EARTHSC 1121 Lecture 6: 1/26

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Finishing up on the age of the Earth; starting up on the structure of the Earth
Absolute Dating (106)
Variants of a chemical element that differ by the number of neutrons in the
nucleus. The number of protons is the same and two isotopes of an element
behave identically to one another chemically.
“Carbon-12” = Carbon with 12 things in the nucleus. Carbon has 6 protons and 6
“Carbon-13” = Carbon with 7 neutrons.
Carbon dating = an experimental technique comparing Carbon-14 to Carbon-12
Decay of Radioactive Elements
“Parent” isotope decays to a “daughter” isotope
Potassium-40 (most is Potassium-39 and is not radioactive as a stable element).
We have all consumed radioactive Potassium-40 if we have eaten a
banana. It is a small amount though, so harmless.
ß- + 40K→ 40Ar
Radioactive Decay
Every radioactive element has a characteristic time for decay: “half life”
After the amount of time of that half life has passed, half of those radioactive
elements have decayed.
N = N0e-(0.693t / t½)
N = number of atoms
N0 = number of atoms at a time (t) = 0
t½ time for the number of atoms to be half N0
Half-Life of Radioactive Isotopes
Parent Isotope; Daughter Isotope; Half-Life
There is no stable nucleus of Uranium.
Earth’s oldest rocks
Zircon, for example.
Super hard, hard to react chemically
If it forms, it will tend to form with Uranium.
Meteorites (112)
Just about all meteorites are ~4.5 Gyr.
Chondrite is a kind of meteorite.
Called chondrite because it has chondrules, or holes, within it.
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