GY 112: Earth History Lecture 7 & 8: Dating

UNIVERSITY OF SOUTH ALABAMA

GY 112: Earth History

Lecture 7 & 8: Dating

Instructor: Dr. Douglas W. Haywick

Last Time

1. Alfred Wegener and “Drifting Continents”

2. The Plate Tectonic Revolution

3. Plate Tectonics Mechanisms

His supporting evidence?

Matching rock types and fossils*

* types and ages

The Plate Tectonics

Revolution

Paleomagnetism shows that the ocean floor youngest near the ridges and oldest near the continents

The Plate Tectonics Revolution

The outer part of the Earth is broken up into several large tectonic plates

The Plate Tectonics Revolution

1963-1968 J. Tuzo Wilson was the first to describe global tectonics in terms of rigid surface "plates“, and recognized ocean evolution (“Wilson Cycle”).

He characterized three basic plates boundaries

Four Major “Geophysical” Layers

1) The Crust

2) The Mantle

3) The Outer Core

4) The Inner Core (1270 km; solid metal)

The Plate Tectonics Mechanism

It is postulated that the convection currents can eventually break up the lithosphere into separate plates

Tension

Cooler

Hotter

Divergent Plate

Boundaries

Convergent Plate

Boundaries

Transform Fault

Plate Boundaries

Today’s Agenda

1. Relative vs. Absolute Dating Techniques a) Magnetostratigraphy b) Fission Track Dating

2. Radiometric Dating

3. Mass spectrophotometers

(Web Lectures 7 & 8)

Dating

Geologists can time events by putting them in order of occurrence.

But, this does not allow you to actually date when those events actually occurred.

Source: http://academic.brooklyn.cuny.edu/geology/leveson/core/topics/time/graphics/history1e.gif

Geological Dating Techniques

Relative Techniques : Assigns an age to a rock that puts it into a narrow range (e.g., mid-Devonian; Late Cretaceous, upper Pliocene).


Relative Techniques: Assigns an age to a rock that puts it into a narrow range (e.g., mid-Devonian; Late Cretaceous, upper Pliocene).

Absolute Techniques : Assigns an age to a rock that is a number (e.g., 354.7 +/- 21.3 MA; 1,453 KA +/- 67 KA).


Relative Techniques : paleontology (biostratigraphy), stable isotope stratigraphy, paleomagnetism/magnetostratigraphy)

Source: http://www.ideofact.com/archives/trilobite.jpg


Relative Techniques: paleontology ( biostratigraphy ), stable isotope stratigraphy, paleomagnetism)


Absolute Techniques: fission track dating, radiometric dating

Source: http://www.geo.umn.edu/people/grads/bair0042/MFT.html

Relative Dating

Magnetic Stratigraphy or Magnetostratigraphy

Recall: Paleomagnetism

S

Magnetostratigraphy

N

•The Earth has a magnetic field

•north is north and south is south, but…

Magnetostratigraphy

S

•The Earth has a magnetic field

•north is north; south is south, but…

…. It hasn’t always been that way

Magnetic reversals

N

Magnetostratigraphy

• Magnetization of ancient rocks at the time of their formation is a good piece of evidence supporting plate tectonics….

Source: http://piru.alexandria.ucsb.edu/collections/geosyst ems/geosystems11-15.jpg

Magnetostratigraphy

• Magnetization of ancient rocks at the time of their formation is a good piece of evidence supporting plate tectonics….

….. and, it allows us to date rocks (kind of)

Source: http://piru.alexandria.ucsb.edu/collections/geosyst ems/geosystems11-15.jpg

Magnetostratigraphy

• Reversals in polarity of field are recorded in rocks when they crystallize and as they settle from water

Magnetometer

Magnetostratigraphy

Magnetometer

• Reversals in polarity of field are recorded in rocks when they crystallize and as they settle from water

• Vertical successions of sedimentary rock record changes in magnetic field over time

Magnetostratigraphy

A portion of the paleomagnetic record from 10 MA to 0 MA (today)

Magnetostratigraphy

• Chron

– Polarity time-rock unit

Magnetostratigraphy

• Chron

– Polarity time-rock unit

– Period of normal or reversed polarity

• Normal interval

– Same as today

– Black

• Reversed interval

– Opposite to today

– White

Magnetostratigraphy

Absolute Dating

Fission Track Dating


The Periodic Table of the elements


Radioactive elements are unstable

Absolute Techniques

• Fission-Track Dating

– Measure decay of uranium 238 by counting number of tracks

Absolute Techniques

• Fission-Track Dating

– Measure decay of uranium 238 by counting number of tracks

Radiometric Dating

Uranium (and others) are unstable

Radioactive Decay

Radioactive Decay

Three modes of decay

Radioactive Decay


1) Alpha Decay

Loss of alpha particle

• Convert parent into element that has nucleus containing two fewer protons

U

235

→ Pb

207

Radioactive Decay


1) Alpha Decay



2) Beta Decay

Loss of beta particle

• Convert parent into element whose nucleus contains one more proton by losing an electron

C

14

→ N

14

Radioactive Decay


1) Alpha Decay



2) Beta Decay

Loss of beta particle

• Convert parent into element whose nucleus contains one more proton by losing an electron

3) Gamma Decay

Capture of beta particle

• Convert parent into element whose nucleus has one less proton K 40

→

Ar 40

Radioactive Decay

Alpha Decay (Uranium)

238 U

→

206 Pb + 8

α

Radioactive Decay

• Radiometric dating

– Radioactive isotopes decay at constant geometric rate

• After a certain amount of time, half of the parent present will survive and half will decay to daughter

Radioactive Decay

• Radiometric dating

– Radioactive isotopes decay at constant geometric rate

• After a certain amount of time, half of the parent present will survive and half will decay to daughter

• Half-life

– Interval of time for half of parent to decay

Absolute Age

• Absolute ages change

– Error increases in older rocks

– Techniques change

• Biostratigraphic correlations may be more accurate

Half Lives

14

235

40

238

232

Parent Isotope

C (Carbon-14)

K (Potassium-40)

U (Uranium-235)

U (Uranium-238)

14

207

40

206

Daughter Isotope

N (Nitrogen-14)

Pb (Lead-207)

Ar (Argon-40)

Pb (Lead-206)

Th (Thorium-232) 208 Pb (Lead-208)

87 Rb (Rubidium-87) 87 Sr (Strontium-87)

147 Sm (Samarium-147) 143 Nd (Neodymium-143)

Half Life (years) Datable Material(s)

Wood, shells and organic material

Metamorphic, igneous rocks,

Zircon, U-bearing minerals

Metamorphic, igneous & sedimentary rocks; feldspar-bearing minerals





Various rocks and minerals

Very old rocks, REE bearing minerals

Half Lives

14

235

40

238

232

Parent Isotope

C (Carbon-14)

K (Potassium-40)

U (Uranium-235)

U (Uranium-238)

14

207

40

206


N (Nitrogen-14)

Pb (Lead-207)

Ar (Argon-40)

Pb (Lead-206)





5,730 Wood, shells and organic material










Half Lives

14

235

40

238

232

Parent Isotope

C (Carbon-14)

K (Potassium-40)

U (Uranium-235)

U (Uranium-238)

14

207

40

206


N (Nitrogen-14)

Pb (Lead-207)

Ar (Argon-40)

Pb (Lead-206)






700,000,000 Metamorphic, igneous rocks,









Half Lives

14

235

40

238

232

Parent Isotope

C (Carbon-14)

K (Potassium-40)

U (Uranium-235)

U (Uranium-238)

14

207

40

206


N (Nitrogen-14)

Pb (Lead-207)

Ar (Argon-40)

Pb (Lead-206)








1,300,000,000 Metamorphic, igneous & sedimentary rocks; feldspar-bearing minerals







Half Lives

14

235

40

238

232

Parent Isotope

C (Carbon-14)

K (Potassium-40)

U (Uranium-235)

U (Uranium-238)

14

207

40

206


N (Nitrogen-14)

Pb (Lead-207)

Ar (Argon-40)

Pb (Lead-206)









4,500,000,000 Metamorphic, igneous rocks,






Half Lives

14

235

40

238

232

Parent Isotope

C (Carbon-14)

K (Potassium-40)

U (Uranium-235)

U (Uranium-238)

14

207

40

206


N (Nitrogen-14)

Pb (Lead-207)

Ar (Argon-40)

Pb (Lead-206)















Half Lives

14

235

40

238

232

Parent Isotope

C (Carbon-14)

K (Potassium-40)

U (Uranium-235)

U (Uranium-238)

14

207

40

206


N (Nitrogen-14)

Pb (Lead-207)

Ar (Argon-40)

Pb (Lead-206)













48,600,000,000 Various rocks and minerals


Half Lives

Parent Isotope

14 C (Carbon-14)

235 U (Uranium-235)


14 N (Nitrogen-14)

207 Pb (Lead-207)

Half Life (years)

5,730

Datable Material(s)

Wood, shells and organic material



40 K (Potassium-40) 40 Ar (Argon-40) 1,300,000,000 Metamorphic, igneous & sedimentary rocks; feldspar-bearing minerals

238 U (Uranium-238) 206 Pb (Lead-206) 4,500,000,000 Metamorphic, igneous rocks,


232 Th (Thorium-232) 208 Pb (Lead-208) 14,000,000,000 Metamorphic, igneous rocks,


87 Rb (Rubidium-87) 87 Sr (Strontium-87) 48,600,000,000 Various rocks and minerals

147 Sm (Samarium-147) 143 Nd (Neodymium-143) 106,000,000,000 Very old rocks, REE bearing minerals

Age Determination

Mass Spectrophotometer

Age Determination

The all important age equation:

N=N

o

e

-

λ t

No is the number of atoms of parent isotope remaining in a substance

N is the number of atoms of daughter isotope produced through decay,

λ is the decay constant (which depend on the isotope in question) t is the amount of elapsed time.

Age Determination

A more useful equation for age determination:

Rock age= 1/

λ

x ln[(Do-D) + 1]

N

Do is the original amount of daughter isotope in the sample

N is the amount of current parent isotope in the sample

D is the amount of current daughter isotope in the sample

λ is the decay constant

Today’s Homework

1) Quiz 3 Thursday

(Short Answer: Compare/Contrast)

Next Time

Lectures 6: Stratigraphy

Heads-up for tomorrow’s lab

Bring Scientific calculator to lab !

GY 112: Earth History

Lectures 7, 8: Dating

Instructor: Dr. Doug Haywick dhaywick@southalabama.edu

This is a free open access lecture, but not for commercial purposes.

For personal use only.

GY 112: Earth History Lecture 7 & 8: Dating

GY 112: Earth History

Divergent Plate

Boundaries

Convergent Plate

Boundaries

Transform Fault

Plate Boundaries

Magnetostratigraphy

Magnetostratigraphy

Magnetostratigraphy

Magnetostratigraphy

U

→ Pb

C

→ N

N=N

e