1. Absolute & Relative Dating Alex Burkett, Eric Francey, Juliet Collados

2. Introduction Geologists estimate that the earth is about 4.6 billion years old James Hutton → Principle of Uniformitarianism: current geologic processes are the same processes that were at work in the past Before Hutton’s work, many believed that the earth was only about six thousand years old and all geological features had been formed at the same time Hutton’s observation and conclusions about the age of the earth encouraged other to learn more about the earths history

3. Relative Age vs. Absolute Age Strata: layers if rocks Stratigraphy: the study of strata Relative age indicates that one layer of rock is older or younger than another layer → it does not indicate the exact age of the rock Absolute Age indicates the actual age of a layer of rock

4. Relative Dating - Superposition Formation of sedimentary rock begins when sediments are deposited horizontally; as sediments accumulate, they are compressed and harden into sedimentary rock layers Law of Superposition: an undeformed sedimentary rock layer is older than the layers above it and younger than the layers below it

5. Unconformities Some rock layers are deformed, causing unconformities Caused by movements in the earths crust There are three kinds of unconformities →All rocks below an unconformity are older than those rocks above it ► Nonconformity: an unconformity in which stratified rock rests upon unstratified rock ► Rocks deposited in horizontal layers are folded or tilted and then eroded; the boundary between the tilted layers and the horizontal layers is called an angular unconformity

6. Unconformities Con’t… ► disconformity: on the ocean floor; the boundary between older, eroded surface, and younger, overlying layers is nearly horizontal

7. Absolute Dating – Cross Cutting Sometimes tectonic activity disturbs rock layers and causes either an intrusion of igneous rock into the strata, or a fault (break or crack) in the earths crust, shifting rocks positions → determining age using the law of superposition may be too difficult in these scenarios Law of Crosscutting Relationships: a fault or an intrusion is always younger than the rock layers that it cuts through

8. Absolute Dating – Rate of Erosion If scientists can determine the rate at which a stream erodes its bed, the approximate age can be estimated → edge of Niagara Falls is eroding at an average rate of 1.3 m/year → based on the average rate of erosion, scientists determined the falls formed approximately 9,900 years ago This method is only accurate with geological features that formed within the past 10,000 to 20,000 years

9. **Older features that have developed over a greater amount of time have varying rates of erosion

10. Absolute Dating – Rate of Deposition Geologists collect data over long periods of time and then estimate the average rates of deposition for common sedimentary rocks (such as limestone, shale, and sandstone) On average 30cm of sedimentary rock are deposited over a period of 1000 years, however, this method is not always accurate → it is possible that any layer may not have been deposited at an average rate → i.e. floods deposit many metres of sediment in just one day

11. Absolute Dating – Varve Count Varve: shows definite annual layers; consists of a light coloured band of course particles and a darker band of fine particles → similar to estimating the age of a tree by counting the growth rings in its trunk Varves usually occur in glacial lakes During the summer snow and ice melt rapidly; this rush of water carries large amounts of sediment into the lake Coarse particles quickly form a layer on the bottom of the lake

12. Varve count con’t… Winter freezes the surface of the lake causing fine clay particles to settle slowly and form a think layer on top of the coarse sediments Each varve represents one year of deposition

13. Absolute Age – Radioactive Decay Rocks often emit radioactive isotopes: particles that have a nuclei that emit particles and energy at a constant rate Radioactive decay occurs when an atom emits particles and energy The atom changes into a different isotope of the same element or an isotope of a different element Radioactive decay continues until a stable, or nonradioactive, form of an element is produced Scientists measure the concentrations of the original radioactive isotope and the newly created isotopes The proportions are compared of the new and the original isotopes to determine the absolute age of the rock

14. Absolute Age – Half Life A half life is the time it takes for half the mass of a given amount of a radioactive element to decay into its daughter elements The half life for U-238 (Uranium) to decay into P-206 (Lead) is 4.5 billion years →if you were to begin with 10g of U-238, in 4.5 billion years you would have 5g →after another 4.5 billion years you would have 2.5g By comparing the amount of the radioactive element and its daughter elements in some rock samples, scientists can determine the age of the sample The greater the percentage of the daughter element present, the older the sample is

15. Half Life con’t… The amount of time that has passed since the rock was formed determines which radioactive element will give the more accurate age measurement U-238 (Uranium) is most useful in dating geologic samples more that 10 million years old K-40 (Potassium-40) is used to date rocks between 50,000 and 4.6 billion years old (has a half life of 1.3 billion years) Rb-87 (Rubidium-87); it can be used to verify age of rocks previously dated with K-40 and has a half life of 47 billion years

16. Absolute Dating – Carbon Dating To determine the age of once living things scientists use carbon dating Living plants and animals absorb Carbon-12 and Carbon-14 in the form of CO2 during photosynthesis While the organisms are alive, the ratio of the two elements remains relatively constant When the organism dies, the amount of C-14 decreases as the radioactive carbon atoms decay to nonradioactive nitrogen-14

17. Carbon Dating con’t… The half life of C-14 is about 5,730 years To establish age, scientists determine the proportion of C-12 to C-14 in the sample and compare it with the proportion of C-14 to C-12 known to exist in a living organism Carbon dating can establish the age of samples up to 70,000 years old

18. Application of Absolute & Relative Dating Olduvai Gorge – the “Cradle of Mankind”

19. Application of Absolute & Relative Dating The Shroud of Turin

20. Final Summary A reoccurring need in scientific areas such as archaeology and geology is the need to accurately date material The different methods of absolute and relative dating provide scientists with various tools to come to these conclusions These methods help us to better understand both the world we live in and how we have lived in it

21. Guessing Game!

22. ~141 to 65 millions years old Started to form ~17 million years ago ~11,000 years old

23. All Done! Thanks for your time

24. Works Cited Fagan, Brian M., People of the Earth An Introduction to World Prehistory Fifth Edition. Boston: Little, Brown and Company, 1986. Jolly, Clifford J., and Plog, Fred. Physical Anthropology and Archaeology Third Edition. New York: Alfred A. Knopf, 1982. Pillips, Clifford R., Ramsey, William L., Sager, Robert J., and Watenpaugh, Frank M. Modern Earth Science. Toronto: Holt, Rinehart, and Winston, 2002.