1. 1100 Ma Absolute Geologic Dating GLG 101 - Physical Geology Bob Leighty

2. These notes and web links are your primary “lecture” content in this class. Additionally, various articles are assigned each week to supplement this “lecture” information. I believe you’ll have enough information to reference without having to purchase a costly textbook. These lecture notes are very similar to the ones I use in my traditional classes. You’ll find they are loaded with imagery and streamlined text that highlight the most essential terms and concepts. The notes provide a framework for learning and, by themselves, are not meant to be a comprehensive source of information. To take advantage of the global knowledge base known as the Internet, I have included numerous hyperlinks to external web sites (like the Wikipedia, USGS, NASA, etc.). Follow the links and scan them for relevant info. The information from linked web sites is meant to supplement and reinforce the lecture notes – you won’t be responsible for knowing everything contained in them. As a distance learning student, you need to explore and understand the content more independently than in a traditional class. As always, I will help guide you through this learning adventure. Remember, email Dr. Bob if you have any questions about today’s lecture (rleighty@mail.mc.maricopa.edu).rleighty@mail.mc.maricopa.edu Leave no questions behind! Explore and have fun!

3.  Isotopic dating measures the decay of radioactive isotopes to date rocks  Dating rocks = GeochronologyGeochronology Absolute Geologic Dating  Certain isotopes of some elements are unstable and decay over timeisotopes Periodic Table of the Elements  Isotopic dating yields specific numerical dates (e.g., 17.6  0.4 Ma = Tempe Butte) Geologic Map of the Tempe Butte area

4. 6 protons & 8 neutrons: mass number = 14 = 14 C 6 protons & 6 neutrons: mass number = 12 = 12 Cprotons 6 protons & 7 neutrons: mass number = 13 = 13 C  In any element, the number of neutrons can varyelementneutrons  These variations of the same element are called isotopesisotopes What’s an Isotope? Absolute Geologic Dating

5.  A unstable nucleus (parent) breaks-down (decays) into a new element (daughter)nucleus Radioactive Decay Absolute Geologic Dating

6.  The “atomic clock” starts at this point (parent atoms begin to decay into daughter atoms)  As an igneous rock crystallizes, parent atoms get locked into their minerals Radioactive Decay Absolute Geologic Dating

7. all parent atomsmore daughter atoms  Over time, parent atoms decay & daughter atoms accumulate time Radioactive Decay Absolute Geologic Dating

8. Which is older & why? Radioactive Decay Absolute Geologic Dating  Example: rock A (basalt) = 2,500 parent & 7,500 daughter rock B (rhyolite) = 5,000 parent & 5,000 daughter Rock ARock B

9. Radioactive Decay Absolute Geologic Dating Linear Decay  Radioactive decay is not linear

10.  Half-life ( ) = the time it takes for half of the parent atoms to decay into daughter atoms Half-life Radioactive Decay Absolute Geologic Dating  Radioactive decay is exponential Exponential Decay

11. After 10 yrs (10 ) 1 ( 99%) 1:999 Example: for an isotope with a half-life of 1 year After 3 yrs (3 ) 125 (12.5%) 875 (87.5%) 1:7 After 2 yrs (2 ) 250 (25%) 750 (75%) 1:3 Rock formed1000 (100%) 0 (0%) - Time # of atoms ParentDaughterP:D ratio After 1 year (1 )500 (50%) 500 (50%) 1:1 Radioactive Decay Absolute Geologic Dating

12.  Isotopes with long ( 238 U, 40 K) - useful for older rocks  Isotopes with short ( 14 C) - useful for younger rocksHalf-life Absolute Geologic Dating

13.  Must have a “closed system” (no parent/daughter atoms added/removed by metamorphism)  Crystal formed with only parent isotopes (no daughter atoms in the system at the beginning) > Look for signs of weathering or alteration  Need measurable amounts of parent material  The rate of decay for various isotopes is constant and can be determined Important Constraints Absolute Geologic Dating

14.  Need: 1) amount of parent & daughter material, and 2) half-life of the parent isotope 2 = 2 * 10 Ma = 20 Ma  Example – An igneous rock has 250 parent atoms & 750 daughter atoms. If the parent isotope = 10 Ma, how old is the rock? Getting a “Number Age” Absolute Geologic Dating # of 0 1 2 parent atoms 1000500250

15. yes (e.g., basalt, granite, etc.) > zircon ( 238 U), mica & K-feldspar ( 40 K), or unfossilized organic material ( 14 C) no no (unless it’s C-rich)  Radioactive isotopes (e.g., 238 U, 40 K, 14 C, etc.) > igneous > sedimentary > metamorphic  Certain minerals  Certain rock types Getting a “Number Age” Absolute Geologic Dating What Do We Need?

16. Interpreting Absolute Dates Absolute Geologic Dating  For igneous rocks, the absolute age represents crystallization age Basaltic lava erupting at Kilaeua Close-up of basaltMinerals in a basalt

17.  For sedimentary rocks, the crystallization age of the certain clasts OR if C-rich organic material, when the plant/animal died Interpreting Absolute Dates Absolute Geologic Dating Conglomerate Carbonized plant material

18.  For metamorphic rocks: metamorphism alters the “atomic clock” by changing the amount of parent & daughter atoms  The age of crystallization appears YOUNGER when daughter atoms are lost during metamorphism (e.g., less daughter = less decay = younger) Interpreting Absolute Dates Absolute Geologic Dating

19.  Absolute ages also tell us (indirectly) how old adjacent rocks are  Age of Layer ? = 200,000-400,000 a (or 200-400 Ka) Relative & Absolute Dating Absolute Geologic Dating

20.  The Geologic Time Scale is a dual scale based on relative ages of rock sequences with isotopic dates added to itGeologic Time Scale Absolute Geologic Dating

21. sill (D) = 100 Ma, dike (F) = 10 Ma, lava flow (H) = 1 Ma Geologic Sequence Diagram Absolute Geologic Dating

22.  Deposition of A  Deposition of B  Deposition of C  Deposition of E  Intrusion of D  Tilting & erosion  Deposition of G  Extrusion of H  Deposition of I, J, then K Y  Intrusion of F O (>100 Ma) (10-1 Ma) (100 Ma) (10 Ma) (1 Ma) (<1 Ma) Geologic Sequence Diagram Absolute Geologic Dating

23. WWW Links in this Lecture > Absolute dating - http://en.wikipedia.org/wiki/Absolute_datinghttp://en.wikipedia.org/wiki/Absolute_dating > Geochronology - http://en.wikipedia.org/wiki/Geochronologyhttp://en.wikipedia.org/wiki/Geochronology > Isotopes - http://en.wikipedia.org/wiki/Isotopeshttp://en.wikipedia.org/wiki/Isotopes > Element - http://en.wikipedia.org/wiki/Chemical_element > Neutrons - http://en.wikipedia.org/wiki/Neutrons > Protons - http://en.wikipedia.org/wiki/Protons > Nucleus - http://en.wikipedia.org/wiki/Atomic_Nucleus > Radioactive decay - http://en.wikipedia.org/wiki/Radioactive_decay > Exponential decay - http://en.wikipedia.org/wiki/Exponential_decay > Half-life - http://en.wikipedia.org/wiki/Half-life > Geologic time scale - http://www.stratigraphy.org/geowhen/timelinestages.htmlhttp://en.wikipedia.org/wiki/Chemical_elementhttp://en.wikipedia.org/wiki/Neutronshttp://en.wikipedia.org/wiki/Protonshttp://en.wikipedia.org/wiki/Atomic_Nucleushttp://en.wikipedia.org/wiki/Radioactive_decayhttp://en.wikipedia.org/wiki/Exponential_decayhttp://en.wikipedia.org/wiki/Half-lifehttp://www.stratigraphy.org/geowhen/timelinestages.html Absolute Geologic Dating