G202
Geologic time
How old is the earth?
4.6 billion years
Used radioactive decay to determine this date (more on this later)
Oldest mineral found were Zircons in Canada
Oldest Rocks found were 3.8 billion
Hard to imagine 4.6 bys
Geologic time scale
What is it (diagram)
How was it created
Why was it created
Geologic time scale…
Created originally using relative age dating techniques
Principle of superposition
Nicolaus Steno (1669)
Principle of original horizontality
Nicolaus Steno (1669)
Principle of cross cutting relationships
Principle of inclusions
Unconformities
Disconformity
Nonconformity
Angular unconformity
At any place and at any time erosion or deposition is occurring
Principle of Faunal Succession
Fossils organisms succeed one another in a definite and determinable order and therefore
anytime period can be recognized by its fossil content
Index fossils
William Smith (1769-1839)
Identified rock layers by fossils
Made 1st geologic map of England
Showed fossils useful for stratigraphy
Used fossils for correlation
George Cuvier (1769-1832)
Recognized extinction of marine fossils
Showed many fossils have no living relative
Showed younger strata had fossils similar to living forms
The research and ideas of Smith and Cuvier lead to the foundation of the geologic systems
Geologic systems: groups of rocks that are recognizable worldwide and are typical of a particular
geologic period
System: we call them periods on the geologic time table
Objective of the system: To find rocks representing continuous time from the first known
fossils to the present
Smith: used fossils to correlate widespread rock units
Cuvier: constructed a composite rock column fro the first appearance of fossils to the present
Used fossil assemblages to correlate rock units
Used sedimentary rocks units that represent time units
Helped divide the Phanerozoic Eon
Ex// Cambrian Period = Cambrian System: represents all the rock deposited on earth
between 570-505 million years ago
How much of the earth's history is represented by the geologic system?
570my / 4.6 billion years = 12.3 % or 1/8 of the earth's history
Why does the rock system not reach further back into Earth's history?
The Geologic Systems provide a base for the relative geologic time Column
No Dates on this scale
Age of earth is 4.6 billion years.
How could this be determined?
History of dating techniques
Edmond Halley (1715)
James Joly (1899)
Lord Kelvin (late 1800)
Discovery of radioactivity:
The Curies (1903) discovered Radium maintains temperature above the ambient (has energy)
Strutt: showed radioactive decay could account for heat flow loss studied by Lord Kelvin
Holmes (1911): Wrote 1st paper on Radiometric age dating techniques
Applied to geologic column
Agreed with stratigraphy of all geologic systems
Applied absolute age dates on all periods, epochs, ages….nothing out of place
Validated relative dating laws.
Radioactive age dating:
Based on the decay of radioactive elements
What is radioactivity?
Unstable nucleus…process of becoming stable creates energy called radiation
Nucleus: Protons and Neutrons
Shells, orbitals, clouds: Electrons
Protons: +1, 1 amu
Neutrons: no charge, 1 amu
Electrons: -1, no mass
P + E creates a N
N can be broken into a P + E (more on this in a minute)
Atomic Mass
Atomic Number
Isotope: two atoms with the same # of protons but different # of neutrons
C12 à 6 P + 6 N = 12 amu
C 13 à 6 P + 7 N = 13 amu
C14 à 6 P + 8 N = 14 amu
When atoms decay, two outcomesà
New atom is created (change the number of protons)
Isotope of the atom is created (same number of protons)
Parent and daughter
½ life
Decay rate is constant and doesn't change due to temp, pressure, or concentration
3 ways decay occurs
Alpha emission
a particle is emitted from nucleus
a particle = 2 p and 2 n
How does it change the mass number?
How does the atomic number change?
Beta emission
b particle is emitted
b particle is an electron that gets emitted from a neutron
N= P + E
How does the mass number change
How does the atomic number change
Electron Capture
Electron is captured by a proton and a neutron is created
How does the mass change?
How does the atomic number change?
Radioactive elements/isotopes used in geology
Parent: Uranium 238 Daughter: Lead 206 ½ life: 4.5 by
Ur 238 undergoes 9 steps to reach Pb206
None of the decay products are stable except Pb206
Parent: Potassium 40 Daughter: Argon 40 ½ life: 1.3 by
Used frequently due to abundance in many mineral structures
Decays by electron capture
Carbon 14 à Nitrogen 14 ½ life 5730 yr
Not common in rocks except limestone
Used only on organic matter
Max age date is 75,000 yrs
Others:
Uranium 235à Lead 207 713 my
Rubidium 87à Strontium 87 47 by
Limitations on Process
Must have a closed system
Dates determined are those of last crystallization
Must use the proper decay pair
Other dating techniques
Dendrochronology
Varves
Pleistocene late sediment from melt waters
Glacial
Pair of layers
Loweràsummer (silts and sands)
Upperà winter clay and organic
Volcanic glass hydration rind
Magnetic reversals
Index fossils