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1100 Ma1100 Ma
Absolute Geologic DatingAbsolute Geologic Dating
GLG 101 - Physical GeologyGLG 101 - Physical Geology Bob LeightyBob Leighty
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 ([email protected]).Leave no questions behind!
Explore and have fun!Explore and have fun!
Isotopic dating measures the decay of radioactive isotopes to date rocks
Dating rocks = Geochronology
Absolute Geologic DatingAbsolute Geologic DatingAbsoluteAbsolute
Geologic DatingGeologic Dating
Certain isotopes of some elements are unstable and decay over time
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
6 protons & 8 neutrons: mass number = 14 = 14C
6 protons & 6 neutrons: mass number = 12 = 12C
6 protons & 7 neutrons: mass number = 13 = 13C
In any element, the number of neutrons can vary
These variations of the same element are called isotopes
What’s an Isotope?What’s an Isotope?AbsoluteAbsolute
Geologic DatingGeologic Dating
A unstable nucleus (parent) breaks-down (decays) into a new element (daughter)
Radioactive DecayRadioactive DecayAbsoluteAbsolute
Geologic DatingGeologic Dating
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 DecayRadioactive DecayAbsoluteAbsolute
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all parent atoms more daughter atoms
Over time, parent atoms decay & daughter atoms accumulate
time
Radioactive DecayRadioactive DecayAbsoluteAbsolute
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Which is older & why?
Radioactive DecayRadioactive DecayAbsoluteAbsolute
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Example:
rock A (basalt) = 2,500 parent & 7,500 daughter
rock B (rhyolite) = 5,000 parent & 5,000 daughter
Rock A Rock B
Radioactive DecayRadioactive DecayAbsoluteAbsolute
Geologic DatingGeologic Dating
Linear DecayLinear Decay
Radioactive decay is not linear
Half-life () = the time it takes for half of the parent atoms to decay into daughter atoms
Radioactive DecayRadioactive DecayAbsoluteAbsolute
Geologic DatingGeologic Dating
Radioactive decay is exponential
Exponential DecayExponential Decay
After 10 yrs (10) 1 (<1%) 999 (>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 formed 1000 (100%) 0 (0%) -
Time # of atomsParent Daughter P:D ratio
After 1 year (1) 500 (50%) 500 (50%) 1:1
Radioactive DecayRadioactive DecayAbsoluteAbsolute
Geologic DatingGeologic Dating
Isotopes with long (238U,40K) - useful for older rocks
Isotopes with short (14C) - useful for younger rocks
Half-lifeHalf-lifeAbsoluteAbsolute
Geologic DatingGeologic Dating
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 ConstraintsImportant ConstraintsAbsoluteAbsolute
Geologic DatingGeologic Dating
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”Getting a “Number Age”AbsoluteAbsolute
Geologic DatingGeologic Dating
# of 0 1 2
parent atoms 1000 500 250
yes (e.g., basalt, granite, etc.)
> zircon (238U), mica & K-feldspar (40K), or
unfossilized organic material (14C)
no
no (unless it’s C-rich)
Radioactive isotopes (e.g., 238U, 40K, 14C, etc.)
> igneous
> sedimentary
> metamorphic
Certain minerals
Certain rock types
Getting a “Number Age”Getting a “Number Age”AbsoluteAbsolute
Geologic DatingGeologic Dating
What Do We Need?What Do We Need?
Interpreting Absolute DatesInterpreting Absolute DatesAbsoluteAbsolute
Geologic DatingGeologic Dating
For igneous rocks, the absolute age represents crystallization age
Basaltic lava erupting at Kilaeua Close-up of basalt Minerals in a basalt
For sedimentary rocks, the crystallization age of the
certain clasts OR if C-rich organic material, when the
plant/animal died
Interpreting Absolute DatesInterpreting Absolute DatesAbsoluteAbsolute
Geologic DatingGeologic Dating
Conglomerate
Carbonized plant material
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 DatesInterpreting Absolute DatesAbsoluteAbsolute
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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 DatingRelative & Absolute DatingAbsoluteAbsolute
Geologic DatingGeologic Dating
The Geologic Time Scale is a dual scale based on relative ages of rock sequences with isotopic dates added to it
Geologic Time ScaleGeologic Time ScaleAbsoluteAbsolute
Geologic DatingGeologic Dating
sill (D) = 100 Ma, dike (F) = 10 Ma, lava flow (H) = 1 Ma
Geologic Sequence DiagramGeologic Sequence DiagramAbsoluteAbsolute
Geologic DatingGeologic Dating
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 KY
Intrusion of F
O
(>100 Ma)
(10-1 Ma)
(100 Ma)
(10 Ma)
(1 Ma)
(<1 Ma)
Geologic Sequence DiagramGeologic Sequence DiagramAbsoluteAbsolute
Geologic DatingGeologic Dating
WWW Links in this LectureWWW Links in this Lecture
> Absolute dating - http://en.wikipedia.org/wiki/Absolute_dating
> Geochronology - http://en.wikipedia.org/wiki/Geochronology
> Isotopes - http://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.html
AbsoluteAbsolute
Geologic DatingGeologic Dating