris from the origin of the solar system, until about
3.8 billion years ago. Unlike the Moon, however, the evi-
dence of this period of impacts has been obliterated on
Earth by weathering and erosion, as well as by plate move-
ments and mountain building. In addition to the accretion
of planetesimals, the Eoarchean Earth was probably hit by a
Mars-sized planetesimal 4.4 to 4.6 billion years ago, causing
an ejection of a huge amount of hot material that eventu-
ally coalesced and formed the Moon.
When Earth first formed, it retained considerable
residual heat from its origin and it had much more heat
generated by radioactive decay, and as result, volcanism was
ubiquitous. An early atmosphere formed, but it was one
very unlike the oxygen-rich one present now, and when the
planet cooled suffi ciently, surface waters began to accumu-
late. If we could somehow go back and visit early Earth, we
would see a rapidly rotating, hot, barren, waterless planet
bombarded by meteorites and comets. We would also see
no continents, intense cosmic radiation, no organisms, and
widespread volcanism (Figure 19.3a). The age of the oldest
continental crust is uncertain, but we can be sure that at
least some was present by 3.8 billion years ago, and detrital
3500
3600
3700
3800
3900
4000
4100
4200
4300
4400
4500
4600
3600
Lower limit
is not
defined
◗
Figure 19.2
The Precambrian Geologic Time Scale This most
recent revision of the geologic time scale was published by
the International Commission on Stratigraphy (ICS) in 2004.
See Figure 1.17 for the complete time scale. Notice the use
of the prefi xes
eo
(early or dawn),
paleo
(old or ancient),
meso
(middle), and
neo
(new or recent). The age columns on the left
and right sides of the time scale are in hundreds and thousands
of millions of years (1800 million years = 1.8 billion years, for
example).
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