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precambrian

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									    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
  Cambrian-aged rocks.
• 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
             Eoarchean?
• 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.
 Continental Foundations—Shields,
       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
  nucleus.
• Cratons are the foundations of continents, and
  along their margins more continental crust was
  added as they evolved to their present sizes and
  shapes.
• Both Archean and Proterozoic rocks are present in
  cratons.
• 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
  Michigan.
• 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.
             Archean Rocks
• Only 22% of Earth’s exposed Precambrian crust is
  Archean, with the largest exposures in Africa and
  North America.
• 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
  plutonic rocks.
            Greenstone Belts
• 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
  synclinal structure.
   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
  arc.
• 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
  granitic magma.
  Archean Plate Tectonics and the
        Origin of Cratons
• Most geologist are convinced that plate tectonics
  took place during the Archean but it differed in
  detail.
• 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
                Evolve?
• 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
  radiation.
• 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
  waste product.
      Earth’s Surface Waters—The
              Hydrosphere
• 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
  ago.
• 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
  reactions.
• 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
  and complexity.
   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
  algae).
• Currently the oldest known stromatolites are 3.0
  billion years old in rocks found in South Africa.

								
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