Geologic Time Scale Development

DEVELOPMENT OF THE GEOLOGIC TIME SCALE

• RELATIVE GEOLOGIC TIME SCALE - Based on PRINCIPLES of SUPERPOSITION, ORIGINAL HORIZONTALITY, LATERAL CONTINUITY, CROSS-CUTTING RELATIONS, UNCONFORMITIES, INCLUSIONS, FOSSIL SUCCESSION
  • yields sequence of events
  • doesn't say how long ago or of what duration
  • developed mostly between 1822 & 1841 from rocks exposed in Europe
  • Paleozoic mostly in England by Murchison & Sedgwick
• ABSOLUTE GEOLOGIC TIME SCALE - Based on RADIOMETRIC DATING TECHNIQUES
  • gives ages in years before present
  • early attempts were not based on radiometric dating techniques
    • Bishop Ussher (mid-1600's) - October 22, 4004 B. C.
    • Georges Buffon (mid-1700's) - cooling of metal & non-metal balls (75,000 years)
    • John Joly (1899) - buildup of salt in the ocean (90 m.y.)
    • various geologists (1800's) - thickness of sedimentary sections (1 b.y.)
    • Lord Kelvin (1824-1907) - attempt similar to Buffon's, but more rigorous = 20-400 m.y. [didn't take radioactivity into consideration]

FUNDAMENTAL GEOLOGIC PRINCIPLES

• SUPERPOSITION - the oldest layer is at the bottom in undisturbed strata
• ORIGINAL HORIZONTALITY - sediment originally deposited in nearly horizontal layers
  • various primary sedimentary structures can be used to determine up
• ORIGINAL LATERAL CONTINUITY - as originally deposited, strata extend continuously in all directions to the depositional basin edge
• CROSS-CUTTING RELATIONSHIPS - igneous intrusion or fault younger than rock intruded or cut
• UNCONFORMITIES - surfaces of nondeposition or erosion encompassing significant amounts of geologic time
  • hiatus = interval of time not represented by strata in an area
    • DIASTEMS = relatively short hiatuses in a continuous, conformable sequence
    • BEDDING PLANES are essentially diastems
  • nonconformity = unconformity cut into metamorphic or igneous rocks & overlain by sedimentary rocks
  • angular unconformity = unconformity where overlying & underlying strata different dips
  • disconformity = unconformity where overlying & underlying strata are parallel to each other
• INCLUSIONS - rock fragments included within a rock mass are older than the enclosing rock mass
• FOSSIL SUCCESSION - fossil assemblages succeed one another through time in a regular & determinable order
• PALEOMAGNETISM - alternating layers of normal & reversed polarity

FOUNDERS OF HISTORICAL GEOLOGY

• NICOLAUS STENO (1638-1686)
  • noted for clear enunciation of Superposition, Original Horizontality, and Original Lateral Continuity
• JOHANN LEHMANN
  • proposed subdivision of rocks of the Earth's crust into 3 categories based on lithology (Ore, Stratified, & Alluvial Mountains)
• GIOVANNI ARDUINO
  • proposed subdivision of rocks near Padua, Italy, into Primary, Secondary, & Tertiary
• ABRAHAM WERNER (1749-1817)
  • founder of Neptunism, a school of thought which held that all crustal rocks were deposited or precipitated from an early, global ocean
  • added Transition & New Stratified Mountains to Lehmann's scheme
• GEORGE CUVIER (1769-1832)
  • founder of Catastrophism, a school of thought which held that the history of life punctuated by catastrophes involving violent flooding of the continents that wiped out all life; new life specially created
  • co-developer of the principle of Fossil Succession
• WILLIAM SMITH (1769-1839)
  • co-developer of the principle of Fossil Succession
• JAMES HUTTON (1726-1797)
  • father of modern historical geology
  • developed Uniformitarianism, that is, "the past history of our globe must be explained by what can be seen to be happening now", or "the present is the key to the past."
  • developed the concept of the geologic cycle, that is, mountains build up, are eroded, & shed sediments that are deposited & eventually incorporated into new mountains, based on the principle of Unconformities
  • clearly envisioned the immensity of geologic time ("no vestige of a beginning, & no prospect of an end")
  • founder of Plutonism, a school of thought which held that igneous rock originated from molten rock derived from deep in the Earth, although sediments clearly were deposited from water, that overcame Neptunist view of igneous rock origins
  • John Playfair popularized Hutton's work
• CHARLES LYELL (1797-1875)
  • principal Uniformitarianist in the debate with the Catastrophists, which ended in the triumph of Uniformitarianism
• UNIFORMITARIANISM
  • reinterpreted & altered since Hutton & Lyell
  • modern view: physical/chemical laws are uniform & operated in the past as they do today, but rates & types of processes have changed
    • the modern view accomodates short-term, catastrophic terrestrial events (for example, earthquakes & hurricanes), the significance of which were played down during the uniformitarianism-catastrophism debate
    • additionally, the modern view accomodates short-term, catastrophic extraterrestrial events (such as meteorite & comet collisions) which Lyell did not accept

Study Questions

1. What are the differences between relative & absolute dating?

2. What are the fundamental geologic principles?

3. How are the fundamental geologic principles used in development of the geologic time scale?

4. What are the differences between the several types of unconformities?

5. Who clearly stated the principles of superposition, original horizontality, and original lateral continuity?

6. Who co-developed the principle of biologic succession?

7. Why is James Hutton considered the father of modern historical geology?

8. How does Uniformitarianism as used today differ from its use during the time of Lyell?

RADIOMETRIC DATING TECHNIQUES

• Radioactivity - Spontaneous change (decay) in the nucleus of an atom
  • ATOMIC NUCLEUS - protons (p⁺, electrical charge = +1, mass = 1) & neutrons (n^o, electrical charge = 0, mass = 1)
    • ATOMIC NUMBER (determines the element) = number of p⁺
    • ATOMIC MASS = number of p⁺ + n^o
    • NOTE: n^o = electron (e^-, electrical charge = -1, mass = 0, called a beta particle) + p⁺
  • ISOTOPES = different types of an element differing in atomic mass
  • FORMS OF DECAY
    • ALPHA DECAY (ejection of an alpha particle from a nucleus)
      • alpha particle = nucleus of a Helium atom (2 p⁺ + 2 n^o, [atomic # = 2, atomic mass = 4])
      • atomic # decreases by 2 & atomic mass decreases by 4
      • example: ₉₂U²³⁸ - alpha particle -> ₉₀Th²³⁴ (daughter product)
    • BETA DECAY (ejection of a beta particle from a nucleus)
      • atomic number increases by 1 & atomic mass remains the same
      • example: ₃₇Rb⁸⁷ - beta particle -> ₃₈Sr⁸⁷
    • ELECTRON CAPTURE DECAY (capture of an e^- [or beta particle] by a nucleus)
      • atomic number decreases by 1 & atomic mass remains the same
      • example: ₁₉K⁴⁰ + beta particle -> ₁₈Ar⁴⁰
  • HALF-LIFE of a radioactive element - TIME FOR ONE-HALF OF ANY AMOUNT of a radioactive element TO DECAY
    • DECAY IS EXPONENTIAL, that is it is faster earlier
• Assumptions & Sources of Error
  • 1. HALF-LIVES DON'T CHANGE & are MEASURED ACCURATELY
    • "CONCORDANT" AGES (same age - 2 different decay series) CONFIRMS
  • 2. Mineral/rock is "CLOSED" SYSTEM (parent & daughter don't leave system)
    • amount of PARENT REMAINING + DAUGHTER = amount of original PARENT
  • 3. NO DAUGHTER present INITIALLY
    • often CAN CORRECT FOR any daughter present initially anyway
    • example: ISOTOPES OF LEAD (Pb) = Pb²⁰⁴, Pb²⁰⁶, Pb²⁰⁷, Pb²⁰⁸
      • Pb²⁰⁴ from original solar nebula only, while Pb²⁰⁶, Pb²⁰⁷ & Pb²⁰⁸ from both original solar nebula & decay of U²³⁸, U²³⁵ & Th232
      • obtain original solar nebula Pb²⁰⁶, Pb²⁰⁷, & Pb²⁰⁸ from amount of Pb²⁰⁴ using fixed ratio of Pb²⁰⁴:Pb²⁰⁶:Pb²⁰⁷:Pb²⁰⁸ in original solar nebula (from meteorites), then subtract to get radiogenic
  • 4. MASS SPECTROMETER MEASUREMENT ERROR is ±0.2-2.0%
• Principle Radiometric Timekeepers
  • HALF-LIVES

    Isotope	Half-life	Isotope	Half-life
    Rb87	47. by	U235	713. my
    U238	4.51 by	C14	5,730. years
    K40	1.3 by

  • LEAD PRODUCERS (U²³⁸ -> Pb²⁰⁶; U²³⁵ -> Pb²⁰⁷)
  • POTASSIUM-ARGON (K⁴⁰ + beta -> Ar⁴⁰ [11%])
    • K⁴⁰ - beta particle -> Ca⁴⁰ (89%), but can't correct for non-radiogenic
  • RUBIDIUM-STRONTIUM (Rb⁸⁷ - beta particle -> Sr⁸⁷)
  • PB PRODUCERS & RB-SR - used for PLUTONIC IGNEOUS ROCKS; K-Ar - used for VOLCANIC IGNEOUS ROCKS
  • C¹⁴ - for dating GEOLOGICALLY YOUNG ORGANIC MATERIAL (less than 100 ky)
    • C¹⁴ is created continously in the atmosphere (N¹⁴ + n^o -> C¹⁴ + p⁺)
    • C¹⁴ - beta particle -> N¹⁴; ratio of C¹⁴ to all carbon gives age

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