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How Old is the Earth

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Often plutons are many km below the surface. Review of Unconformities ... 1) formation of pluton deep underground. 2) erosion of several km of overlying rock ... – PowerPoint PPT presentation

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Title: How Old is the Earth


1
How Old is the Earth?
  • Ussher (Irish archbishop)
  • Creation occurred on Oct 23, 4004 BC.
  • Counted back Biblical generations
  • Lord Kelvin (physicist)
  • Used the rate of cooling of the Earth to
    calculate at what time it would have been
    completely molten
  • Came to 100,000,000 years
  • Current knowledge based on radiometric dating of
    oldest minerals (found on earth) and of
    meteorites to determine age of solar system
  • Age of earth as we understand it 4.6 billion
    years
  • Why was Kelvin wrong?

2
Geologic Time
  • Geologists think of time differently

3
How do we measure time?
  • Absolute time
  • We can tell how old a rock is via radiometric
    dating techniques
  • Work best on igneous rocks
  • When an igneous rock crystallises, minerals
    within it sometimes contain unstable, radioactive
    isotopes
  • These decay at fixed rates, forming different
    elements (aka half-lives)
  • By measuring the ratio of the daughter-product
    to the radioactive isotope, and determining how
    much radioactive isotope was present at the time
    of formation (via the half-life of that isotope),
    you can calculate how old the rock is.
  • I will not ask you to do this (I promised, no
    math!)
  • However

4
Concept of Half-life
  • Half-life
  • the amount of time it takes a radioactive element
    to reach half its original amount

5
Half-lives, daughter products, and effective
dating range of commonly-used isotopes for
radiometric dating
  • K-40
  • Half-life of 1.25 billion years
  • Decays to Ar-40 (argon)
  • Effective in dating rocks that are 100,000
    4,600,000,000 years old
  • U-238
  • Half-life of 4.5 billion years
  • Decays (eventually) to Pb-206 (lead)
  • Effective in dating rocks that are 10,000,000
    4,600,000,000 years old
  • Rb-87
  • Half-life of 49 billion years
  • Decays to Sr-87 (strontium)
  • Effective in dating rocks that are 10,000,000
    4,600,000,000 years old
  • C-14
  • Half life of 5,730 years
  • Decays to N-14 (nitrogen)
  • Effective in dating dead, preserved organic
    matter that died 100 40,000 years ago

6
Carbon dating
  • Useful in determining age of a particular
    volcanic eruption that is lt40,000 years BP
  • Vegetation, while alive, converts CO2 to living
    tissue
  • 12CO2 and 14CO2 maintain constant ratio in the
    atmosphere, and maintain constant ratio in
    vegetation, while it lives
  • Upon death, the C-14 in a plant decays
  • By measuring the radioactivity of dead organic
    matter, we can indirectly determine the current
    ratio of 12C to 14C, and determine when the plant
    died
  • Lava flows kill plants and turn them into
    charcoal that can be easily dated!

7
Relative Time
  • Principles of
  • Original Horizontality
  • Superposition
  • Lateral Continuity
  • Cross-cutting relationships
  • Unconformities
  • Disconformities
  • Angular unconformities
  • Nonconformities

8
Original Horizontality and Superposition
9
Lateral Continuity
10
Cross-Cutting Relationships
11
Unconformities
  • Disconformity
  • Where layers of sediment are apparently missing
  • Missing rock strata separates beds that are
    parallel to each other
  • Usually identified by a gap in the fossil record
  • If in one layer you find fossils that are
    indicative of 320 million years old, and in the
    layer above it, you find fossils circa 200
    million years old
  • Erosion of the missing layers following their
    deposition

12
Angular Unconformity
  • Where a younger horizontal strata overlies older
    strata that are tilted or folded
  • Explained again by erosion

13
Nonconformity
  • A contact between strata where an erosional
    surface on a pluton is covered by younger
    sedimentary or volcanic rock
  • implies millions of years of erosion
  • Often plutons are many km below the surface

14
Review of Unconformities
  • Disconformities
  • 1) deposition of horizontal sedimentary layers
  • 2) erosion of one or more sedimentary layers
  • 3) deposition of new layers atop much older
    layers, out of sequence
  • Angular Unconformities
  • 1) deposition and lithification of sedimentary
    rock
  • 2) folding or tilting of layers
  • 3) erosion
  • 4) renewed deposition on the erosional surface
  • Nonconformities
  • 1) formation of pluton deep underground
  • 2) erosion of several km of overlying rock
  • 3) deposition and lithification (eventually) of
    new sediment on old igneous surface

15
Question
  • Put the letters in order from oldest to youngest
  • Between what letters did the fault occur?
  • Where did erosion take place?
  • Are there any unconformities? If so, where, and
    what type are they?
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