1 Geologic time

2 John Wesley Powell’s expedition to the Grand Canyon in 1869 “Rocks conceal evidence for an ancient Earth” 3 Review: Origin of the solar system 4 Review: the historical view of Aristotle’s view of geology was adhered to until the Middle Ages Rocks created under the “influence” of stars Earthquakes caused by heated air escaping explosively Fishes lived in the Earth – motionless (fossils) 5 Review: Catastrophism 6 Determining geological ages Relative age dates – placing rocks and events in their proper sequence of formation Numerical dates – specifying the actual number of years since an event occurred (known as absolute age dating) 7 Principles of 1) Nicolaus Steno,1669 2)Principle of original horizontality 3)Principle of cross-cutting relationships 4)Inclusions 5)Unconformities 8 Principle of original horizontality 9 Grand Canyon, Fig. 8.2, p. 218 10 Principle of cross-cutting relationships Fig. 8.4, p. 219. 11 containing the inclusions is younger 12 Unconformity Def: a break in the rock record produced by erosion and/or nondeposition of rock units 13 Types of unconformities Angular unconformity – tilted rocks are overlain by more flat- lying rocks Disconformity – strata on either side of the unconformity are parallel Nonconformity – metamorphic or igneous rocks in contact


with sedimentary strata 14 Angular unconformity • ’s “Theory of the Earth,” 1785 () ()

, Scotland 15 Channel deposit showing a cross-cutting relationship 16 Cross-cutting relationship on the moon 17 Grand Canyon, Fig. 8.6 18 Correlation of rock layers Matching of rocks of similar ages in different regions is known as correlation 19 Fig. 8.9 illustration of correlation 20 Correlation often relies upon fossils Fig. 8.10

21 The 22 Isotopes and radioactive decay in dating Number of neutrons can vary Thus, an element can have more than one mass number = isotope

Ex: Oxygen has three isotopes (8,9, and 10 neutrons)

Fig. 2.4, p. 37 23 The Periodic Table of the Elements 24 Radioactivity Spontaneous changes (decay) in the structure of atomic nuclei 25 Using radioactivity in dating Parent – an unstable radioactive isotope Daughter product – isotope resulting from the decay of a parent Half-life – time required for one-half of the radioactive nuclei in a sample to decay 26 U-238 decays to Pb-206 through several steps.

Fig. 8.12, p. 228 27 Table 8.1, p. 231 28 Using radioactivity in dating Fig. 8.13, p. 229 % of atoms that decay during one half-life is always the same (50 %)

Comparing the ratio of parent to daughter yields the age of the sample 29 Using radioactivity in dating

Radioactive decay can be expressed by the formula:


-lt N = Noe

Where: N = number of parent atoms present today in the sample

No= original number of atoms (parent + daughter) e = 2.71828... (this is a constant), l= decay constant = 0.693/(half-life) t = time (years) 30 Figure showing U-238 to Lead- 206 decay 31 radiocarbon dating Half-life of only 5730 yrs Used for recent events C-14 produced in the upper atmosphere

incorporated in CO2, which is absorbed by living matter

Fig. 8.14, p. 233 32 Radiocarbon dating 33 Radiometric dating sources of error A mineral must have remained within a closed system Grains in detrital sedimentary rocks are not the same age as the rock in which they formed Particular minerals in a may not represent the time when the rock formed

34 Geologic time scale 35 Structure of the geologic time scale Eon Era •Period –Epoch

36 End of Chapter 8