Precambrian Time: Vast and Puzzling

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Precambrian Time Vast and Puzzling

• Chapter 13, Section 1

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Precambrian History

• The Precambrian encompasses immense geological
  time, from Earths 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

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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.

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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

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Earth Forms
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Earths Atmosphere Evolves

• Earths original atmosphere was made up of gases
  similar to those released in volcanic eruptions
  todaywater 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

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Evidence of Early Oxygen
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Concept Check

• Why did the amount of oxygen in Earths
  atmosphere increase dramatically?
• The evolution of organisms that used
  photosynthesis caused the release of increasing
  amounts of oxygen into the atmosphere.

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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

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Remnants of Precambrian Rocks
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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
  cherta hard, dense chemical sedimentary rock
• Plant fossils date from the middle Precambrian,
  but animal fossils date to the late Precambrian
  (many are trace fossils)

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Precambrian Fossils
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Assignment

• Read Chapter 13, Section 1 (pg. 364-368)
• Do Section 13.1 Assessment 1-8 (pg. 368)
• Print Out Application Lab 13 Modeling the
  Geologic Time Scale