Transcript GLG101online_03B_AbsoluteDating_MCC_Leighty

Absolute Geologic Dating
1100 Ma
GLG 101 - Physical Geology
Bob 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.
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and, by themselves, are not meant to be a comprehensive source of information.
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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
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Absolute
Geologic Dating
Absolute Geologic Dating
 Dating rocks = Geochronology
 Certain isotopes of some elements are unstable and decay over
time
 Isotopic dating measures the decay of radioactive isotopes to
date rocks
 Isotopic dating yields specific numerical dates
(e.g., 17.6 0.4 Ma = Tempe Butte)
Periodic Table of the Elements
Geologic Map of the Tempe Butte area
Absolute
Geologic Dating
What’s an Isotope?
 In any element, the number of neutrons can vary
6 protons & 6 neutrons: mass number = 12
=
12C
6 protons & 7 neutrons: mass number = 13
=
13C
6 protons & 8 neutrons: mass number = 14
=
14C
 These variations of the same element are called isotopes
Absolute
Geologic Dating
Radioactive Decay
 A unstable nucleus (parent) breaks-down (decays) into a new
element (daughter)
Absolute
Geologic Dating
Radioactive Decay
 As an igneous rock crystallizes, parent atoms get locked into
their minerals
 The “atomic clock” starts at this point (parent atoms begin to
decay into daughter atoms)
Absolute
Geologic Dating
Radioactive Decay
 Over time, parent atoms decay & daughter atoms accumulate
all parent atoms
more daughter atoms
time
Absolute
Geologic Dating
Radioactive Decay
 Example:
rock A (basalt) = 2,500 parent & 7,500 daughter
rock B (rhyolite) = 5,000 parent & 5,000 daughter
Rock A
Rock B
Which is older & why?
Absolute
Geologic Dating
Radioactive Decay
Linear Decay
 Radioactive decay is not linear
Absolute
Geologic Dating
Radioactive Decay
Exponential Decay
 Radioactive decay is exponential
 Half-life () = the time it takes for half of the parent atoms to
decay into daughter atoms
Absolute
Geologic Dating
Radioactive Decay
Example: for an isotope with a half-life of 1 year
Time
# of atoms
Parent
Daughter
Rock formed
1000 (100%) 0 (0%)
After 1 year (1)
500 (50%)
500 (50%)
1:1
After 2 yrs (2)
250 (25%)
750 (75%)
1:3
After 3 yrs (3)
125 (12.5%)
875 (87.5%)
1:7
After 10 yrs (10)
1 (<1%)
999 (>99%)
1:999
P:D ratio
-
Absolute
Geologic Dating
Half-life
 Isotopes with long  (238U,40K) - useful for older rocks
 Isotopes with short  (14C) - useful for younger rocks
Absolute
Geologic Dating
Important Constraints
 Must have a “closed system”
(no parent/daughter atoms added/removed by metamorphism)
> Look for signs of weathering or alteration
 Crystal formed with only parent isotopes
(no daughter atoms in the system at the beginning)
 The rate of decay for various isotopes is constant and can be
determined
 Need measurable amounts of parent material
Absolute
Geologic Dating
Getting a “Number Age”
 Need: 1) amount of parent & daughter material, and
2) half-life of the parent isotope
 Example – An igneous rock has 250 parent atoms & 750 daughter
atoms. If the parent isotope  = 10 Ma, how old is the rock?
# of 
0
1
2
parent atoms
1000
500
250
2 = 2 * 10 Ma = 20 Ma
Absolute
Geologic Dating
Getting a “Number Age”
What Do We Need?
 Radioactive isotopes (e.g.,
238U, 40K, 14C,
etc.)
 Certain minerals
>
zircon (238U), mica & K-feldspar (40K), or unfossilized organic
material (14C)
 Certain rock types
>
igneous yes (e.g., basalt, granite, etc.)
>
sedimentary no (unless it’s C-rich)
>
metamorphic no
Absolute
Geologic Dating
Interpreting Absolute Dates
 For igneous rocks, the absolute age represents crystallization age
Basaltic lava erupting at Kilaeua
Close-up of basalt
Minerals in a basalt
Absolute
Geologic Dating
Interpreting Absolute Dates
 For sedimentary rocks, the crystallization age of the certain
clasts OR if C-rich organic material, when the plant/animal died
Conglomerate
Carbonized plant material
Absolute
Geologic Dating
Interpreting Absolute Dates
 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)
Absolute
Geologic Dating
Relative & Absolute Dating
 Absolute ages also tell us (indirectly) how old adjacent rocks are
 Age of Layer ? = 200,000-400,000 a (or 200-400 Ka)
Absolute
Geologic Dating
Geologic Time Scale
 The Geologic Time Scale is a dual scale based on relative ages of
rock sequences with isotopic dates added to it
Absolute
Geologic Dating
Geologic Sequence Diagram
sill (D) = 100 Ma, dike (F) = 10 Ma, lava flow (H) = 1 Ma
Absolute
Geologic Dating
Y
Geologic Sequence Diagram
 Deposition of I, J, then K (<1 Ma)
 Extrusion of H (1 Ma)
 Deposition of G
 Tilting & erosion
(10-1 Ma)
 Intrusion of F (10 Ma)
 Intrusion of D (100 Ma)
 Deposition of E
 Deposition of C
O
 Deposition of B
 Deposition of A
(>100 Ma)
Absolute
Geologic Dating
WWW 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