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

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!

mailto:[email protected]

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

http://en.wikipedia.org/wiki/Geochronology

http://en.wikipedia.org/wiki/Isotopes

http://en.wikipedia.org/wiki/Absolute_dating

6 protons & 8 neutrons: mass number = 14 = 14C



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


http://en.wikipedia.org/wiki/Protons

http://en.wikipedia.org/wiki/Chemical_element

http://en.wikipedia.org/wiki/Neutrons


A unstable nucleus (parent) breaks-down (decays) into a new element (daughter)

Radioactive DecayRadioactive DecayAbsoluteAbsolute


http://en.wikipedia.org/wiki/Atomic_Nucleus

http://en.wikipedia.org/wiki/Radioactive_decay

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



all parent atoms more daughter atoms

Over time, parent atoms decay & daughter atoms accumulate

time



Which is older & why?



Example:

rock A (basalt) = 2,500 parent & 7,500 daughter

rock B (rhyolite) = 5,000 parent & 5,000 daughter

Rock A Rock B



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 decay is exponential

Exponential DecayExponential Decay

http://en.wikipedia.org/wiki/Half-life

http://en.wikipedia.org/wiki/Exponential_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



Isotopes with long (238U,40K) - useful for older rocks

Isotopes with short (14C) - useful for younger rocks

Half-lifeHalf-lifeAbsoluteAbsolute


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


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


# 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


What Do We Need?What Do We Need?

Interpreting Absolute DatesInterpreting Absolute DatesAbsoluteAbsolute


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



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)



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


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


http://www.stratigraphy.org/geowhen/timelinestages.html

sill (D) = 100 Ma, dike (F) = 10 Ma, lava flow (H) = 1 Ma

Geologic Sequence DiagramGeologic Sequence DiagramAbsoluteAbsolute


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


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





http://en.wikipedia.org/wiki/Chemical_element

http://en.wikipedia.org/wiki/Neutrons

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

Documents

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