The Oldest Rocks on Earth and Calibrating the Geologic Time Scale

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The Oldest Rocks on Earth and Calibrating the
Geologic Time Scale

• What seest thou else in the dark backward and
  abysm of time?
• Shakespeare

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Mile Markers in Deep Time

• The fossil-based framework for Earth history is
  known as the GEOLOGIC TIME SCALE
• Units Eras, Periods, Epochs, Ages

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What do we do with rocksthat dont contain
fossils?

• How do we determine the age of rocks that are
  too old to have abundant fossils?
• How do we determine the age of igneous and
  metamorphic rocks that do not contain fossils?
• How do we put numbers on age?
• What is the oldest rock on Earth and how do we
  know its age?

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Oldest Rock on Earth

• Acasta Gneiss NWT, Canada
• 3.96 Ga /- 3 Ma
• How was this determined?
• Revolutions still more remote appeared in the
  distance of this extraordinary perspective. -
  Playfair

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Oldest Minerals on Earth

• Acasta Gneiss
• 3.96 Ga /- 3 Ma
• Oldest Minerals
• Sand grains of zircon
• Dated at 4.4 Ga /- 8 Ma
• From sandstone in Australia

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How do we know when these events happened?

• Fossils give us the relative age relationships
  (sequence of events)
• Only Radioactive Decay can be used to establish
  numerical age

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How does it work?

• Some chemical elements are unstable
• They break down (radioactive decay) by undergoing
  changes in the nucleus
• This is a Nuclear reaction (not chemical)
• Changes in nucleus produce
• A new chemical element daughter product
• Energy in the form of radiation

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How does it work?

• These nuclear reactions occur at a constant rate
  this is the half life
• Since rate is constant, we can know how long the
  clock has been ticking.
• Time is calculated by measuring the number of
  atoms of the daughter product and the number of
  atoms of the parent element.

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Where do we find these parent and daughter
elements?

• Mineral crystals that are closed systems
• Minerals like zircon are ideal
• Contain small amounts of Uranium
• Very tough and durable a sealed container
• Age that is calculated is the age of
  crystallization

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Can we measure the age of all rocks?

• Ages are typically measured for igneous rocks
  such as layers of volcanic ash, lava flows and
  cross-cutting intrusions.
• Ages may also be measured for some metamorphic
  rocks age of metamorphism.
• Ages are tied to the Geologic Time Scale which is
  based on fossils.

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The Silurian Devonian Boundary

• Boundary is defined at the Global Stratotype at
  Klonk in the Czech Republic
• Index Fossil is the Graptolite Monograptus
  uniformis uniformis
• Auxiliary Index is the conodont Icriodus
  woschmidti woschmidti

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SUNY Oneonta Students and Calibrating the Age of
the Silurian Devonian Boundary
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SUNY Oneonta Students and Calibrating the Age of
the Silurian Devonian Boundary
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Calibrating the Silurian Devonian Boundary

• Helderberg Group Silurian Devonian limestones
  in New York
• Coeymans Formation and Kalkberg Formation yield
  the index conodont I. woschmidti
• K-bentonites (volcanic ashes) in New Scotland
  Formation
• Microphenocrysts of zircon extracted from
  K-bentonite
• Uranium Lead system used to determine age of
  Helderberg ash
• Occurrence within zone of lowest Devonian index
  fossil means age of ash approximates age of
  boundary
• S-D Boundary placed at 418 Ma
• 417.6 /- 1.0 Ma (Tucker, et. al. 1998)