Optional Field Trip ExerciseChanged from March 18 to

March 4Due March 9 at 1:10 pm

Zuhl MuseumAlumni Center, 775 College Av.

Geologic Time Scale

• Precambrian (4.6 Ga to 540 Ma)

• Paleozoic (540 Ma to 250 Ma)• Mesozoic (250-65 Ma)

– Triassic– Jurassic– Cretaceous

• Cenozoic (65 Ma to the present)

LIFE ON EARTH

3.5 billion year old bacteria and algae

Cambrian explosion of life:540 million years ago

Trilobites-Paleozoic arthropods

Today: 80% of species are arthropods

another Trilobite

Crinoids

First land animals:365 Ma

Dimetrodon-Permian reptile

End of the Paleozoic

Major extinction event:96% of marine life was killed70% land vertebrates killed

End of the Paleozoic

Mesozoic-Ammonites

Mesozoic-Ammonites

Mesozoic-Triceratops

Cretaceous-Tyrannosaurus

Rex

End of the Mesozoic

Major extinction event:85% of all species died.

Cenozoic-Mammoth

Cenozoic-Sabre tooth tiger

Summary

Principle of superpositionPrinciple of original horizontality

Principle of faunal successionUnconformities

Time scaleEvolution of life

ISOTOPES

Elements with SAME number of protons and different number of neutrons

ISOTOPES

SOME isotopes are radioactive...

Example:

12C is stable

14C is radioactive, and decays into 14N

Radioactivity

Radioactive decay--constant rateGives off energy

Constant rate of decay gives us a geologic clock

Half-lives

Different elements decay at different rates.

The time it takes for HALF of the atoms to decay is the “half-life”

Parents decay into daughters…

Need both parents and daughters

0halflives:

100 parents

0 daugh-ters

Ratio:

100:0

1halflives:

50 parents

50 daugh-ters

Ratio:

1:1

1halflives:

50 parents

50 daugh-ters

Ratio:

1:1

2halflives:

25parents

75daugh-ters

Ratio:

1:3

3halflives:

13 parents

87daugh-ters

Ratio:

1:7

Half-lives of elements

Parent Daughter Half Life14C 14N 5730 yrs

40K 40Ar 1.3 Ga

238U 206Pb 4.4 Ga

Pick the best isotope for the job

• C: Young, organic material– Ex.: Charcoal, peat

• K: Young or old minerals with K– Ex.: Feldspar, biotite

• U: Old minerals containing U– Ex.: Zircon

To get the age of a mineral...

Need to know:

1) Ratio of parent to daughter atoms (measured with Mass-Spectrometer)

2) Half-life of the element (known by experiment or estimated)

Example

You have 1000 parent atoms to start.

After one 1/2 life, you have how many left?

If the half life is 700 million years, the rock is 700 Ma.

If you have 250 parent atoms left, how old is the rock?

ZIRCONS ARE FOREVER

Zircons have Uranium and makeexcellent geologic clocks

4.364 Ga zircon:Jack Hills Quartzite,Australia

This zircon is about 0.1 mm long

Igneous Zircons

Metamorphic zircons

Zircons in sedimentary rock

Zircons

• Igneous: age of cooling of magma• Metamorphic: age of heating• Sedimentary: age of source rocks

Carbon dating

Short half life: 5730 yrs.

Why is it still here? Cosmic rays convert N to C

Photosynthesis and animal food: C exchange with environment

After death, C exchange stops, “clock starts”

14C decays and forms 14N

Applications

Useful for archeology

Shroud of Turin:

3 labs determined an age of 1260-1390 A.D.