Geologic Time— Concepts and Principles

1. Chapter 8 Geologic Time—Concepts and Principles

2. What is time? • We are obsessed with time, using • clocks • calendars • appointment books • Mostly we don’t have enough of it. • Our common time units are • seconds • hours • days • weeks • months • years • Ancient history involves • hundreds of years • thousands of years • But geologic time involves • millions of years • even billions of years

3. Grand Canyon • When looking down into the Grand Canyon, we are really looking all the way back to the early history of Earth

4. Grand Canyon • More than 1 billion years of history are preserved, • like pages of a book, • in the rock layers of the Grand Canyon • Reading this rock book we learn • that the area underwent episodes of • mountain building • advancing and retreating shallow seas • We know these things by • applying the principles of relative dating to the rocks • and recognizing that present-day processes • have operated throughout Earth history

5. Concept of Geologic Time • Geologists use two different frames of reference • when discussing geologic time • Relative dating involves placing geologic events • in a sequential order as determined • from their position in the geologic record • It does not tell us how long ago • a particular event occurred • only that one event preceded another • For hundreds of years geologists • have been using relative dating • to establish a relative geologic time scale

6. 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

7. Geologic Column and the Relative Geologic Time Scale Absolute ages (the numbers) were added much later.

8. Concept of Geologic Time • The second frame of reference for geologic time • is absolute dating • Absolute dating results in specific dates • for rock units or events • expressed in years before the present • It tells us how long ago a particular event occurred • giving us numerical information about time • Radiometric dating is the most common method • of obtaining absolute ages • Such dates are calculated • from the natural rates of decay • of various natural radioactive elements • present in trace amounts in some rocks

9. Geologic Time Scale • The discovery of radioactivity • near the end of the 1800s • allowed absolute ages • to be accurately applied • to the relative geologic time scale • The geologic time scale is a dual scale • a relative scale • and an absolute scale

10. Changes in the Concept of Geologic Time • The concept and measurement of geologic time • has changed through human history • Early Christian theologians • conceived of time as linear rather than circular • James Usher (1581-1665) in Ireland • calculated the age of Earth based • on recorded history and genealogies in Genesis • He announced that Earth was created on October 22, 4004 B.C. • A century later it was considered heresy to say Earth was more than about 6000 years old.

11. Changes in the Concept of Geologic Time • During the 1700s and 1800s Earth’s age • was estimated scientifically • Georges Louis de Buffon (1707-1788) • calculated how long Earth took to cool gradually • from a molten beginning • using melted iron balls of various diameters • Extrapolating their cooling rate • to an Earth-sized ball, • he estimated Earth was 75,000 years old

12. Changes in the Concept of Geologic Time • Others used different techniques • Using rates of deposition of various sediments • and thickness of sedimentary rock in the crust • gave estimates of <1 million • to more than 2 billion years. • Using the amount of salt carried • by rivers to the ocean • and the salinity of seawater • John Joly in 1899 • obtained a minimum age of 90 million years

13. 5 Relative-Dating Principles • Six fundamental geologic principles are used in relative dating • 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 • This method is used for determining the relative age • of rock layers (strata) and the fossils they contain

14. Relative-Dating Principles • 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

15. Principle of Horizontality • Illustration of the principles of superposition • and original horizontality • Horizontality: These sediments were originally • deposited horizontally • in a marine environment • This outcrop is Chattanooga Shale, Tennessee

16. Principle of Superposition • Illustration of the principles of superposition • and original horizontality • Superposition: The youngest • rocks are at the top • of the outcrop • and the oldest rocks are at the bottom

17. Relative-Dating Principles • Principle of lateral continuity • Nicolas Steno • Sediment extends laterally in all direction • until it thins and pinches out • or terminates against the edges • of the depositional basin • Principle of cross-cutting relationships • James Hutton (1726-1797) • An igneous intrusion or a fault • must be younger than the rocks • it intrudes or displaces

18. 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.

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

20. Early Geology- Neptunism • Neptunism • All rocks, including granite and basalt, • were precipitated in an orderly sequence • from a primeval, worldwide ocean. • proposed in 1787 by Abraham Werner (1749-1817) • Werner was an excellent mineralogist, • but is best remembered • for his incorrect interpretation of Earth history

21. Neptunism • Werner’s geologic column was widely accepted • Alluvial rocks • unconsolidated sediments, youngest • Secondary rocks • rocks such as sandstones, limestones, coal, basalt • Transition rocks • chemical and detrital rocks, some fossiliferous rocks • Primitive rocks • oldest including igneous and metamorphic

22. …later….Catastrophism • 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

23. Neptunism and Catastrophism Were Eventually abandoned • These hypotheses were abandoned because • they were not supported by field evidence • Basalt was shown to be of igneous origin • Volcanic rocks interbedded with sedimentary • and primitive rocks showed that igneous activity • had occurred throughout geologic time • More than 6 catastrophes were needed • to explain field observations • The principle of uniformitarianism • became the guiding philosophy of geology

24. Uniformitarianism • Principle of uniformitarianism • Present-day processes have operated throughout geologic time. • Developed by James Hutton, advocated by Charles Lyell (1797-1875) • Term uniformitarianism was coined • by William Whewell in 1832 • 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

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

26. Unconformity at Siccar Point

27. Uniformitarianism erosion erosion • Hutton viewed Earth history as cyclical deposition uplift • He also understood • that 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

28. Crisis in Geology • Lord Kelvin (1824-1907) • knew about high temperatures inside of deep mines • and reasoned that Earth • is losing heat from its interior • Assuming Earth was once molten, he used • the melting temperature of rocks • the size of Earth • and the rate of heat loss • to calculate the age of Earth as • between 400 and 20 million years

29. Crisis in Geology • This age was too young • for the geologic processes envisioned • by other geologists at that time, • leading to a crisis in geology • Kelvin did not know about radioactivity • as a heat source within the Earth

30. Absolute-Dating Methods • The discovery of radioactivity • destroyed Kelvin’s argument for the age of Earth • and provided a clock to measure Earth’s age • Radioactivity is the spontaneous decay • of an atom’s nucleus to a more stable form • The heat from radioactivity • helps explain why the Earth is still warm inside • Radioactivity provides geologists • with a powerful tool to measure • absolute ages of rocks and past geologic events

31. Atoms • Understanding absolute dating requires • knowledge of atoms and isotopes • All matteris made up of atoms • The nucleus of an atom is composed of • protons – particles with a positive electrical charge • neutrons – electrically neutral particles • with electrons – negatively charged particles – encircling the nucleus • The number of protons(= the atomic number) • helps determine the atom’s chemical properties • and the element to which it belongs

32. 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

33. Radioactive Decay • Radioactive decay is 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.

34. 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.

35. 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

36. Uranium 238 decay

37. 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

38. Half-Lives • The length of half-lives for different isotopes • of different elements • can vary from • less than 1/billionth of a second • to 49 billion years • Radioactive decay • is geometric not linear, • so has a curved graph

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

40. Determining Age • By measuring the parent/daughter ratio • and knowing the half-life of the parent • which has been determined in the laboratory • geologists can calculate the age of a sample • containing the radioactive element • The parent/daughter ratio • is usually determined by a mass spectrometer • an instrument that measures the proportions • of atoms with different masses

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

42. 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 • Because they fit, some radioactive parents • are included in the crystal structure • of certain minerals

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

44. Not Sedimentary Rocks • Generally, sedimentary rocks cannot be radiometrically dated • because the date obtained • would correspond to the time of crystallization of the mineral, • when it formed in an igneous or metamorphic rock, • not the time that it was deposited as a sedimentary particle • Exception: dating the mineral glauconite, • because it forms in certain marine environments as a reaction with clay • during the formation of the sedimentary rock

45. 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.

46. 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

47. 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) Most of these are useful for dating older rocks Uranium 238 Lead 206 4.5 billion Uranium 234 Lead 207 704 million Thorium 232 Lead 208 14 billion Rubidium 87 Strontium 87 48.8 billion Potassium 40 Argon 40 1.3 billion

48. II. 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 • and the amount of uranium • with older samples having more tracks • This method is useful for samples between 1.5 and 0.04 million years old

49. III. 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

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