Review – Relative Dating Principle of Original Horizontality Principle of Superposition Principle of Cross-Cutting Rela

1. I. Principles of Relative Dating Review – Relative Dating Principle of Original Horizontality Principle of Superposition Principle of Cross-Cutting Relationships Inclusions Principle of Faunal Successions

2. I. Principles of Numerical Dating Numerical Dating Determining the age of a rock or the Earth’s surface in years Based on some type of natural clock (e.g., annual tree or coral growth rings)

3. I. Principles of Numerical Dating • Isotope Dating—based on the rate of decay of radioactive isotopes within rocks • A. Structure of atoms—very small, one hundred-millionth of a centimeter • Atoms: smallest particles of an element that retain all of the element’s chemical properties • Nucleus: • Protons, positive charge • Neutrons, neutral charge • Electronsorbit: if nucleus is a basketball, electrons up to 3 km away

4. I. Principles of Numerical Dating I. Isotope Dating: based on the rate of decay of radioactive isotopes within rocks 1. Charge: Protons + Neutrons (neutral) Electrons - Atoms always have the same # of protons and electrons so the charge is balanced. 2. Mass: Protons 1 AMU or atomic mass unit = 1.67x10-24 gms Neutrons = 1 AMU Electrons = mass negligible

5. I. Principles of Numerical Dating I. Isotope Dating: based on the rate of decay of radioactive isotopes within rocks 3. Atomic Number = number of protons Atomic number determines an atoms identity H=1, Mg=12, O=8, Fe=26 4. Atomic Mass = protons + neutrons Can change in different atoms of the same element 5. Isotopes: atoms of the same element (same atomic #), with different # of neutrons For example Oxygen (8 protons): 8 neutrons 16O 9 neutrons 17O 10 neutrons 18O

6. I. Principles of Numerical Dating • I. Isotope Dating: based on the rate of decay of radioactive isotopes within rocks • B. What happens during radioactive decay • Radioactive isotopes: have nuclei that spontaneous decay by emitting or capturing a variety of subatomic particles (protons, electrons, etc.) • Unstable isotopes  stable isotopes

7. C. How it works (radioactive decay) 1. Decay rates of radioactive atoms are constant 2. Unaffected by temperature, pressure, water, etc. 3. Half Life: time it takes for half the atoms of the parent isotope to decay, ranges from tens of billions of years to thousandths of a second. Percentage of parent atoms that decay in each half life is the same (50%) The actual number of atoms that decay with each passing half-life continually decreases I. Principles of Numerical Dating Increase in daughter = decrease in parent

8. I. Principles of Numerical Dating • How it works (radioactive decay) • 4. When mineral crystallizes (e.g., zircon mineral) • a. some amount of radioactive parent may get incorporated into the mineral structure • b. but daughter is different element and doesn’t “fit” in the structure(or inert and won’t combine), so at the time of formation there is 100% parent and 0% daughter

9. I. Principles of Numerical Dating • How it works (radioactive decay) • 4. When mineral crystallizes (e.g., zircon mineral) • c. With a given time, the amount of parent slowly declines and amount of daughter increases within the mineral. • d. So we can go back and measure the ratio of parent to daughter (knowing the half life) and determine when the mineral crystallized.

10. I. Principles of Numerical Dating • Radioactive isotopes are incorporated in minerals and rocks in a variety of ways. • As minerals crystallize from magma, radioactive isotopes are included in mineral crystal structure. • At the time of crystallization, only parent isotopes are included in the mineral. • Radioactive parent isotopes then begin to decay producing daughter isotopes.

11. I. Principles of Numerical Dating ISOTOPE DATING uses this process to measure the amount of time elapsed since the mineral’s formation. With time, the amount of parent isotope will decrease and the amount of daughter isotope will increase. The DECAY RATE is constant and acts like a “clock”. Decay rates are not affected by temperature, pressure, or chemical reaction with the parent isotope. By measuring the ratio of parent to daughter isotopes in the mineral and comparing it with the rate of radioactive decay, we can determine the numerical age of a rock.

12. I. Principles of Numerical Dating • D. Dating Rocks • Igneous rocks – the best! Dates when the minerals formed • 2. Metamorphic: during metamorphism ions can migrate, so dating tells us when metamorphism ended. • 3. Sedimentary rocks: more errors because it dates the age of the individual pieces, gives maximum age

13. I. Principles of Numerical Dating • How it works (radioactive decay)

14. I. Principles of Numerical Dating • How it works (radioactive decay)

15. I. Principles of Numerical Dating • How it works (radioactive decay)

16. I. Principles of Numerical Dating • How it works (radioactive decay)

17. I. Principles of Numerical Dating E. Sources of Error Groundwater can bring in new ions and carry old ions away  Sample unfractured and unweatheredrock, makes the age of the rock too young) Very young rocks: not enough time to accumulate daughter isotopes to be measured accurately So we date many different samples and make sure the ages agree We date using different dating methods to make sure they agree

18. II. Types of Isotope Dating We pick dating methods based on approximate age of the rock and the rock composition Old rocks – long half lives Young rocks – short half lives Composition: have to use a dating method corresponding to what is in the rock

19. I. Principles of Numerical Dating • II. Types of Isotope Dating (page 252, table 8.1) • 1. Uranium-thorium-lead (granite) • 2. Rubidium-Strontium • plagioclase feldspar (igneous and metamorphic rocks) • 3.Potassium-Argon • lots of minerals (plagioclase, biotite, muscovite, amphibole) • Argon is inert gas and will not combine into any minerals

20. I. Principles of Numerical Dating II. Types of Isotope Dating 4. Carbon 14 (radiocarbon dating) 14C  14N 5730 year ½ life Useful between 100 and about 50,000 years old Can date things that contain organic carbon (Used to be living): bones, shells, wood, charcoal, plants, paper, cloth, pollen, seeds)

21. I. Principles of Numerical Dating II. Types of Isotope Dating 4. Carbon 14 (radiocarbon dating) Anything living takes in carbon in the form of CO2 (stable and unstable) by eating and drinking and photosynthesis As long as the organism is living, it is taking in more 14C but when it dies, it’s 14C starts declining (12C remains constant) Longer time since it died  less 14C

22. I. Principles of Numerical Dating III. Other Dating Techniques: Besides isotopic methods A. Dendrochronology (Tree-ring dating) Trees grow rings for each year We can count rings to get ages of trees Pronounced changes in climate (i.e. drought) causes distinct patterns that can then be correlated between trees Useful for dating: landslides, avalanches, or mudflows or wooden artifacts

23. I. Principles of Numerical Dating III. Other Dating Techniques: Besides isotopic methods A. Dendrochronology (Tree-ring dating) Trees grow rings for each year We can count rings to get ages of trees Pronounced changes in climate (i.e. drought) causes distinct patterns that can then be correlated between trees Useful for dating: landslides, avalanches, or mudflows or wooden artifacts

24. I. Principles of Numerical Dating III. Other Dating Techniques: Besides isotopic methods B. Varve chronology (lake deposits) Lakes produce annual layers of sediment much like tree rings Spring & summer  high sediment input  thick, coarse, light-colored layers Winter little to no sediment, especially when covered in ice—very fine stuff deposited which has been floating around for months  dark, thin layers Useful for dating: landslides into the lake

25. Origin of Lake Varves (Summer)

26. Origin of Lake Varves (Winter)

27. I. Principles of Relative Dating III. Other Dating Techniques: Besides isotopic methods C. Lichenometry (dating lichen colonies) Lichen—simple plant-like colonies the grow on exposed rock For similar rocks and similar climate: the larger the lichen colony, the longer the time since the growth surface was exposed Develop a growth curve based on measuring lichen of known age (tombstones, buildings) then extrapolate/interpolate to age of unknown rock Useful for dating: glacial deposits, rockfalls, mudflows (expose new rock to surface)

28. Review Principles of Absolute Dating • Radioactive isotopes are incorporated in minerals. • As minerals crystallize from magma, radioactive isotopes are included in mineral crystal structure. • At the time of crystallization, only parent isotopes are included in the mineral. • Radioactive parent isotopes then begin to decay producing daughter isotopes. Isotope Dating: based on the rate of decay of radioactive isotopes within rocks

29. Review Principles of Absolute Dating Carbon 14 14C  14N Short ½ life Useful between 100 and about 50,000 years old Can date things that contain organic carbon (Used to be living): bones, shells, wood, charcoal, plants, paper, cloth, pollen, seeds) Isotope Dating: based on the rate of decay of radioactive isotopes within rocks

30. I. Principles of Numerical Dating III. Other Dating Techniques: Besides isotopic methods A. Dendrochronology (Tree-ring dating) Useful for dating: landslides, avalanches, or mudflows or wooden artifacts B. Varve chronology (lake deposits): Useful for dating: landslides into the lake C. Lichenometry (dating lichen colonies): Useful for dating: glacial deposits, rockfalls, mudflows (expose new rock to surface)