Radiometric Dating

1
Radiometric Dating
2
Landscape Development

• Mountains, valleys, oceans
• How and when did they form?
• How old is Earth?

3
Static Theory

• Bishop Ussher
• mid-1600s
• Constructed a chronology of society and Earth
  history based on the Bible
• Concluded that the Earth formed in 4004 B.C.
• Earth is only a few thousand years old
• Todays landscape created instantaneously
• Earths birthday is 23 OCT

4
Dynamic Theories

• Observations
• Rocks form at volcanoes
• Landscape affected by floods

5
Catastrophism

• 1600-1700s
• Landscape formed through a series of catastrophes
• Mountains, canyons formed during sudden
  world-wide disasters triggered by unknowable
  causes that no longer operate
• Fit rate of Earth processes to Usshers age for
  the Earth

6
Uniformitarianism

• 1700 to 1800s
• James Hutton, Charles Lyell
• Physical, chemical, and biological laws that
  operate today have also operated in the past
• The present is the key to the past

7
Uniformitarianism - Implications

• Geological processes operate over extremely long
  periods of time
• Example erosion
• Catastrophism - everything happened at once
• Uniformitarianism - everyday process, operating
  over a sufficiently long period of time
  (thousands to millions of years) can shape the
  landscape

8
Dating

• Answer questions like
• When did the volcano erupt?
• When did the mountains form?
• Numerical Dating (X million years ago)
• Need Radioactive dating methods
• Relative Date (order of events)
• Geologists use both together

9
Stenos Principles

• Original horizontality
• Sedimentary rocks are formed in layers which were
  originally horizontal
• Observe flat layers - rocks undisturbed
• Observe tilted layers - tectonics at work
• Lateral continuity
• The horizontal layers continue in all directions
  at the time that they form

10
Stenos Principles

• Superposition
• Oldest rocks are on the bottom,
• Youngest rocks are on the top
• Cross-cutting relationships
• Igneous intrusions and faults are younger than
  the sedimentary rocks that they cut across

11
Unconformities - Importance

• Represent significant geologic events
• Represent intervals of time when layers are
  missing from the geologic record
• You will see them in lab

12
Grand Canyon Example
Grand CanyonExample
13
Applying Relative Dating
14
Principle of Fossil Succession

• Fossil organisms succeed one another in a
  definite determinable order
• Any time period can be recognized by its fossil
  content
• Examples age of trilobites, ages of fishes, age
  of dinosaurs, etc.
• Application of Principle of Superposition
• Important for Correlation

15
Index Fossils

• Fossils useful as time indicators
• Short age range
• Easily recognized
• Widespread distribution

16
Atomic Structure Review

• Atoms are comprised of
• A nucleus containing protons and neutrons
• Electrons that orbit the nucleus
• The atomic number (elements ID) the number of
  protons in the nucleus
• The mass number of protons of neutrons
• When the number of neutrons changes we have
  isotopes of the element

17
Radioactivity

• Normally, the forces that bind the nucleus
  together are strong.
• Radioactivity - process by nuclei spontaneously
  break apart, when forces holding them together
  are not strong enough.
• Types
• Alpha emission
• Beta emission
• Electron Capture

18
Radiometric Dating

• Procedures that count the products of
  radioactivity to date rocks
• Example
• Uranium (parent) included in crystal structure of
  certain minerals as they crystallize from a magma
  (start the clock)
• Lead (daughters) decay products accumulate
• Rate is known (Half Life)
• Count number of parents and daughters to get age

19
Isotopes and Half Lives

• 238U --gt 206Pb 4.5 x 109 years
• 87Rb --gt 87Sr 47 x 109 years
• 40K --gt 40Ar 1.3 x 109 years
• 14C 5730 years
• Isotope half lives vary
• Source of isotopes depends on mineralogy

20
GeologicTime Scale

• Eon/Era/Period names - based on fossil
  assemblages, superposition
• Absolute ages, from radiometric dating

21
Radioactive Decay Example

• If you have an orange slip of paper come down
  front
• The half-life in our example is 30 seconds
• As this activity progresses, answer the following
  questions
• How many half-lives have taken place?
• What is happening to the amount of parent versus
  daughter?

22
A second example

• You start with 1000 in your bank account
• Every week your spouse or boyfriend/girlfriend
  removes half of the money
• The half-life of your account 1 week
• Questions to think about
• When is the most money taken out?
• Will the amount in your bank account ever go to
  zero?

23
Your Bank Account

• Start 1000
• Week 1 500
• Week 2 250
• Week 3 125
• Week 4 62.50
• Week 5 31.25
• Week 6 15.63
• Week 7 7.81
• Week 8 3.90
• Week 9 1.95

• Week 10 0.98
• Week 11 0.49
• Week 12 0.24
• Week 13 0.12
• Week 14 0.06
• Week 15 0.03
• Week 16 One and a half cents left in your bank
  account

24
Lateral Continuity
25
Superposition
Younger
26
Cross-cutting Relationships
27
Index Fossils
28
The Atom
29
Types ofRadioactiveDecay
Alpha Emission
Beta Emission
(-)
Electron Capture
(-)
30
Radioactive Decay Curve
31
Carbon-14