Telling Geologic Time Trilobite fossils these arthropods went extinct during the great extinction at the end of the Permian Period ~250 Ma ka = kilo-annum Ma = Mega-annum Ga = Giga-annum Relative dating telling if one layer or geologic feature is older or younger than another Absolute dating calculating a numerical age using radiometric dating (with unstable isotopes) or other techniques (TL, amino acids, tree rings) James Hutton The Father of Geology Uniformitarianism 1788 James Hutton: English farmer and intellectual: envisioned geologic deep time: "no vestige of a beginning, no prospect of an end." Principles of Relative Dating Early Numerical Estimates of Earth s Age Principle of Original Horizontality Superposition Cross Cutting Relationships Inclusions Faunal Succession million years Cooling of Earth.075-1,280 Cooling of Sun 4.4-500 Orbital physics <10-3,700,000 Salt in ocean 25-2,350 Sediment accumulation 3-15,000 Radioactive elements 1,340-4,600 Copyright Houghton Mifflin Company. All rights reserved. Essentials of Geology, 8a 4 Buffon s Age of Earth (1749) years Surface of Earth consolidated 1 Earth consolidated to its center 2,936 Earth cool enough to be touched 34,270 Beginning of life 35,983 Temperature of present reached 74,832 End of life 168,123 Copyright Houghton Mifflin Company. All rights reserved. Essentials of Geology, 8a 5 History of thought on geotime Estimating an absolute age for the Earth 1654 Archbishop Ussher, biblical literalist: added ages in the Bible: Earth born: October 26, 4004 B.C 9:00 AM 1749 George de Buffon - Historie Naturelle: Iron ball hypothesis. Earth ~ 75,000 yr old 1775 Abraham Werner,, Freiberg Germany. Neptunist: all rocks precipated. 1788 James Hutton: English farmer: deep time: "no vestige of a beginning,--no prospect of an end." Copyright Houghton Mifflin Company. All rights reserved. Essentials of Geology, 8a 6 1
1812 George Cuvier,, French zoologist working in Paris basin: recognized fossil life and breaks in stratigraphic record. Inferred catastrophes caused extinctions. Embraced by biblical literalists (Biblical deluge) 1867 Chas. Lyell: : 20 Ma needed for complete change of molluscan species. 12 of these events since Ordovician time, hence, Earth >240 Ma 1897 Lord Kelvin: : used thermodynamics to estimate cooling of Earth, and age from molten state: 24-40 40 Ma 1899 John Joly, Irish geologist: used amount of salt in ocean and rate of salt added annual to infer age of the Earth at 90 Ma Copyright Houghton Mifflin Company. All rights reserved. Essentials of Geology, 8a 7 the breakthrough RADIOACTIVITY!! 1895 => Bequerel discovered uranium activated his photo plates in total darkness. 1903 Marie and Pierre Curie => isolated radium 1905 => Ernest Rutherford and B.B. Boltwood=>radioactive elements disintegrate ( parent and daughter ). Both dated rocks, but early ages were too high 1913 => Frederick Soddy clarified the nature of isotopes leading to refined methods and accurate ages 1947 Libby => radiocarbon dating 1947 Libby The age of Geochronology was born!! Copyright Houghton Mifflin Company. All rights reserved. Essentials of Geology, 8a 8 (1) Principle of Original Horizontality 2. Principle of Superposition: the rocks on top are younger (unless??) Youngest Strata Oldest Strata Because of gravity, layers form horizontally. Thus tilted layers imply passage of geologic time as well as structural deformation (3) Principle of Crosscutting Relationships e.g. dikes and faults must be younger than the rocks they cut through (4) Principle of Inclusions The inclusions have to be older than the rocks that contain them. 2
(5) Principle of Faunal Succession Example of Relative Time 1 Sketch by Baron Cuvier (1769-1832) Sedimentary strata contain fossils that succeed each other vertically in a specific, reliable order that can be correlated over wide horizontal distances Example 2 of Relative Time Unconformities Gaps in the Rock = Gaps in Time Unconformity an angular unconformity to be specific! For practice, write the order of the units, faults, or intrusions. Types of Conformity - Angular Unconformity Formation of an Angular Unconformity 3
Angular Unconformity in the Grand Canyon The Angular Unconformity at Siccar Point, Scotland This is one of the outcrops that blew Hutton s mind in the mid to late 1780s! Source: Edward A. Hay, De Anza College, Cupertino, CA Example of Correlation Colorado Plateau Correlation Colorado Plateau Correlation Dating using unstable isotopes Isotopes and Radioactive Decay Isotopes Varieties of same element with differing number of neutrons, but same number of protons Radioactive Decay - Spontaneous nuclear disintegration of unstable isotopes Daughter Product results from decay of parent product Half-life time over which half the original parent isotopes decays. Various techniques (e.g. U-Pb, K-Ar, radiocarbon dating) Copyright Houghton Mifflin Company. All rights reserved. Essentials of Geology, 8a 24 4
Half life: The time in which half of the parent elements will decay into the daughter elements The time period in which each atom has a 50% chance of decaying. Figure 7.17b Radioactive Clocks Half-life Radioactive parent Stable daughter ( 238 U) ( 206 Pb) now One half-life Two half-lives Copyright Houghton Mifflin Company. All rights reserved. Essentials of Geology, 8a 25 Copyright Houghton Mifflin Company. All rights reserved. Essentials of Geology, 8a 26 Radiometric Dating Methods NUCLIDES METHOD Potassium-Argon Rubidium-Strontium Samarium-Neodymium Lutetium-Hafnium Rhenium-Osmium Uranium-Lead Uranium-Lead Lead-Lead 40 K - 40 Ar 87 Rb - 87 Sr 147 Sm - 143 Nd 176 Lu - 176 Hf 187 Re - 187 Os 235 U - 207 Pb 238 U - 206 Pb ---- HALF-LIFE (billion years) 1.25 48.8 106 35.9 43.0 0.704 4.47 ---- Copyright Houghton Mifflin Company. All rights reserved. Essentials of Geology, 8a 27 A typical lab where this is done Copyright Houghton Mifflin Company. All rights reserved. Essentials of Geology, 8a 28 What can be dated? Copyright Houghton Mifflin Company. All rights reserved. Essentials of Geology, 8a 29 Copyright Houghton Mifflin Company. All rights reserved. Essentials of Geology, 8a 30 5
Fig. 08.25 Age of the eras and Earth Oldest Earth rock 4 b.y. Moon rocks dated at 4.2 b.y. Dating of meteorites Between 4.5 & 4.6 billion years old Copyright Houghton Mifflin Company. All rights reserved. Essentials of Geology, 8a 31 Copyright Houghton Mifflin Company. All rights reserved. Essentials of Geology, 8a 32 http://pubs.usgs.gov/fs/2007/3015/f s2007-3015.pdf Link to updated geologic time scale. The Geologic Time Scale see also pp. x xi in book Comprehending geologic time Copyright Houghton Mifflin Company. All rights reserved. Essentials of Geology, 8a 33 Copyright Houghton Mifflin Company. All rights reserved. Essentials of Geology, 8a 34 6