Optional Field Trip Exercise Changed from March 18 to March 4 Due March 9 at 1:10 pm
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Transcript of Optional Field Trip Exercise Changed from March 18 to March 4 Due March 9 at 1:10 pm
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Optional Field Trip ExerciseChanged from March 18 to
March 4Due March 9 at 1:10 pm
Zuhl MuseumAlumni Center, 775 College Av.
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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)
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LIFE ON EARTH
3.5 billion year old bacteria and algae
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Cambrian explosion of life:540 million years ago
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Trilobites-Paleozoic arthropods
Today: 80% of species are arthropods
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another Trilobite
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Crinoids
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First land animals:365 Ma
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Dimetrodon-Permian reptile
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End of the Paleozoic
Major extinction event:96% of marine life was killed70% land vertebrates killed
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End of the Paleozoic
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Mesozoic-Ammonites
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Mesozoic-Ammonites
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Mesozoic-Triceratops
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Cretaceous-Tyrannosaurus
Rex
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End of the Mesozoic
Major extinction event:85% of all species died.
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Cenozoic-Mammoth
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Cenozoic-Sabre tooth tiger
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Summary
Principle of superpositionPrinciple of original horizontality
Principle of faunal successionUnconformities
Time scaleEvolution of life
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ISOTOPES
Elements with SAME number of protons and different number of neutrons
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ISOTOPES
SOME isotopes are radioactive...
Example:
12C is stable
14C is radioactive, and decays into 14N
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Radioactivity
Radioactive decay--constant rateGives off energy
Constant rate of decay gives us a geologic clock
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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
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0halflives:
100 parents
0 daugh-ters
Ratio:
100:0
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1halflives:
50 parents
50 daugh-ters
Ratio:
1:1
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1halflives:
50 parents
50 daugh-ters
Ratio:
1:1
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2halflives:
25parents
75daugh-ters
Ratio:
1:3
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3halflives:
13 parents
87daugh-ters
Ratio:
1:7
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Half-lives of elements
Parent Daughter Half Life14C 14N 5730 yrs
40K 40Ar 1.3 Ga
238U 206Pb 4.4 Ga
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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
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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)
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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?
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ZIRCONS ARE FOREVER
Zircons have Uranium and makeexcellent geologic clocks
4.364 Ga zircon:Jack Hills Quartzite,Australia
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This zircon is about 0.1 mm long
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Igneous Zircons
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Metamorphic zircons
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Zircons in sedimentary rock
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Zircons
• Igneous: age of cooling of magma• Metamorphic: age of heating• Sedimentary: age of source rocks
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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
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Applications
Useful for archeology
Shroud of Turin:
3 labs determined an age of 1260-1390 A.D.