Hutton’s Unconformity
Siccar Point, Scotland
• During a time when many claimed the age of the Earth to be about 6,000 years, came a voice of reason:
– “We find no sign of a beginning-no prospect for an end”
– James
Hutton
(1788),
Scotland
• Original Horizontality
– sedimentary beds deposited near horizontal
• Superposition
– layers young from bottom to top
• Lateral Continuity
– tapering, thinning near edges
• Cross-cutting Relationships
– disrupted pattern is older than disruption
– truncation of older rock bodies
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• Unconformity surface that represents a gap in the geologic record
– Disconformity - contact representing missing parallel beds
– Angular unconformity - younger strata overlie an erosion surface on tilted or folded layers
– Nonconformity - erosion surface on plutonic or metamorphic rock
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Great Unconformity: The Grand Canyon
• In detail a nonconformity with Cambrian (ca.
530 Ma) Tapeats sandstone deposited on 1.7 Ga
Vishnu schist)
Reading the rock record:
Timing of events on the local scale
What is the relative age of granite ?
When did tilting take place ?
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• Physical continuity
• Similarity of rock types
• Superposition
• Correlation by fossils
– Principle of faunal succession
– Concept of index fossil
– Fossil assemblage
• Radioisotopic dating-comparisons
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• Physical continuity
• Similarity of rock types
• Superposition
• Correlation by fossils
– Principle of faunal succession
– Concept of index fossil (short lived, widespread)
– Fossil assemblage
• Radioisotopic dating-comparisons
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Henri Becquerel b. 1852 / d. 1908
Becquerel (1896) Sur les radiations emises par phosphorescence. Comptes Rendues Acad. Sci. Paris, v. 122,
420-21.
• Discovered natural radioactivity (x-rays fog photographic plate)
• Nobel Prize in Physics 1903 (with Pierre and Marie Curie)
Ernest Rutherford b. 1871 / d. 1937
Rutherford & Soddy (1903) Radioactive Change. Phil Mag ser 6, v 576-91.
• Atomic "disintegration" theory of radioactivity
• Nobel Prize in Chemistry 1908
• Isotopic Dating (or radioisotopic dating)
• Isotopes and Radioactive Decay
–Isotopes
• Atoms of an element with differing number of neutrons, but same number of protons
–Radioactive Decay
• Spontaneous nuclear disintegration of unstable isotopes
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Radioactive decay of Uranium to Lead
Helium atoms neutron > proton
–Parent isotope decays to daughter product
• rate of decay measured by physicists
• decay process is “exponential”, not linear
• half-life
– Time it takes for half the parent isotope to decay
– After one half-life, equal amounts of parent and daughter isotopes are present in a mineral
• decay constant
– Portion of isotope that decays per unit time
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Exponential
Decay of parent isotope and
Increase of daughter isotope
Linear
Decay of burning candle
• Relationship between time and radioactive decay
Constant = 2.718
Number of atoms of isotope at time “t”
N = N o e λ t
Decay constant: portion of isotope that decays per unit time
Number of atoms of isotope when clock was set
• How do we measure λ ? Half-life concept:
λ = ln 2/t half-life
= 0.693/t half-life
• Age of mineral: t = (1/ λ )*ln(N/N o
) we measure N (# atoms parent isotope) we measure D (# atoms daughter) to get N o
[N o
= N + D]
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• Uses of Isotopic Dating
– Age of eruption of lava, ash
• ash can allow dating of sediments in many cases
– Age of intrusion
– Age of metamorphism
• Reliability of Isotopic Dating ?
– Multiple methods: U-Pb, Rb-Sr, K-Ar, etc.
– Combine with Relative Dating Principles
– Powerful means to understand time scales and rates of geological processes
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• Arthur Holmes b. 1890 / d. 1965
Holmes (1913) The Age of the Earth, Harper & Brothers
• First geologist to apply radioactivity to dating rocks
• First numerical timescale (counters Lord Kelvin)
• Combined U-Pb date from Norway with Boltwood's results
• Phanerozoic >400 myr long
• Earth is >4 billion yrs old
Holmes (1928) Radioactivity and Earth movements.
Transactions Geological Society of Glasgow, v. 18, 559-
606.
•Suggested that mantle flows due to radioactive heating, supporting Wegner's theory of continental drift, 30 years before plate tectonic theory !
K-Ar dating of the Green River Formation, Wyoming
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K-Ar dating feldspar crystals in volcanic ash beds
• Based on Fossil Assemblages & Radioisotopic dating
• Eras, Periods, and Epochs
– Cenozoic Era
• Tertiary & Quaternary Periods
– Recent (Holocene) Epoch
– Mesozoic Era
– Paleozoic Era
• Precambrian - All time before Paleozoic Era
– 88% of Earth History
– no megascopic fauna (bacteria)
– radioisotopic dating essential
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– 4.4 billion year old crust
4,560,000,000
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Geologic Time Scale
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