Geology Reference
In-Depth Information
Chapter Summary
Geologists divide the Precambrian into two eons, the
Archean and the Proterozoic.
Each continent has an ancient, stable craton made up of a
Precambrian shield and platform. The Canadian shield in
North America is made up of several subunits.
Archean rocks are mostly granite-gneiss complexes and
subordinate greenstone belts. One model for the origin
of greenstone belts holds that they formed in back-arc
marginal basins.
The amalgamation of Archean cratons and continental
accretion along their margins account for the origin of a
large landmass known as Laurentia.
Many geologists think that Archean plates moved faster
than plates do now because Earth possessed more radio-
genic heat.
Sandstone-carbonate-shale assemblages deposited on
passive continental margins and in intracratonic basins
are the most common Proterozoic-age rocks.
Widespread glaciers were present during the Early and
Late Proterozoic.
Earth's earliest atmosphere lacked free oxygen, but was
rich in carbon dioxide. It was derived by the release of
gases during volcanism, a process called outgassing.
Meteorite and comet impacts and outgassing yielded the
hydrosphere.
Deposition of widespread banded iron formations
between 2.5 and 2.0 billion years ago and the fi rst red
beds about 1.8 billion years ago indicate that some free
oxygen was present in the atmosphere.
Energy such as lightning and ultraviolet radiation acting
on chemical elements present on early Earth may have
yielded the fi rst living things. Some investigators think
that RNA molecules were the fi rst molecules capable of
reproduction.
All known Archean fossils represent prokaryotic bacteria.
Stromatolites formed by photosynthesizing bacteria may
date from 3.5 billion years ago.
Endosymbiosis practiced by prokaryotic cells was prob-
ably responsible for the fi rst eukaryotic cells.
The oldest megafossils are carbonaceous impressions,
probably of algae, in rocks more than 2 billion years old.
The Neoproterozoic Ediacaran faunas include the oldest
well-documented animal fossils. None had durable skel-
etons, so their fossils are not common.
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Important Terms
abiogenesis (p. 512)
anaerobic (p. 514)
autotrophic (p. 514)
back-arc basin (p. 499)
banded iron formation (p. 506)
Canadian shield (p. 496)
continental accretion (p. 496)
continental red bed (p. 506)
craton (p. 496)
Ediacaran faunas (p. 516)
endosymbiosis (p. 515)
eukaryotic cell (p. 514)
geologic record (p. 494)
granite-gneiss complex (p. 496)
greenstone belt (p. 497)
Grenville orogeny (p. 503)
heterotrophic (p. 514)
Laurentia (p. 500)
Midcontinent rift (p. 503)
monomer (p. 513)
multicelled organism (p. 515)
orogen (p. 500)
outgassing (p. 507)
Pannotia (p. 504)
photochemical dissociation (p. 507)
photosynthesis (p. 510)
platform (p. 496)
polymer (p. 513)
prokaryotic cell (p. 514)
Rodinia (p. 504)
sandstone-carbonate-shale
assemblage (p. 502)
shield (p. 496)
stromatolite (p. 513)
Wilson cycle (p. 502)
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