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.