Earth Sciences 240A - Lecture 3
Age Dating
Age of Earth: Early
Attempts
Empirical Attempt: Archbishop James Ussher
4004 BC; [modified by others to
Ussher’s Date: Accepted by
Christian church; Became ‘dogma’ for 200 years
Creation: a molten ball which formed wrinkles
(mountains/basins) on cooling
All changes assumed catastrophic in nature (i.e. not
explainable)
Nicholas Steno
Avid geologist; recognized fossilized sharks teeth in rocks
Three fundamental principles of geology; published about 1700
1. Principle of Superposition
2. Principle of Original
Horizontality
3. Principle of Original Lateral
Continuity
Persuaded to join
Catholic Church in later life; encouraged to renounce his science
Relative Age Dating
Apply Steno’s Principles
Add: James Hutton’s
Principle of Cross-Cutting Relationships
William Smith’s Principle of Faunal
Succession
Divisions of Time
Four Eons: Hadean, Archeozoic,
Proterozoic, Phanerozoic
Three Eras in Phanerozoic Eon; Separated
by major extinction events
Paleozoic Era: ‘ancient life’
Mesozoic Era: ‘middle life’
Cenozoic Era: ‘recent life’
Absolute Dating
Radioactivity; discovered by Henri Bequerel (1896)
‘Atoms of some
chemical elements are inherently unstable and breakdown in a predictable
fashion’
Atomic Nomenclature
Nucleus: Protons + neutrons (mass
number)
Electrons: Beyond nucleus; Negative
charge; hardly any mass
Isotope (varying number of neutrons)
E.g. 16O, 17O,
18O
Radioactive Isotope
Too many particles in nucleus;
unstable
Parent – Daughter:
Half-life: Length
of time it takes for any quantity of parent to decay to half the original mass
Alpha-decay; Beta-decay; Electron capture
Radioactive Age Dating
U, Th, Sr-Rb: Long half-lives; Used
for very old events
Carbon-14: Conversion of 14N by
cosmic radiation by neutron capture (and loss of 1 proton); Incorporation by
all living organisms; Decay begins at death
Half-life: 5730
years
Next: Philosophy
Uniformitarianism
vs. catastrophism