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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

Principles used to determine relative age

• 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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Unconformities

• 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 GaVishnu schist)

What is the relative age of granite ?When did tilting take place ?

Reading the rock record:Timing of events on the local scale

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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

Timing of event on the regional scale: Correlation

Correlation of strata: Colorado Plateau

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Correlation

• 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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Radioactivity and GeochronologyRadioactivity and Geochronology

Ernest Rutherford b. 1871 / d. 1937

Rutherford & Soddy (1903) Radioactive Change. Phil Magser 6, v 576-91.

• Atomic "disintegration" theory of radioactivity

• Nobel Prize in Chemistry 1908

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)

Numerical Age• 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 atomsneutron > proton

Numerical Age• Radioactive Decay

–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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ExponentialDecay of parent isotope

and Increase of daughter isotope

LinearDecay of burning candle

Calculating the age of a Mineral• Relationship between time and radioactive decay

N = Noe-λtDecay constant: portion of isotopethat decays per unit timeNumber of atoms of

isotope at time “t” Number of atoms of isotopewhen clock was set

Constant = 2.718

• How do we measure λ ? Half-life concept:λ = ln 2/thalf-life = 0.693/thalf-life

• Age of mineral:t = (1/λ)*ln(N/No)

we measure N (# atoms parent isotope)we measure D (# atoms daughter) to get No [No = N + D]

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Numerical Ages• 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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Modern GeochronologyModern Geochronology

•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'sresults

• 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

Evolving Geologic Time Scale• 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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Age of the Earth• Early speculation• Dating of meteorites• U-Pb dating of zircon crystals

– 4.4 billion year old crust• The Earth is 4.56 billion years old

4,560,000,000• Comprehending Geologic Time

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Geologic Time Scale

Mill

ions

of Y

ears