Grand Canyon

1. Grand Canyon • When looking down into the Grand Canyon, we are really looking all the way back to the early history of Earth Chapter 4: Geologic Time—Concepts and Principles

2. Relative Geologic Time Scale • The relative geologic time scale has a sequence of • eons • eras • periods • epochs • but no numbers indicating how long ago each of these times occurred

3. Concept of Geologic Time • Relative Dating – putting rock layers and events in order relative to when they occurred. • Absolute dating which results in specific numerical dates for rock units or events • Such dates are calculated from the natural rates of decay of various natural radioactive elements present in trace amounts in some rocks

4. The discovery of radioactivity near the end of the 1800s allowed absolute ages to be accurately applied to the relative geologic time scale The most recent geologic time scale model is a dual scale a relative scale and an absolute scale Geologic Time Scale -- today Fig. 4-1, p. 62

5. Changes in the Concept of Geologic Time • Attempts to give an age to the earth • James Usher (1581-1665) in Ireland • calculated the age of Earth based on genealogies in Genesis • Announced that Earth was created on October 22, 4004 B.C. • A century later it was still considered heresy to say Earth was more than about 6000 years old.

6. Changes in the Concept of Geologic Time • Georges Louis de Buffon (1707-1788) • calculated how long Earth took to cool gradually from molten iron balls • Earth about 75,000 years Others calculated the rate of sediment deposition Also the rate of salt build-up in the oceans from the continental rivers Ages in millions to billions of yearsIn 1953, the dating of meteorites was accomplished. Age ~ 4.5 by

7. Relative-Dating Principles • Six fundamental geologic principles are used today in relative dating 1. Principle of superposition • Nicolas Steno (1638-1686) • In an undisturbed succession of sedimentary rock layers, the oldest layer is at the bottom and the youngest layer is at the top • (note: Steno lived contemporaneously with Usher) • This method is used for determining the relative age of rock layers (strata) and the fossils they contain

8. Illustration of the principles of superposition

9. Relative-Dating Principles 2. Principle of original horizontality • Nicolas Steno • Sediment is deposited in essentially horizontal layers • Therefore, a sequence of sedimentary rock layers that is steeply inclined from horizontal must have been tilted after deposition and lithification

10. Illustration of the principles of original horizontality

11. Principle of Lateral ContinuityNicholas StenoSediment extends laterally in all direction until it thins and pinches out or terminates against the edges of the depositional basin

12. Principle of Cross-Cutting RelationshipsJames Hutton (1726-1797An igneous intrusion or a fault event must be younger than the rocks it intrudes or cuts across

13. Cross-cutting Relationships North shore of Lake Superior, Ontario Canada • A dark-colored dike has intruded into older light colored granite. • The dike is younger than the granite.

14. Cross-cutting Relationships Templin Highway, Castaic, California • A small fault displaces tilted beds. • The fault is younger than the beds.

15. Principle of inclusionsThat which is included is _________(older? Younger?)

16. Principle of inclusions

17. Principle of fossil succession

18. Catastrophism • Proposed by Georges Cuvier (1769-1832) • Dominated European geologic thinking! • The physical and biological history of Earth resulted from a series of sudden widespread catastrophes which accounted for significant and rapid changes in Earth and exterminated existing life in the affected area • Six major catastrophes occurred, corresponding to the six days of biblical creation.The last one was the biblical flood • (also relatively modern, and built on Usher’s Biblical age of the Earth)

19. Uniformitarianism • Principle of uniformitarianism • Present-day processes have operated throughout geologic time. This includes the physical, chemical and biological processes • Developed by James Hutton, advocated by Charles Lyell (1797-1875) • Hutton applied the principle of uniformitarianism when interpreting rocks at Siccar Point Scotland • We now call what he observed an unconformity but he properly interpreted its formation • Term uniformitarianism was coined by William Whewell in 1832

20. Unconformity at Siccar Point • the tilted, lower rocks resulted from severe upheavals that formed mountains • The mountains were then worn away and covered by younger flat-lying rocks • the erosional surface represents a gap in the rock record

21. Uniformitarianism erosion • Hutton viewed Earth history as cyclical uplift deposition • Old Earth: geologic processes operate over a vast amount of time • Modern view of uniformitarianism • Today, geologists assume that the principles or laws of nature are constant but the rates and intensities of change have varied through time

22. Sequence of Events Key to Rock Types

23. Unconformities: 3 Types 1 2 3

24. Using Radioactive Decay to obtain numerical age • Understanding absolute dating requires knowledge of atoms and isotopes • The nucleus of an atom is composed of • protons – particles with a positive electrical charge • neutrons – electrically neutral particles • electrons – the negatively charged particles – encircling the nucleus • atomic number • Equal to the number of protons • helps determine the atom’s chemical properties and the element to which it belongs

25. Isotopes • Atomic mass number= number of protons + number of neutrons • The different forms of an element’s atoms with varying numbers of neutrons are called isotopes • Different isotopes of the same element have different atomic mass numbers but behave the same chemically • Most isotopes are stable, but some are unstable • Geologists use decay rates of unstable isotopes to determine absolute ages of rocks

26. Radioactive Decay • Radioactive decay -the process whereby an unstable atomic nucleus spontaneously changes into an atomic nucleus of a different element • Three types of radioactive decay: • In alpha decay, two protons and two neutrons (alpha particle) are emitted from the nucleus.

27. Radioactive Decay • In beta decay, a neutron emits a fast moving electron (beta particle) and becomes a proton. • In electron capture decay, a proton captures an electron and converts to a neutron.

28. Radioactive Decay • Some isotopes undergo only one decay step before they become stable. • Examples: • rubidium 87 decays to strontium 87 by a single beta emission • potassium 40 decays to argon 40 by a single electron capture • But other isotopes undergo several decay steps • Examples: • uranium 235 decays to lead 207 by 7 alpha steps and 6 beta steps • uranium 238 decays to lead 206 by 8 alpha steps and 6 beta steps

29. Uranium 238 decay

30. Half-Lives • The half-life of a radioactive isotope is the time it takes for one half of the atoms of the original unstable parent isotope to decay to atoms of a new more stable daughter isotope • The half-life of a specific radioactive isotope is constant and can be precisely measured • Can vary from less than 1/billionth of a second to 49 billion years • Is geometric not linear, so has a curved graph

31. Uniform Linear Change • In this example of uniform linear change, water is dripping into a glass at a constant rate

32. Geometric Radioactive Decay • In radioactive decay, during each equal time unit, one half-life, the proportion of parent atoms decreases by 1/2

33. Determining Age • For example: • If a rock has a parent/daughter ratio of 1:3 = a parent proportion of 25%, • and the half-live is 57 million years, • 25% means it is 2 half-lives old. • the rock is 57 x 2 =114 million years old.

34. What Materials Can Be Dated? • Most radiometric dates are obtained from igneous rocks • As magma cools and crystallizes, • radioactive parent atoms separate from previously formed daughter atoms • Some radioactive parents are included in the crystal structure of certain mineralsDating of sedimentary rocks RARE: dating the mineral glauconite, because it forms in certain marine environments as a reaction with clay during the formation of the sedimentary rock

35. Igneous Crystallization • Crystallization of magma separates parent atoms • from previously formed daughters • This resets the radiometric clock to zero. • Then the parents gradually decay.

36. Sources of Uncertainty • In glauconite, potassium 40 decays to argon 40 • because argon is a gas, it can easily escape from a mineral • A closed system is needed for an accurate date • that is, neither parent nor daughter atoms can have been added or removed from the sample since crystallization • If leakage of daughters has occurred • it partially resets the radiometric clock and the age will be too young • If parents escape, the date will be too old. • The most reliable dates use multiple methods.

37. Sources of Uncertainty • During metamorphism, some of the daughter atoms may escape • leading to a date that is too young. • However, if all of the daughters are forced out during metamorphism, then the date obtained would be the time of metamorphism—a useful piece of information. • Dating techniques are always improving. • Presently measurement error is typically <0.5% of the age, and even better than 0.1% • A date of 540 million might have an error of ±2.7 million years or as low as ±0.54 million

38. Dating Metamorphism a.A mineral has just crystallized from magma. b. As time passes, parent atoms decay to daughters. c. Metamorphism drives the daughters out of the mineral as it recrystallizes. Dating the whole rock yields a date of 700 million years = time of crystallization. d. Dating the mineral today yields a date of 350 million years = time of metamorphism, provided the system remains closed during that time.

39. Long-Lived Radioactive Isotope Pairs Used in Dating • The isotopes used in radiometric dating • need to be sufficiently long-lived so the amount of parent material left is measurable • Such isotopes include: Parents Daughters Half-Life (years) Uranium 238 Lead 206 4.5 billion Uranium 235 Lead 207 704 million Thorium 232 Lead 208 14 billion Rubidium 87 Strontium 87 48.8 billion Potassium 40 Argon 40 1.3 billion

40. Fission Track Dating • Uranium in a crystal will damage the crystal structure as it decays • The damage can be seen as fission tracks under a microscope after etching the mineral • The age of the sample is related to • the number of fission tracks • the amount of uranium

41. Radiocarbon Dating Method • Carbon is found in all life • It has 3 isotopes • carbon 12 and 13 are stable but carbon 14 is not • Carbon 14 has a half-life of 5730 years • Carbon 14 dating uses the carbon 14/carbon 12 ratio of material that was once living • The short half-life of carbon 14 • makes it suitable for dating material < 70,000 years old • It is not useful for most rocks, • but is useful for archaeology • and young geologic materials

42. Carbon 14 • Carbon 14 is constantly forming in the upper atmosphere • When a high-energy neutrona type of cosmic ray strikes a nitrogen 14 atomit may be absorbed by the nucleus and eject a proton changing it to carbon 14 • The 14C formation rate • is fairly constant • has been calibrated against tree rings

43. Carbon 14 • The carbon 14 becomes part of the natural carbon cycle and becomes incorporated into organisms • While the organism lives it continues to take in carbon 14 but when it dies the carbon 14 begins to decay • without being replenished • Thus, carbon 14 dating • measures the time of death

44. Tree-Ring Dating Method • The age of a tree can be determined by counting the annual growth rings in lower part of the stem (trunk) • The width of the rings are related to climate can be correlated from tree to tree • a procedure called cross-dating • The tree-ring time scale now extends back 14,000 years

45. Tree-Ring Dating Method • In cross-dating, tree-ring patterns are used from different trees, with overlapping life spans

46. Summary • James Hutton viewed Earth history as cyclical and very long • His observations were instrumental in establishing the principle of uniformitarianism • Charles Lyell articulated uniformitarianism in a way that soon made it the guiding doctrine of geology • Uniformitarianism holds that • the laws of nature have been constant through time and that the same processes operating today have operated in the past, although not necessarily at the same rates