Chapter 9 - Geologic Time: Concepts Principles

1. Chapter 9 - Geologic Time: Concepts & Principles Introduction: John Wesley Powell expeditions through the Grand Canyon � 1869 first record-ed passage through the canyon. 1871 exped-ition included a photographer, a surveyor, and three topographers. John Wesley Powell bio Grand Canyon is fascinating because of the exposure of a long period of geologic time. Relative dating � Geologic events placed in order. Absolute dating � Specific dates for rock units or events.

6. Correlation allows us to relate events within a given region, by verifying the similar ages of rock strata and sequences. Correlation is done by Physical Criteria and Fossil Criteria Physical Criteria � Similarities in lithology, distinctive �marker beds�, sequences of contrasting beds. Figure 9.9 shows how rock exposures in the Grand Canyon, Zion National Park, and Bryce Canyon National Park are �tied together� producing the �Colorado Plateau sequence�.

7. Distinctive series of �marker beds�

8. Same sequence of �marker beds�

9. Concepts of time and the changes thereof� St. Augustine of Hippo (354 to 430 A.D.) helped establish the concept of linear time, B.C. and A.D. time scale. Bishop James Ussher (1600s) estimated Earth�s creation took place on 10/22/4004 B.C., used recorded history and Genesis geneology. John Joly (1899) estimated 90 million years for oceans to reach current salinity.

12. How we use Correlation

13. Correlation example

14. More correlation examples � Moenkopi & Organ Rock Shale

15. Moenkopi & Organ Rock Fm. at Canyonlands

20. Neptunism & Catastrophism � Biblically-based concepts of Earth creation Neptunism � Abraham Werner (1787) � suggested that all rocks precipitated from global flood (seawater), even igneous rocks (see p. 61, Table 4.1). Catastrophism � Georges Cuvier (late 18th century) � suggested Earth�s landforms were due to a series of catastrophies. Assumptions were abandoned for lack of field evidence.

24. Nonconformity � Cambrian Tapeats Ss overlying the Brahma Schist.

26. Disconformity � between the Cambrian Muav Ls and the Mississippian Redwall Ls, the entire Ordovician, Silurian, and Devonian are missing.

27. All three types are present in the Grand Canyon � see Fig. 9.6, pg. 252.

28. William Smith was the first to recognize the concept of �index fossils� or �guide fossils�, i.e., fossils that are widespread, but restricted to short geologic ranges, e.g., Inoceramus, a Cretaceous clam.

30. Radioactive isotopes, with known decay rates & parent/daughter products are used for absolute dating.

31. Atom � the smallest particle of an element with all of the characteristics of that element. Atoms consist of a nucleus composed of protons and neutrons and a shell of orbiting electrons. Atoms are neutrally charged. Isotopes - Variations of atoms with one or more extra neutrons, resulting in slightly different characteristics. Atomic number - # of protons. Atomic mass - # of protons and neutrons. Types of radioactive decay on page 64.

34. Prior to discovery of radioactive decay, James Hutton, Charles Darwin, John Wesley Powell, et al, inferred that the Earth was of immense age. Early estimates were based on how long it would take to deposit given sedimentary layers, how long would it take to reach the present salinity of the ocean, etc.. Absolute Time � Radiometric age-dating measuring parent product/daughter product ratios & known decay rates.

35. Atomic structures � shell of electrons orbiting nucleus of protons and neutrons. Atomic number - # of protons � determines position on the periodic table. Atomic mass = protons + neutrons. When there are extra neutrons present in the nucleus, these are isotopes. Atomic mass changes by 1 for each extra neutron, atomic number does not change. Isotopes may have different character-istics. In some isotopes, the nuclei are unstable and will break down over time. (Fig. 9.14).

37. Each decay has a known rate. With this known rate and the quantity of parent element vs. the daughter element measured, we can estimate the length of time of decay. The �clock� starts when the mineral solidifies from the magma. Half-Life � the amount of time needed for half of a given amount of parent product to decay to daughter product. Decay occurs on an exponential curve (Figure 9.15, pg. 262).

39. Carbon 14 (C14) is useful for dating of events <75,000 yrs. Newer information suggests, though, that atmospheric carbon 14 may vary due to solar activity. Dendrochronology � use of tree-ring data to estimate ages. Width and density of ring suggests climatic conditions (amount of rainfall, temperature, length of growing season). Reference chronologies established for given areas. Wood fragments with a few rings may be matched against reference datum.

40. Fig. 9.14 (pg. 261) shows decay of U238 through 13 intermediate, unstable daughter products until stable daughter product Pb206 is reached. Radiometric ages are �best available science� estimates. Oldest granite � South Africa � dated at 3.2 billion years, contains quartzite inclusions (older rock). Quartzite is likely the product of metamorphism of sandstone, which was derived from the weathering of still older rock.

42. Next smaller units � Eras Precambrian � Hadean (5.4 to 3.8 b.y.); Archaean (3.8 to 2.5 b.y.); and Proterozoic (2.5 b.y. to 540 m.y.) Phanerozoic � Paleozoic (540 to 248 m.y.); Mesozoic (248 to 65 m.y.); Cenozoic (65 m.y. to present) Eras divided into Periods, Periods are divided into Epochs. We live in the Holocene (Recent) Epoch � last 12,000 yrs.