PRECAMBRIAN EARTH AND LIFE
HISTORY THE ARCHEAN EON
• The duration of geologic time is beyond
comprehension. If 24 hours represented all
geologic time, the Precambrian would be more
than 21 hours long, more than 88% of the total.
• Precambrian is a widely used term that refers to
both time and rocks. In regard to time it includes all
geologic time from Earth’s origin 4.6 billion years
ago to the beginning of the Phanerozoic Eon 542
million years ago.
• Precambrian encompasses all rocks lying below
• Precambrian is difficult to interpret, particularly for
the older part of the Precambrian because many of
the rocks have been metamorphosed and
deformed and in many areas they lie deeply buried
beneath younger rocks.
• Establishing formal subdivisions of the Precambrian is
difficult because of the complexities and the fact
that Precambrian contain few fossils and are of little
use in stratigraphy.
• Most Precambrian subdivisions are based on
absolute ages rather than time-stratigraphic units.
What Happened During the
• The Eoarchean refers to all geologic time from the
Earth’s origin until the onset of the Paleoarchean 3.6
billion years ago. Also the oldest rocks on Earth are
3.96 billion years old, so we have no geologic
record for most of the Eoarchean. Nevertheless
some events took place during this time.
• For one thing, it was during the Eoarchean that the
Earth accreted from planetesimals and
differentiated into a core and mantle, and at least
some of the crust was present.
• Earth was bombarded by comets and meteorites
and volcanic activity.
• An atmosphere formed but it was different from the
oxygen-rich one we have now, and surface waters
began to accumulate as the Earth cooled.
• The oldest known rocks on Earth are 3.96 billion
years—Acasta Gneiss in Canada and 3.8 billion-
year old rocks from Montana and Greenland,
indicating that some continental crust had evolved
by the Eoarchean time. In addition some
sedimentary rocks in Australia contain 4.4 billion
year old zircons (ZrSiO4) so source rocks at least that
old must have existed.
• Shortly after Earth formed, it was exceedingly hot
and volcanism was widespread. However, rather
than being a fiery orb for half a billion years as was
formerly accepted , some geologist think that the
Earth cooled sufficiently by 4.4 billion years ago for
surface waters to accumulate.
• The first crust that formed was probably thin and
made of ultramafic rock. Upwelling mantle currents
of magma disrupted this early crust and numerous
subduction zones developed to form the first island
arcs. Collisions between island arcs eventually
formed continental cores. Larger groups of merged
island arcs, or protocontinents, grew faster by
accretion along their margins and eventually the
first continental nuclei or cratons formed.
Platforms and Cratons
• Continents consist of rocks with an overall
composition similar to that of granite, and
continental crust is thicker and less dense than
oceanic crust, which is made of basalt and gabbro.
• A Precambrian shield is found on all continents.
Continuing outward from the shields are broard
platforms of buried Precambrian rocks that underlie
much of each continent. Collectively a shield and
platform make up a craton—a continents ancient
• Cratons are the foundations of continents, and
along their margins more continental crust was
added as they evolved to their present sizes and
• Both Archean and Proterozoic rocks are present in
• In North America, the exposed part of the craton is
the Canadian shield, which occupies most of
northeastern Canada, a large part of Greenland,
Adirondack Mountains of New York, and parts of
Lake Superior region in Minnesota, Wisconsin, and
• The Canadian sheild as well as the adjacent
platform are made up of numerous units of smaller
cratons that amalgamated along deformation belts
during the Paleoproterozoic.
• Only 22% of Earth’s exposed Precambrian crust is
Archean, with the largest exposures in Africa and
• Archean crust is made up of a variety of rocks but
are geologist characterize them as greenstone belts
and granite-gneiss complexes.
• Granite-gneiss complexes are actually composed
of a variety of rocks with granitic gneiss and granitic
• An ideal greenstone belt has three major rock
associations; volcanic rocks are most common in
the lower and middle parts whereas the upper
rocks are mostly sedimentary. They typically have a
Evolution of Greenstone Belts
• Greenstone belts probably developed in back-arc
marginal basins. Back-arc marginal basins are
found between continent and a volcanic island
• There is an early stage of extension when the back-
arc marginal basin forms, which is accompanied by
volcanism, emplacement of plutons and
sedimentation, followed by an episode of
compression when the basin closes. During this
latter stage, the greenstone belt rocks are
deformed, metamorphosed, and intruded by
Archean Plate Tectonics and the
Origin of Cratons
• Most geologist are convinced that plate tectonics
took place during the Archean but it differed in
• Because the Earth had more residual and
radiogenic heat, plates moved faster and magma
was generated more rapidly. As a result continents
grew more rapidly along their margins by a process
called continental accretion.
The Atmosphere and Hydrosphere
How Did the Atmosphere Form and
• Today Earth’s atmosphere is composed mostly of
nitrogen and free oxygen, meaning oxygen not
combined with other elements as in carbon dioxide
(CO2 ) and water vapor (H2O). It also has small but
important amounts of other gases such as ozone
(O3) which blocks most of the Sun’s ultraviolet
• Earth’s earliest atmosphere was probably
composed of hydrogen and helium, the most
abundant gasses in the universe.
• This atmosphere would have quickly been lost into
space because the Earth’s gravitational attraction
is too weak to retain gasses with such low molecular
weights. Second, the Earth at the time had no
magnetic field—magnetosphere to keep solar
winds from sweeping away the atmosphere.
• Water vapor is the most common gas emitted by
volcanoes today including carbon dioxide, sulfur
dioxide, carbon monoxide, sulfur, hydrogen and
nitrogen. Archean volcanoes emitted the same
gases and thus an atmosphere developed, but an
atmosphere lacking free oxygen.
• Two processes account for introducing free oxygen
into the atmosphere.
• Photochemical dissociation—involves ultraviolet
radiation from the Sun in the upper atmosphere
disrupting water molecules, thus releasing their
oxygen and hydrogen.
• Photosynthesis—a metabolic process in which
organisms use carbon dioxide and water to make
organic molecules and oxygen is released as a
Earth’s Surface Waters—The
• Water vapor is the most abundant gas released by
volcanoes, so once the Earth had cooled
sufficiently, water vapor condensed and began to
accumulate, perhaps as early as 4.4 billion years
• Oceans were present during the Eoarchean.
Although volumes and the geographic extent can
not be determined.
The Orgin of Life
• Scientist have found fossils in 3.3 to 3.5 billion-year-
old Archean rocks and chemical evidence in 3.85-
billion-year-old rocks in Greenland that have
convinced investigators that organisms were
present by this early date.
• There is unequivocal evidence for Archean
organisms but compared to the present, the
Archean was biologically impoverished.
• How might have life originated? Abiogenesis—how
life originated from nonliving matter.
• Before discussing abiogenesis let us be clear on
what is living and nonliving. In most cases the
distinction is straightforward: dogs and trees are
alive, but rocks and water are not.
• Bacteria are living, but in some circumstances, they
can go for long periods without showing signs of life
and then go about living again.
• Viruses behave like living organisms in the
appropriate host cell, but when outside a host cell
they neither metabolize nor reproduce.
• Also, microspheres (carbon-based molecules) from
spontaneously and grow and divide but these
processess are more like random chemical
• So what do viruses and microspheres have to do
with the origin of life? First they show that living
versus nonliving distinction is not always easy to
make. Second, if life originated by natural
processes from nonliving matter (abiogenesis), it
must have passed through prebiotic stages—that is,
stages in which the entities would have shown signs
of living organisms but were not truly living.
Abiogenesis holds that several small steps took
place, each leading to an increase in organization
The Oldest Known Organisms
• As far back as the early 1900’s, Charles Walcott
described layered moundlike structures from
Paleoproterozoic-aged Gunflint Iron Formation in
Ontario, Canada. He proposed that these
structures, now called stomatolites, represented
reefs constructed by algae, but not until 1954 did
paleontologist demonstrate that stromatolites are
the product of organic activity.
• Present-day stromatolites form and grow as
sediment grains are trapped on sticky mats of
photosynthesizing cyanobacteria (blue-green
• Currently the oldest known stromatolites are 3.0
billion years old in rocks found in South Africa.