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Precambrian Time Vast and Puzzling

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					Precambrian Time: Vast
     and Puzzling
     Chapter 13, Section 1
        Precambrian History
• The Precambrian encompasses immense
  geological time, from Earth’s distant beginnings
  4.56 billion years ago until the start of the Cambrian
  period, over 4 billion years later
• The Precambrian comprises about 88% of the
  geologic time scale
• Most Precambrian rocks do not contain fossils,
  making correlating rock layers difficult
• Many rocks are metamorphosed and deformed,
  extremely eroded, and hidden by overlaying strata
         Concept Check

• Why are specific events in Precambrian
  history difficult to determine?
• Most Precambrian rocks do not contain
  fossils, which makes correlating rock
  layers difficult. Many Precambrian rocks
  are metamorphosed and deformed,
  extremely eroded, or hidden by overlying
  strata.
             Earth Forms
• Scientists hypothesize that Earth formed
  as gravity pulled together dust, rock, and
  ice in space; gravity increased as Earth
  grew and began pulling in more materials
• The high velocity impacts caused the
  planet to melt and divide into specific
  layers based on density
• Over several hundred million years, the
  crust and mantle cooled and hardened,
  forming rock
Earth Forms
          Earth’s Atmosphere Evolves
• Earth’s original atmosphere was made up of gases
  similar to those released in volcanic eruptions today—
  water vapor, carbon dioxide, nitrogen, and several trace
  gases, but no oxygen
• Torrential rains continued and slowly filled low areas,
  forming the oceans; this reduced the water vapor and
  carbon dioxide in the atmosphere, leaving behind a
  nitrogen rich atmosphere
• Primitive organisms evolved that used photosynthesis
  and released oxygen
• Slowly, the oxygen content increased, seen in the banded
  iron formations (which took the free oxygen out of the
  atmosphere)
• Oxygen began to accumulate in the atmosphere about
  2.5 billion years ago
Evidence of Early Oxygen
         Concept Check

• Why did the amount of oxygen in Earth’s
  atmosphere increase dramatically?
• The evolution of organisms that used
  photosynthesis caused the release of
  increasing amounts of oxygen into the
  atmosphere.
       Precambrian Rocks
• The lack of Precambrian rock illustrates the law
  of superposition, it is covered by much younger
  strata
• Precambrian rocks do show through the
  surface where younger strata are extensively
  eroded, such as in the Grand Canyon and in
  some mountain ranges
• Shield – a large relatively flat expanse of
  ancient metamorphic rock within the stable
  continental interior
• Much of what we know about Precambrian
  rocks comes from ores mined from shields
• The mining of iron, nickel, gold, and other
  metals has provided Precambrian rock
  samples for study
Remnants of Precambrian
        Rocks
      Precambrian Fossils
• The most common Precambrian fossils are
  stromatolites
• Stromatolite – structure produced by algae
  trapping sediment and forming layered mounds
  of calcium carbonate
• Remains of cyanobacteria have been found
  that extend the record of life back beyond 3.5
  billion years
• Many of these ancient fossils are preserved in
  chert—a hard, dense chemical sedimentary
  rock
• Plant fossils date from the middle Precambrian,
  but animal fossils date to the late Precambrian
  (many are trace fossils)
Precambrian Fossils
            Assignment

• Read Chapter 13, Section 1 (pg. 364-368)
• Do Chapter 13 Assessment #1-26 (pg. 389-
  390)
• For Section 1: Do #’s 2, 11-13, 20-24

				
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posted:9/28/2012
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