Geosciences
Geologic Time
Rates in geologic time
Relative Time
Numerical Time
Key events in Earth history
HW 1 – due next week on Monday, 10:59 am. Please note: HW should be completed in YOUR OWN WORDS! Copying/pasting from the internet or other students is a violation of the UA Code of Academic Integrity and will result it no credit for the HW and a report to the dean of students.
Remember these principles: “Relative Dating”
youngest
oldest
Original Horizontality- rocks are deposited in horizontal layers
Superposition – rocks at the
bottom are the oldest
Tilting/folding is younger than deposition
Inclusion – younger rocks may incorporate pieces of older rocks
Cross-cutting relationships – older rocks
may be cut by younger rocks or features
(examples: intrusions of magma, faults)
older
older
younger
older
older
younger
younger
older
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We also use… Fossils Certain fossils exist in a narrow time range but over a wide distribution on Earth. Index Fossils
Example: Dinosaurs
Example – T Rex exists in Cretaceous rocks
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Now we have an order of events, but what about numerical ages?
Where did the numbers on the geologic time scale come from?
have nuclei that spontaneously decay
daughter
parent
Radioactive isotopes
-- emit or capture subatomic particles
parent: unstable, decaying radioactive isotope
daughter: stable radiogenic isotope
K40 Ar40
19 protons 18 protons
Proton converts to neutron via
electron capture
(Half-life is 1.25 billion years)
Half-life: time it takes for half the parent isotopes to decay (change) to daughter isotopes.
Example:
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Example – Potassium decays to Argon via electron capture
Carbon (14C) decaying to Nitrogen (14N)
Half-life of 14C = 5,730 ± 40 years
Half life = time it takes for half of the parent atoms to spontaneously decay to daughter atoms.
Half-lives are constant and known (have been measured).
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Decays by beta decay (emits a beta particle, either an electron or a positron). In the case of C14 it is electron emission. This is beta negative decay in which a neutron converts to a proton while emitting an electron and an antineutrino. Willard Libby – 1960 Nobel Prize
# half lives
Parent remaining
Daughter present
0
1
2
3
4
600 (total at start)
0
Handout – #s 1-10 only - let’s do it!
Once you have above filled in, method to find age:
1) Figure out how many ½ lives have passed based on amount of parent remaining and/or daughter present
2) Multiply the half life (known) by the number of half lives that have passed
Parent 300, 150, 75, 37.5. Daughter 300, 450, 525, 562.5.
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Rocks as clocks?
A mountain – made of rock
Half dome – used to be a magma chamber!
A rock like granite is chock full of minerals that incorporate radioactive elements when they form!
Zircon – a fantastic clock!
Incorporates Uranium into its structure when it forms from magma.
U-Pb age analyses
Rocks crushed, zircons extracted, and analyzed for their isotopic compositions, from which an age is calculated.
Images by George Gehrels, UA Geosciences
How can we date a very old rock?
Choose a chronometer with appropriate
time scale:
Isotope Half-lives Max ages:
14C: 5730 y 50 Ka
K-Ar: 1.3 Ga 4.5 Ga
Rb-Sr: 48 Ga 4.5 Ga
238U-206Pb: 4.6 Ga 4.5 Ga
235U-207Pb: 704 Ma 4.5 Ga
Sm-Nd: 106 Ga 4.5 Ga
Ka = thousand years
Ga = billion years
Ma = million years
Turns out, Pb is a great isotope to use to date rocks on Earth.
Handout #s 11-18 – GO!
4.1-4.3 Ga ages from Jack Hills of western Australia
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Formation of the Earth
Earth is about 4.6 billion years old, based on the
radiometric ages of meteorites and lunar rocks.
There were no rocks on earth at that time.
Type Number Dated Method Age (billions of years)
Chondrites (undisturbed H, LL, E) 38 Rb-Sr 4.50 +/- 0.02
H Chondrites (undisturbed) 17 Rb-Sr 4.52 +/- 0.04
H Chondrites 15 Rb-Sr 4.59 +/- 0.06
LL Chondrites (undisturbed) 13 Rb-Sr 4.49 +/- 0.02
E Chondrites (undisturbed) 8 Rb-Sr 4.51 +/- 0.04
Eucrites (polymict) 23 Rb-Sr 4.53 +/- 0.19
Eucrites 13 Lu-Hf 4.57 +/- 0.19
Iron (plus iron from St. Severin) 8 Re-Os 4.57 +/- 0.21
Oldest rock known on Earth dates at ~4.2 bill yrs. Solid crust existed by 4.2 bill yrs ago
So…why do we say Earth is 4.6 billion years old and not 4.2 billion years old?
Recall our solar system discussion…meteorites…
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